JP5580251B2 - Wafer bonding strength inspection apparatus and method - Google Patents

Description

  The present invention relates to an apparatus and method for inspecting the bonding strength of a bonded wafer, and more particularly to an apparatus and method for inspecting the bonding strength using terahertz waves.

  In the manufacturing process of MEMS (Micro Electro Mechanical Systems) wafers, NEMS (Nano Electro Mechanical Systems) wafers, SOI (Silicon on Insulator) wafers, 3D stacked devices, power devices, etc., bonding of multiple wafers (substrates) ( Bonding) is performed.

  When the bonding strength of the bonded wafer is weak, peeling of the bonded portion may occur in the next process including dicing, and the yield deteriorates. Moreover, even if it does not peel in the next process, it may peel off due to secular change, thereby reducing the reliability of the product.

  Factors affecting the bonding strength include chemical bonding force of materials such as covalent bond, ionic bond, hydrogen bond, van der Waals force, residual stress, strength of base material, bonding quality and the like. Patent Documents 1, 2, and 3 describe examples of methods for evaluating bonding strength.

  In the method described in Patent Document 1, destructive inspection by a tensile test is used. In the method described in Patent Document 2, an unbonded portion (void or gap) generated at the bonding interface from interference fringes generated due to the optical path difference of light irradiated to the bonding portion, the capacitance of the bonding portion, or a change in impedance. , The gap) is evaluated. In the method described in Patent Document 3, the bonded wafer is diced, and the bonded surface is exposed, soaked in an etching solution such as a hydrofluoric acid aqueous solution, and the amount etched and the amount soaked into the bonding interface, Assess the bonding state.

Japanese Patent No. 2846973 JP-A-9-289238 Japanese Patent No. 4569058

  In the method described in Patent Document 1, since the sample is destroyed by a tensile test, it is not possible to inspect the bonding strength of the shipment in the production line. Furthermore, in this method, when investigating environment resistance, aging, durability against stress in the manufacturing process, etc., it is necessary to prepare a large number of samples and perform sampling inspection at each stage. In addition, the tensile test is dependent on the size of the joint area, and is not suitable for local joint inspection.

  In the method described in Patent Document 2, if there is no gap at the bonding interface, a bonding strength defect cannot be detected.

  In the method described in Patent Document 3, since a bonded wafer (bonded substrate) is immersed in an etching solution, a destructive inspection may be performed in manufacturing MEMS or the like.

  An object of the present invention is to provide an inspection apparatus and method capable of inspecting the bonding strength of a local bonded portion even when there is no gap at the bonded interface without destroying the bonded wafer. It is in.

  A wafer bonding strength inspection apparatus according to the present invention includes a sample stage that holds a bonded wafer, a terahertz wave generator that generates a terahertz wave, a terahertz wave detector that detects a terahertz wave transmitted or reflected by the bonded wafer, and a terahertz wave. An arithmetic unit that calculates the THz wave characteristic of the bonded wafer from the terahertz wave detected by the detector.

  The calculation unit calculates the bonding strength corresponding to the THz wave characteristic of the bonded wafer to be inspected from the relationship between the THz wave characteristic of the reference sample obtained in advance and the bonding strength.

  According to the present invention, it is possible to provide an inspection apparatus and method capable of inspecting the bonding strength of a local bonded portion even when there is no gap at the bonded interface without destroying the bonded wafer. Can do.

It is a figure which shows the structural example of the wafer bonding strength inspection apparatus of this invention. It is a figure explaining the outline | summary of wafer level chip size packaging. It is a figure explaining the state before joining of two Si (silicon) wafers. It is a figure explaining the 1st joining state of two Si (silicon) wafers. It is a figure explaining the 2nd joining state of two Si (silicon) wafers. It is a figure explaining the 3rd joining state of two Si (silicon) wafers. It is a figure explaining the example of the relationship between heat processing temperature and tensile strength about a joining wafer. It is a figure explaining the example of the time waveform (time waveform of a terahertz wave) of the electric field strength at the time of making a THz wave permeate | transmit a bonded wafer. It is a figure explaining the frequency power spectrum waveform obtained by carrying out the Fourier transform of the time waveform of FIG. It is a figure explaining the relationship between heat processing temperature and THz wave transmittance | permeability about a joining wafer. It is a figure explaining the relationship between terahertz wave transmittance | permeability and joining intensity | strength about a joining wafer. It is a figure explaining the example of the transmission measuring method. It is a figure explaining the time waveform of the electric field strength of the THz wave obtained by the transmission measurement method. It is a figure explaining the example of the reflection measuring method. It is a figure explaining the time waveform of the electric field strength of the THz wave obtained by the reflection measuring method. It is a figure explaining the other example of the reflection measuring method. It is a figure explaining the example of the measurement result by the reflection measuring method of FIG. It is a figure explaining the example of the result of having measured the bonding strength of the bonded wafer by the wafer bonding strength test | inspection apparatus of this invention. FIG. 14 is a diagram illustrating an example in which the result of FIG. 13 and another wafer bonding information image are displayed in an overlapping manner. It is a figure explaining the example of the method of detecting only the scattered light of a terahertz wave by the transmission measurement method in the wafer bonding strength inspection apparatus of this invention. It is a figure explaining the other example of the method of detecting transmitted light other than the scattered light of a terahertz wave by the transmission measurement method in the wafer bonding strength inspection apparatus of this invention. It is a figure explaining the example of the method of detecting only the scattered light of a terahertz wave by the reflection measurement method in the wafer bonding strength inspection apparatus of this invention. It is a figure explaining the other example of the method of detecting reflected light other than the scattered light of a terahertz wave by the reflection measurement method in the wafer bonding strength test | inspection apparatus of this invention. It is a figure explaining the example of the wafer bonding strength test | inspection method of this invention.

  An example of an embodiment of a wafer bonding strength inspection apparatus 100 according to the present invention will be described with reference to FIG. The bonded wafer 200 to be inspected is formed by bonding a plurality of wafers (substrates), that is, by bonding them together. The bonding method of the bonded wafer 200 includes anodic bonding, direct bonding, plasma activated bonding, hybrid bonding, surface activated bonding, solder bonding, fusion bonding, eutectic bonding, glass frit bonding, indirect bonding with an adhesive, and the like. There are various ways. In direct bonding or the like, an adhesive is not used. For example, in eutectic bonding, gold (Au), solder, or the like is used as an adhesive. Among these methods, an optimal joining method is selected according to the device or material to be manufactured.

The wafer material and adhesive before bonding differ depending on the device and method to be manufactured. Examples of the wafer material and adhesive include silicon (Si), gallium arsenide (GaAs), indium phosphide (InP), gallium phosphide (GaP), indium arsenide (InAs), and lithium niobate (LiNbO ₃ ). , Lithium tantalate (LiTaO ₃ ), sapphire (Al ₂ O ₃ ), gold (Au), platinum (Pt), silver (Ag), copper (Cu), aluminum (Al), tin (Sn), lead (Pb) ), Zinc (Zn), solder, aluminum oxide (Al ₂ O ₃ ), silicon carbide (SiC), silicon nitride (Si ₃ N ₄ ), aluminum nitride (AlN), gallium nitride (GaN), crystal, quartz (SiO ₂ ) and titanium (Ti).

  In the wafer bonding strength inspection apparatus 100 according to the present invention, the bonding strength of the bonded portion (bonded interface) of the bonded wafer 200 is measured and inspected by terahertz waves. A terahertz wave (hereinafter referred to as a THz wave) refers to an electromagnetic wave of 0.1 to 100 THz. In the wafer bonding strength inspection apparatus 100 of this example, a THz wave having an optimum frequency band obtained by an experiment is selected corresponding to the bonding method of bonded wafers.

  In the wafer bonding strength inspection apparatus 100 of this example, generation and detection of THz waves are performed using a basic configuration of terahertz time domain spectroscopy (THz-TDS). Terahertz time-domain spectroscopy is a spectroscopic method that obtains a spectrum of an electromagnetic wave by Fourier-transforming a time waveform of an oscillating electric field intensity obtained by directly measuring the waveform of a terahertz wave. Terahertz time-domain spectroscopy is applied to various fields because it can obtain the amplitude, waveform and phase for each frequency of electromagnetic waves. The time waveform of the oscillating electric field intensity may be simply referred to as a THz wave time waveform. In the present invention, in order to detect the bonding strength, the time waveform of the THz wave may be obtained by terahertz time domain spectroscopy, but without obtaining the time waveform, only the peak value of the time waveform, that is, the maximum or minimum value. May be detected.

  In terahertz time domain spectroscopy, a transmission measurement method for detecting a THz wave transmitted through a sample and a reflection measurement method for detecting a THz wave reflected from the sample are known. In the present invention, either the transmission measurement method or the reflection measurement method may be used. Although the THz wave depends on conditions, it is permeable to compounds such as semiconductors and ceramics like radio waves, but is not permeable to metals. When the transmission measurement method is used, at least a part of the plurality of wafers constituting the bonded wafer 200 is formed of a material that transmits THz waves. When the reflection measurement method is used, among the plurality of wafers constituting the bonded wafer 200, the incident-side wafer is formed of a material that transmits THz waves, and the inner wafer is formed of a material that does not transmit THz waves. The Therefore, in the wafer bonding strength inspection apparatus 100 of this example, a bonded wafer 200 made of two or more wafers including at least one wafer made of a material that transmits THz waves is used as an inspection target. For example, a bonded wafer including a Si wafer that transmits THz waves to the outside is an inspection target. Examples thereof include a bonded wafer composed of an Si wafer and an Si wafer, a bonded wafer composed of an Si wafer and an Al wafer, and a bonded wafer composed of an Si wafer, an Au wafer, and an Si wafer.

  A wafer bonding strength inspection apparatus 100 includes a sample chamber 101, a personal computer (hereinafter referred to as PC) 102, a system controller 104, a femtosecond laser (ultrashort pulse laser) 106 including a beam splitter 108, and a bias power supply 110 with a modulation function. And a time delay stage 112 having a movable mirror 114 capable of linear movement 116, a lock-in amplifier 118, a preamplifier 120, a sample stage controller 122, and a sample chamber adjustment management device 124.

  Each unit in the wafer bonding strength inspection apparatus 100, that is, a femtosecond laser 106, a bias power supply 110 with a modulation function, a time delay stage 112, a lock-in amplifier 118, a sample stage controller 122, and a sample chamber adjustment management device 124, It is connected to the system controller 104 via a cable 130. Dashed lines represent electrical connections.

  The system controller 104 performs control for operating the function of each unit in the wafer bonding strength inspection apparatus 100 in accordance with a command from the PC 102 or a program or trigger incorporated in the PC 102 in advance. The operator inputs a command related to the inspection of the bonding strength of the bonded wafer 200 to the system controller 104 via the PC 102. The wafer bonding strength inspection apparatus 100 inspects the bonding strength of the bonded wafer 200 in accordance with a command from the system controller 104. The result of the joint strength inspection is sent to the PC 102 via the system controller 104.

  In this example, the PC 102 is connected to the system controller 104, but a system higher than the PC 102 may be connected to the system controller 104. The display unit is not necessarily the PC 102, and may be an arbitrary device such as a monitor device or a display panel. In this case, the display unit is controlled by the system controller 104.

  The system controller 104 may include functions of the sample stage controller 122 and the sample chamber adjustment management device 124. In that case, the sample stage controller 122 and the sample chamber adjustment management device 124 are omitted. Furthermore, the system controller 104 and the PC 102 may be replaced by one control arithmetic device.

  In the sample chamber 101, a THz wave optical system 150 that generates and detects THz waves and a sample stage 160 that supports the bonded wafer 200 to be inspected are provided. The THz wave optical system 150 includes a THz wave generator 151, a THz wave projecting optical system 153, a THz wave receiving optical system 154, and a THz wave detector 157. The THz wave generator 151 is provided on the side wall of the sample chamber 101, and the THz wave detector 157 is provided on the bottom wall of the sample chamber 101. In this example, the optical axis of the THz wave generator 151 and the optical axis of the THz wave detector 157 are arranged so as to be orthogonal to each other so as not to match.

  The THz wave generator 151 includes a condenser lens 151a, a THz wave generation element 151b, and a lens 151c. The THz wave generating element 151b is configured by a photoconductive switch, a nonlinear optical crystal, or the like. The THz wave projecting optical system 153 includes an ellipsoidal mirror, a parabolic mirror, and the like. The THz wave receiving optical system 154 includes a parabolic mirror, a Winston cone mirror, and the like. A dotted line 138 in the THz wave optical system 150 indicates an optical path of the THz wave. The THz wave detector 157 includes a condenser lens 157a, a THz wave detection element 157b, and a lens 157c. The THz wave detection element 157b is configured by a photoconductive switch, a nonlinear optical crystal, or the like.

  The illustrated THz wave optical system 150 is an optical system for transmission measurement. In the transmission measurement method, the THz wave detector 157 is configured and arranged to detect a THz wave transmitted through the sample. In the case of the reflection measurement method, the THz wave detector 157 is configured and arranged to detect the THz wave reflected from the sample. For example, the THz wave detector 157 may be arranged on the same side of the sample as the THz wave generator 151 so that the THz wave reflected from the sample can be detected. Alternatively, an optical system for guiding the THz wave reflected from the sample to the THz wave detector 157 may be provided.

  In this example, the THz wave optical system 150 is provided with the THz wave light projecting optical system 153 and the THz wave light receiving optical system 154, so that the bonding strength of the local bonding portion of the bonded wafer 200 can be inspected.

  Between the femtosecond laser (ultra short pulse laser) 106 and the THz wave generator 151, an optical fiber 132 is provided as a femtosecond laser optical path. Between the femtosecond laser (ultrashort pulse laser) 106 and the time delay stage 112, an optical fiber 134 is provided as a femtosecond laser optical path. An optical fiber 136 is provided as a femtosecond laser optical path between the THz wave detector 157 and the time delay stage 112. In this example, femtosecond laser light is propagated through an optical fiber. Therefore, compared with the case of propagating through space, the alignment of the femtosecond laser optical system can be simplified, and at the same time, the apparatus can be miniaturized.

  Although a single THz wave optical system 150 may be provided in the sample chamber 101, a plurality of THz wave optical systems may be provided as long as the space of the sample chamber 101 allows. By providing a plurality of THz wave optical systems, the measurement time for scanning the entire surface of the bonded wafer 200 by THz wave irradiation is shortened.

  The sample stage 160 includes a chuck mechanism 161, a rotation mechanism 162, an elevating mechanism 163, and a horizontal mechanism 164, and is controlled by the sample stage controller 122 or the system controller 104. The chuck mechanism 161 is configured to place and fix the bonded wafer 200, and an adsorption method that sucks the back surface of the bonded wafer 200 with a differential pressure, a gripping mechanism method that sandwiches the edge of the bonded wafer 200, and the like may be used. The rotation mechanism 162 rotates and moves the bonded wafer 200 on the chuck mechanism 161. The lifting mechanism 163 moves the bonded wafer 200 on the chuck mechanism 161 up and down along the vertical direction. The horizontal mechanism 164 moves the bonded wafer 200 on the chuck mechanism 161 in a plane along the horizontal direction. The bonded wafer 200 is positioned by these mechanisms. That is, the irradiation position of the THz wave on the bonded wafer 200 is arranged at an arbitrary position in the sample chamber 101.

  When the entire surface of the bonded wafer 200 is scanned by THz wave irradiation, the bonded wafer 200 needs to be quickly moved with respect to the THz wave optical system. In this example, when the entire surface of the bonded wafer 200 is scanned by THz wave irradiation, the bonded wafer 200 can be linearly moved in the horizontal direction by the horizontal mechanism 164 while being rotated by the rotating mechanism 162. In this example, since the horizontal mechanism 164 and the rotating mechanism 162 are provided on the sample stage 160, the bonded wafer 200 is more resistant to the THz wave optical system than when only the horizontal mechanism 164 is provided. It can be moved quickly. Therefore, it is possible to scan at high speed within the range allowed by the sampling time of the THz wave.

  According to the present invention, the bonded wafer 200 is not in contact with anything other than the mounting portion of the chuck mechanism 161. Therefore, the bonded wafer 200 is not deformed or broken while the bonded strength of the bonded wafer 200 is measured by the wafer bonded strength inspection apparatus 100.

  The sample chamber 101 is further provided with a sample chamber adjustment management device terminal 172 and a sample chamber opening / closing mechanism 174. The sample chamber 101 has a sealed structure like a vacuum chamber. The internal space of the sample chamber 101 is maintained in an atmosphere of a vacuum state, a nitrogen filling state (nitrogen purge), or a dry air filling state (dry air purge) by the sample chamber adjustment management device 124 or an external device. When the bonded wafer 200 is loaded into the sample chamber 101 or when the bonded wafer 200 is unloaded from the sample chamber 101, the sample chamber opening / closing mechanism 174 is opened. When the bonded wafer 200 is loaded or unloaded, the sample chamber opening / closing mechanism 174 is closed.

  The sample chamber adjustment management device terminal 172 adjusts the atmosphere inside the sample chamber 101 such as a heater and an electronic cooler, and sensors for measuring the atmosphere inside the sample chamber 101 such as a pressure sensor, a thermometer, and a hygrometer. Has adjustment equipment. These sensors and adjustment devices are monitored and controlled by the sample chamber adjustment management device 124 installed outside the sample chamber 101, and further monitored and controlled by the system controller 104. For example, data measured by sensors is sent to the system controller 104 via the signal path 130. These data are displayed on the PC 102 or a monitor. The command sent to the system controller 104 via the PC 102 is sent to the sample room adjustment management device 124. A control signal from the sample chamber adjustment management device 124 is sent to the sample chamber adjustment management device terminal 172.

  By providing the sample chamber adjustment management device terminal 172 and the sample chamber adjustment management device 124, the internal space of the sample chamber 101 is maintained in a desired atmosphere. That is, since disturbances such as water vapor and temperature changes can be removed, the THz wave is stabilized.

  Next, the operation of the wafer bonding strength inspection apparatus 100 of the present invention will be described. The laser light oscillated by the femtosecond laser 106 is branched into the pump light 132a and the probe light 134a by the beam splitter 108, and propagates through the optical fibers 132 and 134, respectively.

  The pump light 132a is guided into the THz wave generator 151 as excitation light for generating THz waves. In other words, the pump light 132a is condensed by the condenser lens 151a and irradiated to the THz wave generating element 151b. A THz wave is generated by the THz wave generating element 151b. A modulated bias voltage from the bias power supply 110 with a modulation function is applied to the THz wave generating element 151b. Instead of modulating the bias voltage, a THz wave or femtosecond laser pump light 132a may be modulated by providing an optical chopper. The THz wave generating element 151b generates a white pulse wave having a predetermined frequency bandwidth. This frequency width depends on the THz wave generating element 151b.

  The THz wave generated from the THz wave generating element 151 b is radiated through the lens 151 c, collected by the THz wave projecting optical system 153, and enters the bonded wafer 200. The THz wave transmitted through the bonded wafer 200 is collected by the THz wave receiving optical system 154 and enters the THz wave detector 157. The THz wave incident on the THz wave detector 157 enters the THz wave detection element 157b via the lens 157c.

  On the other hand, the probe light 134 a is guided to the time delay stage 112 via the optical fiber 134. The function of the time delay stage 112 will be described later. Probe light 136 a from the time delay stage 112 propagates through the optical fiber 136. The probe light 136a is guided to the THz wave detector 157 as excitation light for THz wave detection. Similar to the pump light 132a, the probe light 136a is condensed by the condenser lens 157a and applied to the THz wave detecting element 157b.

  Thus, each pulse of the THz white pulse wave generated by the pump light 132a repeatedly reaches the one surface of the THz wave detection element 157b at a predetermined cycle. Each pulse of the probe light 136a from the femtosecond laser 106 repeatedly reaches the surface on the opposite side of the THz wave detecting element 157b with a predetermined period. The THz wave detection element 157b operates only when the probe light 136a is irradiated. That is, the THz pulse wave that has reached the THz wave detection element 157b is sampled by the probe light 136a.

  For each pulse of the THz pulse wave, sampling is performed by one pulse of the probe light 136a. The phase of the sampling point with respect to each pulse of the THz pulse wave corresponds to the time point when one pulse of the probe light 136a reaches the THz wave detection element 157b.

  A time delay stage 112 including a movable mirror 114 is provided in the optical path of the probe light 136a. The length of the optical path of the pump light 132a is constant, but the length of the optical path of the probe light 136a is movable.

  When there is an optical path difference between the optical path of the pump light 132a and the optical path of the probe light 136a, the time when the pump light 132a reaches the THz wave detecting element 157b and the probe light 136a reaches the THz wave detecting element 157b. Differences occur between time points. That is, a time delay occurs. When the movable mirror 114 is fixed at a predetermined position in the time delay stage 112, the time delay is constant. In this case, the phase of the sampling point is the same for each pulse of the THz pulse wave. For example, when the position of the movable mirror 114 of the time delay stage 112 is fixed so that the optical path of the pump light 132a and the optical path of the probe light 136a have the same length, the peak value of each pulse of the THz pulse wave, that is, The maximum or minimum value is the sampling point. This will be described in detail later.

  The optical path difference between the pump light 132a and the probe light 136a is continuously changed by continuously moving the movable mirror 114 straightly in the time delay stage 112. That is, the time delay changes continuously. In this case, the phase of the sampling point continuously changes with respect to each pulse of the THz pulse wave. Therefore, a plurality of data having different sampling point phases for each pulse of the THz pulse wave is obtained. By plotting this data, a time waveform representing each pulse of the THz pulse wave is obtained.

  When the probe light 136a is irradiated, the THz wave detection element 157b generates a detection current proportional to the oscillating electric field of the THz wave. The detection current is amplified by the preamplifier 120 and converted into a voltage. The amplified voltage signal from the preamplifier 120 is sent to the lock-in amplifier 118 in order to improve the S / N ratio. The lock-in amplifier 118 performs synchronous detection by using a modulation function of the bias power supply 110 or a modulation signal by an optical chopper provided in the THz wave generator 151.

  The system controller 104 obtains the time waveform or peak value of the THz wave from the detection signal from the lock-in amplifier 118. That is, when measurement is performed with the position of the movable mirror 114 of the time delay stage 112 fixed so that the optical path of the pump light 132a and the optical path of the probe light 136a have the same length, the peak value of the THz wave is obtained. . When measurement is performed while the movable mirror 114 of the time delay stage 112 is continuously moved, a time waveform of a THz wave, that is, a profile is obtained.

  With reference to FIG. 17, a wafer bonding strength inspection method using the wafer bonding strength inspection apparatus 100 of the present invention will be described. In step S101, THz wave characteristic data is acquired for the standard sample. The operator prepares a plurality of bonded wafers 200 or bonded portions having different bonding strengths as standard samples. This standard sample is irradiated with a THz wave by the wafer bonding strength inspection apparatus 100 of this example, and the time waveform or peak value of the THz wave is measured. Various THz wave physical characteristics with respect to the standard sample can be obtained from the time waveform or the peak value. This is calculated as THz wave characteristic data.

  Here, the THz wave characteristic data includes terahertz wave transmittance, reflectance, absorption spectrum, absorbance, polarization state, phase, complex refractive index, complex dielectric constant, complex conductivity, scattering intensity, scattering range, and the like. .

  In step S102, the bonding strength of the standard sample is measured by a tensile tester or other inspection method. When the THz wave characteristic data and the bonding strength data are obtained for the standard sample in this way, they are stored in the memory of the system controller 104 via the PC 102 or the host device.

  In step S103, the relationship between the THz wave characteristic data and the bonding strength is calculated for the standard sample. The system controller 104 calculates and records the relationship between the THz wave characteristic data and the bonding strength for the standard sample. The relationship between the THz wave characteristic data and the bonding strength may be expressed by a graph, a formula, or a table. An example of a method for calculating the relationship between THz wave characteristic data and bonding strength will be described later with reference to FIG.

  In step S <b> 104, the operator obtains THz wave characteristic data of the bonded wafer 200 or the bonded portion to be inspected using the wafer bonding strength inspection apparatus 100 of the present example. The worker sends the THz wave characteristic data to be inspected to the system controller 104 via the PC 102 or the host device.

  In step S105, the bonding strength to be inspected is obtained using the relationship between the THz wave characteristic data and the bonding strength obtained for the standard sample. That is, the bonding strength corresponding to the THz wave characteristic data to be inspected is read from the relationship between the THz wave characteristic data and the bonding strength obtained for the standard sample.

  In step S106, the system controller 104 determines whether or not the THz wave characteristic data obtained from the standard sample is within the normal range. The operator sets in advance a normal range of THz wave characteristic data corresponding to normal bonding strength via the PC 102 or a host device, and stores it in the system controller 104. The determination result is displayed on the PC 102 or the display device.

  The standard sample and the sample to be inspected are preferably the same in material and bonding method, but the structure, dimensions, etc. are not necessarily the same. For example, when the joint portion of the standard sample is the entire surface, the joint portion of the sample to be inspected may have a lattice shape.

  According to the present invention, since the relationship between the THz wave characteristic data obtained in advance for the standard sample and the bonding strength is used, the bonding strength of the inspection target sample can be obtained without destroying the inspection target sample. That is, the standard sample may be destroyed, but it is not necessary to destroy the sample to be inspected.

  According to the present invention, the sample to be inspected is supported by the sample stage 160. The sample to be inspected does not come into contact with the constituent members of the wafer bonding strength inspection apparatus 100 except for the support portion by the sample stage 160. Furthermore, it is not necessary to immerse the inspection target sample in a chemical such as an etching solution. Therefore, it is possible to measure the bonding strength of the inspection target sample without deforming or destroying the inspection target sample.

  According to the present invention, the THz wave characteristic of the sample to be inspected is obtained using the basic configuration of terahertz time domain spectroscopy. Therefore, it is possible to measure the bonding strength of the sample to be inspected regardless of whether a gap is generated at the bonding interface.

  According to the present invention, a sample to be examined is irradiated with terahertz waves using the basic configuration of terahertz time domain spectroscopy. Therefore, it is possible to measure the bonding strength of any local region of the sample to be inspected.

  An overview of wafer level chip size packaging (wafer level CSP) will be described with reference to FIG. First, a sealing wafer 201 for forming a sealing resin layer and a MEMS wafer 202 that has been processed are prepared. Two wafers 201 and 202 are bonded to form a bonded wafer 200. Next, the bonded wafer 200 is diced to form a chip size package 203. Through wiring is performed on this. In the wafer level CSP, external terminals and a sealing resin layer are formed before dicing, that is, at the wafer stage.

  The principle of direct bonding of two Si wafers 301 and 302 will be described with reference to FIGS. 3A, 3B, 3C, and 3D. In the direct bonding, the two Si wafers 301 and 302 are bonded by hydrophilic treatment and heat treatment (heating). FIG. 3A shows the state S0 before joining. The surfaces of the Si wafers 301 and 302 are slightly oxidized with water and chemicals to form thin oxide films 301a and 302a. Further, a surface treatment (hydrophilization treatment) is performed to attach a large number of hydroxyl groups (OH groups) 311 to the surfaces of the oxide films 301a and 302a.

  The first bonding state S1 shown in FIG. 3B shows a state immediately after bonding at room temperature or a state where low-temperature heat treatment is performed. The surfaces of the two wafers 301 and 302 that have been subjected to hydrophilic treatment are bonded to each other at room temperature or by low-temperature heat treatment. Thereby, hydrogen bonds 312 are generated between the surfaces of the oxide films 301a and 302a and the OH groups.

  The second bonding state S2 shown in FIG. 3C shows a state in which an intermediate temperature heat treatment is performed. When an intermediate temperature heat treatment is performed, the dehydration condensation reaction 313 proceeds and the bonding strength increases.

  A third bonding state S3 shown in FIG. 3D shows a state in which a high-temperature heat treatment is performed. When heat treatment is performed at a high temperature, the dehydration condensation reaction 313 further proceeds, oxygen (O) is bonded to Si 314, and the bonding strength is that of Si itself (covalent bond).

  FIG. 4 shows a curve 401 representing the relationship between the heat treatment temperature and the tensile strength obtained for the standard bonded wafer 200. The vertical axis represents the tensile strength (MPa) measured using a tensile tester. The horizontal axis is the heat treatment temperature (° C.). A plurality of bonded wafers 200 are prepared and bonded by direct bonding, and heat treatment is performed so that the first, second, and third bonded states described with reference to FIGS. 3B, 3C, and 3D are obtained. did. The tensile strength of these bonded wafers 200 was measured with a tensile tester. The tensile strength is weak in the first bonding state S1 (the heat treatment temperature is lower than T1), but the tensile strength is strong in the second bonding state S2 (the heat treatment temperature is T1 or more and T2 or less). Furthermore, in the third bonding state S3 (the heat treatment temperature is higher than T2), the tensile strength is higher and is approximately the same as the bonding strength P2 of the base material Si. It has been shown that the bonding strength increases with increasing heat treatment temperature.

  FIG. 5 shows a curve representing the time waveform (waveform of the oscillating electric field) of the electric field strength obtained for the bonded wafer 200 of the standard sample. The vertical axis represents electric field strength (arbitrary unit au), and the horizontal axis represents time (picoseconds). A plurality of bonded wafers 200 were prepared, and heat treatment was performed so that the first and third bonded states described with reference to FIGS. 3B and 3D were obtained. With respect to these bonded wafers 200, a THz wave of a predetermined frequency band was transmitted by a wafer bonding strength inspection apparatus according to the present invention, and the electric field strength was measured. The electric field strength is an average value at a plurality of arbitrary points on each bonded wafer 200.

  In the measurement experiment of this example, in order to obtain the time waveform of the electric field strength, the movable mirror 114 of the time delay stage 112 was continuously moved to measure the electric field strength.

  A dashed curve 501 shows the measurement result of the bonded wafer 200 in the first bonded state S1. A solid curve 502 indicates a measurement result of the bonded wafer 200 in the third bonded state S3. At the time point t1, the optical path lengths of the pump light and the probe light coincide with each other, the THz wave intensity reaches a peak, that is, maximum or minimum, and each electric field intensity reaches a peak value. The THz wave has energy corresponding to an intermolecular interaction due to hydrogen bonding or the like. When the bonded wafer 200 is irradiated with THz waves, it is absorbed by hydrogen bonds and OH groups. Therefore, the peak value E1 of the time waveform 501 obtained from the bonded wafer 200 in the first bonded state S1 is smaller than the peak value E2 of the time waveform 502 obtained from the bonded wafer 200 in the third bonded state.

  FIG. 6 is a curve showing a power spectrum obtained by Fourier transform of the time waveform of the electric field strength shown in FIG. The vertical axis represents amplitude (arbitrary unit au), and the horizontal axis represents frequency (THz). A broken curve 601 indicates a power spectrum obtained by performing Fourier transform on the time waveform 501 (FIG. 5) obtained from the bonded wafer 200 in the first bonded state S1. A solid curve 602 indicates a power spectrum obtained by performing Fourier transform on the time waveform 502 (FIG. 5) obtained from the bonded wafer 200 in the third bonded state S3.

  As shown in the figure, the power spectrum 601 obtained from the bonded wafer 200 in the first bonded state is broader in each frequency band than the power spectrum 602 obtained from the bonded wafer 200 in the third bonded state. There is.

  FIG. 7 shows a curve 701 representing the relationship between the heat treatment temperature and the THz wave transmittance obtained for the standard bonded wafer 200. The vertical axis represents THz wave transmittance (%), and the horizontal axis represents heat treatment temperature (° C.). A plurality of bonded wafers 200 were prepared and heat-treated so as to be in the first, second, and third bonded states described with reference to FIGS. 3B, 3C, and 3D. With respect to these bonded wafers 200, THz waves were transmitted by a wafer bonding strength inspection apparatus according to the present invention, and THz wave transmittance was measured. The transmittance is a relative value (%) of the measured value of the electric field intensity of the THz wave obtained from each bonded wafer 200 with respect to the reference value of the electric field intensity of the THz wave transmitted through the bonded wafer 200. The reference value may be a measurement value in the case of no sample, or a measurement value in the case of a non-bonded wafer of the same material having the same thickness as the bonded wafer. Further, in this measurement experiment, the electric field strength was measured in a state where the movable mirror 114 of the time delay stage 112 was fixed at a position where the optical path lengths of the pump light 132a and the probe light 136a coincided. Therefore, the maximum value or the minimum value of the electric field strength was obtained. For example, the transmittance was obtained by calculating the ratio of the measured value to the reference value using the peak values E1 and E2 of the time waveform at the time point t1 shown by the curve in FIG.

  As shown in the figure, in the first bonding state S1 (the heat treatment temperature is lower than T1), the THz wave transmittance is low, and in the second bonding state S2 (the heat treatment temperature is T1 or more and T2 or less), the THz wave is transmitted. In the third bonding state S3 (the heat treatment temperature is higher than T2), the transmittance is slightly higher, and the transmittance of the THz wave is higher and is similar to the THz wave transmittance TR2 of the base material (Si). As described above, the THz wave transmittance varies depending on the heat treatment temperature of the bonded wafer 200.

  When measurement is performed with the movable mirror 114 of the time delay stage 112 fixed, there is an advantage that mechanical driving is only the operation of the sample stage 160. Furthermore, since only the peak value of the electric field strength is obtained, the transmittance calculation process can be performed at a higher speed than when the time waveform of the electric field strength is obtained.

  However, the transmittance in an arbitrary frequency band can be obtained by continuously moving the movable mirror 114 of the time delay stage 112 and obtaining the time waveform of the electric field intensity. For example, after obtaining the spectrum intensity of the THz wave shown in FIG. 6, the transmittance can be obtained by calculating the ratio of the measured value to the reference value at an arbitrary frequency.

  FIG. 8 shows a curve 801 representing the relationship between the THz wave transmittance and the bonding strength obtained for the standard bonded wafer 200. The vertical axis represents the bonding strength (Mpa), and the horizontal axis represents the THz wave transmittance (%). Note that the bonding strength (Mpa) on the vertical axis is the tensile strength measured by a tensile test. That is, the tensile strength was regarded as the bonding strength. The solid curve 801 was obtained from the relationship between the heat treatment temperature and tensile strength in FIG. 4 and the relationship between the heat treatment temperature and THz wave transmittance in FIG.

  From this curve 801, an expression representing a function of bonding strength with THz wave transmittance as a variable can be obtained approximately. In the wafer level chip size packaging described with reference to FIG. 2, a lower heat treatment temperature is preferable. For this reason, the entire surface of the bonded wafer 200 may not be heated to such a high temperature (heat treatment temperature T2 in FIG. 4) that the third bonded state S3 is obtained. Even in such a case, a bonding strength of a local portion of the bonded wafer 200 can be obtained by creating an expression representing the curve 801. For example, the junction strength P3 when the transmittance of the THz wave is TR3 is obtained on the curve of FIG. Thus, since the bonding strength of the local part of the bonded wafer can be obtained, the reliability of the bonded portion can be ensured in the manufacturing process.

  The accuracy of the curve 801 depends on the experimental accuracy of the tensile test of FIG. 4 and the THz wave transmittance measurement test of FIG. Therefore, it is preferable that the number of bonded wafers that are standard samples is statistically sufficient. The standard sample may be a part of the bonded wafer 200, but preferably has no gap. As shown in the curves of FIGS. 4 and 7, in the second bonding state S2, the fluctuations in the tensile test and the THz wave transmittance are large. In such a case, the bonding strength of the bonded wafer differs for each local site. Therefore, statistical processing such as obtaining a plurality of THz wave characteristic data from a plurality of measurement times or a plurality of measurement sites and calculating an average value thereof is required.

  In this example, the case where the transmittance of the terahertz wave was measured for the standard sample joined by direct joining was described. From the transmittance of the standard sample thus obtained, it is possible to detect the hydrogen bond and OH group THz wave absorption, phase change, difference from the covalent bond, and the like for the bonded wafer 200 by wafer level chip size packaging. For example, it is known that when a hydrogen bond and an OH group are present, a sharp peak (spectrum peak) is obtained with respect to a THz pulse wave in a specific frequency region.

  However, in the case of a bonded wafer bonded by another bonding method, parameters other than the terahertz wave transmittance may be measured as THz wave characteristic data. As THz wave characteristic data, select at least one of reflectance, absorption spectrum, absorbance, polarization state, phase, complex refractive index, complex dielectric constant, complex conductivity, scattering intensity, scattering range of terahertz wave, and these The curve 801 or a relational expression thereof may be obtained by statistical processing of the data. Further, it is preferable to design an optimum THz wave optical system for the bonding method and material.

  Here, an example was described in which the tensile strength (MPa) was measured using a tensile tester and was used as the joint strength. However, the bonding strength may be calculated by other bonding strength measurement methods, and for example, may be calculated from the bonding strength measurement result using an etching solution as in Patent Document 3.

  4 to 8 show the results obtained for the standard sample bonded wafer 200. FIG. However, a curve similar to the curves in FIGS. 5 and 6 is also obtained for the bonded wafer 200 of the sample to be inspected.

  With reference to FIG. 9A, a transmission measurement method for detecting a THz wave transmitted through a sample will be described. The bonded wafer 900 was formed by bonding wafers 901 and 902 made of two different materials that can transmit THz waves. The THz wave is reflected at the boundary surface where there is a large difference in the refractive index of the THz band. When there is a large difference in refractive index in the THz band between the materials of the two wafers 901 and 902, the THz wave is reflected at the boundary surface 903. There is a large difference in the refractive index in the THz band even at the boundary surface 904 between the first wafer 901 and the space above it. Even at the boundary surface 905 between the second wafer 902 and the space below it, there is a large difference in refractive index in the THz band. Even at these boundary surfaces 904 and 905, the THz wave is reflected. The upper and lower spaces of the bonded wafer 900 are, for example, the atmosphere in the sample chamber 101.

  In this example, a THz wave is incident from the upper boundary surface 904. THz waves are transmitted through or reflected from the boundary surfaces 904, 903, and 905, respectively, and radiate from the lower boundary surface 905.

  The optical path L1 indicates a case where the THz wave is not reflected at the boundary surface and passes through the bonded wafer 900 as it is. The optical path L2 shows a case where the THz wave is reflected by the boundary surface 903, and the reflected wave is reflected by the upper boundary surface 904 and then radiated from the lower boundary surface 903. In the optical path L3, the THz wave passes through the two boundary surfaces 904 and 903, is reflected by the lower boundary surface 905, and the reflected wave is reflected by the boundary surface 903 and then radiated from the lower boundary surface 905. Shows the case. The optical path L4 shows a case where the THz wave is reflected from the lower boundary surface 905, and the reflected wave is reflected from the upper boundary surface 904 and then radiated from the lower boundary surface 905.

  Since the boundary surface 903 is sufficiently thin, for example, the THz wave optical path is incident on the boundary surface 903 from the upper side and is reflected on the upper surface of the boundary surface 903, and is incident on the boundary surface 903 from the upper side and is reflected on the lower surface of the boundary surface 903. The optical paths of the THz waves are drawn overlapping each other.

  FIG. 9B shows a time waveform 910 of the electric field strength of the THz wave obtained by the transmission measurement method. The electric field strength peak 911 at time t1 corresponds to the THz wave that has passed through the optical path L1 in FIG. 9A. The electric field strength peak 912 at time t2 corresponds to the THz wave that has passed through the optical path L2 in FIG. 9A. The electric field strength peak 913 at time t3 corresponds to the THz wave that has passed through the optical path L3 in FIG. 9A. The electric field strength peak 914 at time t4 corresponds to the THz wave that has passed through the optical path L4 in FIG. 9A.

  When there is a large difference in refractive index in the THz band between the materials of the two wafers 901 and 902, a plurality of optical paths L1 to L4 are generated. There is substantially an optical path difference between these optical paths L1 to L4. Therefore, the time points t1, t2, t3, and t4 at which the peaks 911 to 914 appear correspond to the optical path difference. Further, the amplitude and shape of the waveforms of these peaks 911 to 914 are different from each other. For example, the peak 914 waveform is inverted with respect to the peak 911 waveform.

  When the movable mirror 114 of the time delay stage 112 is fixed, only the maximum value or the minimum value of the electric field strength of the THz wave is obtained. In order to obtain the transmittance, the relative value (%) of the measured value with respect to the reference value is calculated using the maximum value or the minimum value. Here, the maximum value or the minimum value may be the maximum peak 911, but may be other peak values 912 to 914.

  When the materials of the two wafers 901 and 902 are the same and the thicknesses of both are the same, the peaks 912 and 913 of the time waveform due to the THz wave propagating through the two optical paths L2 and L3 overlap. Therefore, by fixing the position of the movable mirror 114 of the time delay stage 112 at that time, the joint surface can be focused. In this case, the state of the joint can be detected sensitively.

  Even when the shape and thickness of the wafer before bonding are not uniform, the amplitude and shape of the waveform of the electric field strength of the THz wave vary irregularly. In particular, in the case of a material having a high refractive index, the waveform disturbance caused by the variation in the thickness of the wafer before bonding is larger than the waveform disturbance caused by the difference in refractive index at the bonded portion. In this case, it is difficult to find the maximum peak 911. Therefore, it is preferable to measure the electric field strength while moving the movable mirror 114 of the time delay stage 112 little by little to find the peak value of the electric field strength.

  When the movable mirror 114 of the time delay stage 112 is moved, a time waveform of the electric field strength of THz wave is obtained. In order to obtain the transmittance, the relative value (%) of the measured value with respect to the reference value may be calculated using the maximum value or the minimum value obtained from the time waveform. Furthermore, the frequency spectrum shown in FIG. 6 can be obtained from the time waveform. Therefore, the transmittance can be obtained for each frequency region.

  However, when a frequency spectrum is obtained by performing Fourier transform on a time waveform 910 including irregular fluctuation components as shown in FIG. 9B, the amplitude fluctuates in a plurality of frequency regions instead of the monotonous curve as shown in FIG. It becomes a wavy curve. In such a curve, it is necessary to remove interference components caused by differences in wavelength, sample thickness, and refractive index. After removing the interference component in this way, the transmittance at any frequency can be obtained by calculating the ratio of the measured value to the reference value at any frequency.

  A reflection measurement method for detecting a THz wave reflected from a sample will be described with reference to FIG. 10A. The bonded wafer 1000 of this example is formed by bonding a wafer 1001 made of a material that can transmit THz waves and a material that cannot transmit THz waves, for example, a wafer 1002 made of metal.

  In this example, THz waves are incident from the upper boundary surface 1004. The THz wave is incident on the upper boundary surface 1004 at a predetermined incident angle. The incident THz wave reflects off the boundary surfaces 1004 and 1003 and radiates from the upper boundary surface 1004.

  The optical path L11 shows a case where the THz wave is reflected by the upper boundary surface 1004 and the reflected wave is radiated to the incident side as it is. The optical path L12 shows a case where a THz wave is refracted and incident on the upper boundary surface 1004, reflected by the boundary surface 1003, and the reflected wave is refracted by the upper boundary surface 1004 and emitted to the incident side. In the optical path L13, the THz wave is refracted and incident at the upper boundary surface 1004, reflected by the boundary surface 1003, the reflected wave is reflected by the upper boundary surface 1004, and the reflected wave is reflected again by the boundary surface 1003. In this case, the reflected wave is refracted at the upper boundary surface 1004 and radiated to the incident side.

  FIG. 10B shows a time waveform 1010 of the electric field strength of the THz wave obtained by the reflection measurement method. The electric field intensity peak 1011 at time t11 corresponds to the THz wave that has passed through the optical path L11 in FIG. 10A. The electric field strength peak 1012 at time t12 corresponds to the THz wave that has passed through the optical path L12 in FIG. 10A. The electric field intensity peak 1013 at time t13 corresponds to the THz wave that has passed through the optical path L13 in FIG. 10A.

  Even if the THz wave characteristic data is obtained using the peak value 1011 of the time waveform at time t11 in FIG. 10B, the state of the joint cannot be detected, but for example, obtaining a reference value such as reflectance it can. The state of the joint can be detected by obtaining the THz wave characteristic data using the peak values 1012, 1013 of the time waveform at time points t12, t13 in FIG. 10B. For example, by fixing the position of the movable mirror 114 of the time delay stage 112 at the time points t12 and t13, the joint surface can be focused. In this case, the state of the joint can be detected sensitively.

  FIG. 11 illustrates another example of a high-speed reflection measurement method for detecting a THz wave reflected from a sample. The bonded wafer 1100 of this example is formed by bonding a wafer 1101 made of a material that can transmit THz waves and a wafer 1102 made of a material that cannot transmit THz waves. Further, a mirror 1106 is disposed on the upper surface of the bonded wafer 1101.

  In this example, a THz wave is incident from the upper boundary surface 1104. The THz wave is reflected after being sequentially reflected by the boundary surface 1103 and the mirror 1106.

  An optical path L15 indicates a case where a THz wave is refracted and incident on the upper boundary surface 1104, reflected one after another from the boundary surface 1103 and the mirror 1106, and radiated to the opposite side to the incident side. In this example, the THz wave can be detected at the end of the bonded wafer 1100 and the state of the bonded portion 1103 can be detected.

  With reference to FIG. 12, an example of the measurement result by the high-speed reflection measurement method will be described. FIG. 12 schematically illustrates the bonded wafer 200 as viewed from above. Here, the bonding strength of the bonded wafer 200 was determined by the high-speed reflection measurement method using the THz wave optical system shown in FIG. In a MEMS wafer, a three-dimensional laminated device, and the like, the joints are often in a lattice shape rather than the entire surface. Therefore, it is possible to reduce the time for inspection of the bonding strength by scanning only the lattice-shaped region rather than scanning the entire surface of the bonded wafer 200. The first optical path Lx is incident along the xz plane and shows a state where the optical path of the emitted THz wave is viewed from above. The second optical path Ly is incident along the yz plane and shows a state where the optical path of the emitted THz wave is viewed from above.

  The arrow indicates the direction of the THz wave. The THz wave incident on the bonded wafer 200 is radiated after reflecting the bonded surface and the mirror in order along the arrow a plurality of times. A measurement result of the bonding strength is obtained along the first and second optical paths Lx and Ly on the bonded wafer 200. A circle mark at the tip of the arrow indicates that the measurement result is in a normal range of the bonding strength set in advance. The x mark at the tip of the arrow indicates that the measurement result is not within the normal range of the bonding strength set in advance. For example, the measurement result 1201 along the first optical path Lx and the measurement result 1202 along the second optical path Ly are not in the normal range. Therefore, the bonding strength of the intersection 1203 between the two optical paths is not in the normal range. Therefore, it is necessary to examine in detail the bonding state of this portion 1203. While moving the position of the movable mirror 114 of the time delay stage 112, that is, by obtaining a waveform profile including the peak of the time waveform, it can be confirmed that the bonding strength of this portion 1203 is weak.

  In this example, since the THz wave reflects the bonding surface of the bonded wafer 200 and the mirror disposed thereon multiple times, the THz wave reaching the THz wave detector 157 is attenuated. Therefore, the detection accuracy by the THz wave detector 157 decreases. Furthermore, the information obtained by detecting the THz wave is obtained by averaging information at each reflection point. Therefore, it may be devised to reduce the number of reflections of THz waves on the joint surface and the mirror. For example, the measurement is performed not along the optical path from one edge to the other edge of the bonded wafer 200 but along the half of the optical path. Thereby, half of the bonded wafer 200 is measured. Furthermore, the number of reflections can also be reduced by reducing the incident angle of the THz wave or reducing the size of the mirror.

  FIG. 13 shows an example of the measurement result of the bonding strength of the bonded wafer 200 to be inspected. In the measurement experiment of this example, the position of the movable mirror 114 of the delay unit 112 is fixed so that the electric field strength of the THz wave becomes a peak value during the reference measurement. The transmittance was obtained by scanning the entire surface of the bonded wafer 200 placed on the sample stage 160 by THz wave irradiation. The bonding strength corresponding to the transmittance of the THz wave thus obtained was calculated from the curve 801 shown in FIG. The distribution of the bonding strength obtained in this way is shown by contour lines. It is shown that the bonding strength is strong in the region 1301, the bonding strength is slightly weak in the region 1302, and the bonding strength is weak in the region 1303.

  In addition, you may display the area | region where joining strength is normal, and the area | region where it is not normal by a different color. For example, an area where the bonding strength is not normal may be displayed in red or the like. Thus, by distinguishing whether or not the bonding strength is normal, the operator can easily recognize which region of the bonded wafer 200 has the normal bonding strength and which region has the normal bonding strength. can do. Furthermore, it may be displayed by the host device so that the normal or abnormal bonding strength can be easily recognized.

  When measuring the bonding strength, the movable mirror 114 of the time delay stage 112 was fixed at a predetermined position as described above. That is, it is not necessary to obtain only the peak value of the time waveform of the electric field strength and obtain the profile of the time waveform of the electric field strength. Therefore, it is not always necessary to use the basic configuration of terahertz time-domain spectroscopy (THz-TDS). You may measure with apparatuses other than the THz wave generator 151 and the THz wave detector 157 used by the basic composition of terahertz time domain spectroscopy.

  FIG. 14 shows the result of superimposing the void image 1401 on the bonding strength image 1300 in FIG. A void image 1401 is obtained by installing a camera in the wafer bonding strength inspection apparatus 100 of this example and imaging the same bonded wafer 200 using illumination of visible light or near infrared light. By superimposing the void image 1401 on the bonding strength image 1300 in FIG. 13, distribution information of gaps (voids, gaps) in the bonded wafer 200 can be obtained. For example, by overlapping the void 1402 and the void 1403 on the contour line 1300 of the bonding strength distribution, it can be confirmed that the portion 1303 having a low bonding strength is distributed around the void 1402.

  Instead of the void image 1401, an image having various information may be used. Such an image may be obtained by photographing with an external device. For example, it may be an image having information such as the thickness and shape of the wafer. Thus, by superimposing other images having various information on the bonding strength image 1300, the relationship between the bonding strength distribution and other information can be easily understood.

  When this image is displayed on the PC 102 or the monitor, a cursor 1404 indicated by an arrow is displayed. The operator places the cursor 1404 at a desired position, for example, an abnormal part or an anxious position, and clicks it. Thereby, the time waveform (FIG. 5) and spectrum waveform (FIG. 6) by THz-TDS can be measured for the position, or the already measured result can be displayed. The measurement results shown in FIGS. 12, 13, and 14 are displayed on the PC 102, the display unit, and the like.

  In the wafer bonding strength inspection apparatus and method according to the present invention, THz wave characteristics include terahertz wave transmittance, reflectance, absorption spectrum, absorbance, polarization state, phase, complex refractive index, complex dielectric constant, complex conductivity, and scattering. Intensity, scattering range, etc. are obtained. The method for obtaining the transmittance of the terahertz wave has already been described. The transmittance of the terahertz wave is obtained by the transmission measurement method shown in FIG. 9A. Hereinafter, a method for obtaining other THz wave characteristics will be described. The reflectivity of the terahertz wave is obtained by the reflection measurement method shown in FIGS. 10A and 11. The reflectance of the terahertz wave is a relative value (%) of the measured value of the electric field strength of the THz wave obtained from each bonded wafer 200 with respect to the reference value of the electric field strength of the THz wave reflected from the bonded wafer 200. The reference value may be a measurement value in the case of no sample, or a measurement value in the case of a non-bonded wafer of the same material having the same thickness as the bonded wafer.

  The absorption spectrum of the terahertz wave is obtained by taking a common logarithm of the ratio of the spectrum obtained from each bonded wafer 200 with a reference spectrum as a baseline, and attaching a positive or negative sign thereto. The reference spectrum may be a measurement value in the case of no sample or a measurement value in the case of a non-bonded wafer of the same material having the same thickness as the bonded wafer.

  The absorbance of the terahertz wave can be obtained by a transmission measurement method, similarly to the transmittance. It is obtained by taking the common logarithm of the ratio of the measured value to the reference value of the electric field intensity of the THz wave and attaching a positive or negative sign thereto.

  The polarization state of the terahertz wave is measured by using a polarizing plate. For example, a polarizing plate such as wire grit is mounted on a THz light receiving system, and a change in electric field strength is measured when the polarizing plate is rotated. Thereby, it is possible to detect a state of a light wave having a regular vibration direction. In this case, a change in the polarization state of the THz wave due to the residual stress at the joint is detected.

  The phase of the terahertz wave is a dimensionless quantity indicating the temporal and spatial positions of the terahertz wave. It can be measured by the peak position or pulse width in the time waveform and the spatial waveform.

  The complex refractive index of the terahertz wave is obtained from the time waveform and the amplitude spectrum of the reference value and the measured value. However, the time waveform is composed of a single waveform as shown in FIG. 5A and does not include a reflected wave as shown in FIG. 9B. The complex refractive index is expressed by the following equation.

  Here, ω is each frequency, n is the refractive index, and k is the wave number. Each term of this equation can be obtained from the following amplitude transmittance t (ω).

  Here, E: electric field (electric field strength), d: thickness of sample, c: speed of light, ω: each frequency, subscript sam, ref represents a measured value and a reference value (presence / absence of sample). In general, Equation 2 cannot be solved explicitly. Therefore, initial values of the real component and the imaginary component of the complex refractive index are set, and the difference is calculated until the real component converges. That is, the complex refractive index is obtained sequentially by appropriately modifying the formula.

  The complex permittivity is expressed by the following formula 3, and the complex conductivity is expressed by the following formula 4.

  Each term of these equations is obtained from the following relational expression.

ただしε_∞は十分に高周波での試料の誘電率、ε₀は真空誘電率である。

However epsilon _∞ represents the dielectric constant of the sample at a sufficiently high frequency, epsilon ₀ is the vacuum permittivity.

  With reference to FIG. 15A, an example of a method for detecting only the scattered light of the terahertz wave in the transmission measurement method will be described. The THz wave receiving optical system 154 of this example includes a light-shielding diffuser plate 154A and a mirror 154B arranged so as to surround it. The mirror 154B includes a parabolic mirror, a Winston cone mirror, and the like, and has a center hole. The central holes of the light shielding diffuser 154 </ b> A and the mirror 154 </ b> B are disposed along the optical axis of the THz wave detector 157. The THz wave 141 from the THz wave generator 151 is applied to the bonded wafer 200. The transmitted light 142 other than the transmitted scattered light 143 is blocked by the light shielding diffuser 154 </ b> A and is prevented from reaching the THz wave detector 157. The transmitted scattered light 143 is reflected by the mirror 154B and reaches the THz wave detector 157. Of the transmitted scattered light 143, the THz wave reflected from the outside of the light shielding diffuser 154A is reflected again by the mirror 154B and reaches the THz wave detector 157. In this example, the THz wave detector 157 detects only transmitted scattered light. In this example, the influence of the intensity of the transmitted scattered light is detected as a change in transmittance. Transmitted scattered light can be detected as THz wave characteristics.

  With reference to FIG. 15B, another example of a method for detecting transmitted light other than the scattered light of the terahertz wave in the transmission measurement method will be described. The THz wave receiving optical system 154 of this example has an iris 154C. The central hole of the iris 154C is disposed along the optical axis of the THz wave detector 157. The THz wave 141 from the THz wave generator 151 is applied to the bonded wafer 200. The transmitted light 142 other than the transmitted scattered light 143 reaches the THz wave detector 157 via the center hole of the iris 154C. The transmitted scattered light 143 is blocked by the iris 154C and is prevented from reaching the THz wave detector 157. In this example, the THz wave detector 157 detects transmitted light other than transmitted scattered light. In this example, the transmittance can be detected without being affected by the intensity of the transmitted scattered light.

  With reference to FIG. 16A, an example of a method for detecting only the scattered light of the terahertz wave in the reflection measurement method will be described. The THz wave receiving optical system 154 of this example includes a light-shielding diffuser plate 154A and a mirror 154B arranged so as to surround it. The mirror 154B includes a parabolic mirror, a Winston cone mirror, and the like, and has a center hole. The central holes of the light shielding diffuser 154 </ b> A and the mirror 154 </ b> B are disposed along the optical axis of the THz wave detector 157. The THz wave 141 from the THz wave generator 151 is applied to the bonded wafer 200. The reflected light 144 other than the reflected scattered light 145 is blocked by the light shielding diffuser 154 </ b> A and is prevented from reaching the THz wave detector 157. The reflected scattered light 145 reflects the mirror 154B and reaches the THz wave detector 157. Of the reflected scattered light 145, the THz wave reflected from the outside of the light shielding diffuser 154 </ b> A is reflected again by the mirror 154 </ b> B and reaches the THz wave detector 157. In this example, the THz wave detector 157 detects only the reflected scattered light. In this example, the influence of the intensity of the reflected scattered light is detected as a change in transmittance. Reflected scattered light can be detected as THz wave characteristics.

  With reference to FIG. 16B, another example of a method of detecting reflected light other than the scattered light of the terahertz wave in the reflection measurement method will be described. The THz wave receiving optical system 154 of this example has an iris 154C. The central hole of the iris 154C is disposed along the optical axis of the THz wave detector 157. The THz wave 141 from the THz wave generator 151 is applied to the bonded wafer 200. The reflected light 144 other than the reflected scattered light 145 reaches the THz wave detector 157 via the center hole of the iris 154C. The reflected scattered light 145 is blocked by the iris 154C and is prevented from reaching the THz wave detector 157. In this example, the THz wave detector 157 detects reflected light other than the reflected scattered light. In this example, the transmittance can be detected without being affected by the intensity of the reflected scattered light.

  The wafer bonding strength inspection apparatus and method according to the present invention can be realized by the basic configuration of terahertz time domain spectroscopy (THz-TDS), that is, the THz wave optical system 150 shown in FIG. . However, if the bonding strength of the bonded wafer 200 can be inspected, it is not necessary to use the same configuration as the basic configuration of the terahertz time domain spectroscopy. For example, a configuration of another THz wave spectroscopic device such as a Fourier transform infrared spectrophotometer (FT-IR) may be used. For example, the THz generator (light source) may be a parametric oscillator, a THz quantum cascade laser, a backward wave tube, a Gunn diode, a resonant tunnel diode, a high-pressure mercury lamp, a far-infrared heater, a black body furnace, or the like. The THz detector may be a pyroelectric sensor, a bolometer, a Golay cell, a THz camera, or the like.

  Although the examples of the present invention have been described above, the present invention is not limited to the above-described examples, and it is easy for those skilled in the art to make various modifications within the scope of the invention described in the claims. Will be understood.

100: Wafer bonding strength inspection device,
101: Sample chamber,
102: PC,
104: system controller,
106: femtosecond laser,
108: Beam splitter,
110: Bias power supply with modulation function
112: Time delay stage,
114: movable mirror,
116: Operation direction of the movable mirror (straight operation),
118: Lock-in amplifier,
120: Preamplifier
122: Sample stage controller,
124: Sample chamber adjustment management device,
130: Electrical connection with system controller,
132: optical fiber (femtosecond laser beam path),
132a: pump light,
134: optical fiber (femtosecond laser beam path),
134a: probe light,
136: optical fiber (femtosecond laser beam path),
136a: probe light,
138: THz wave optical path 141: THz wave,
142: transmitted light,
143: transmitted scattered light,
145: reflected scattered light,
144: reflected light,
150: THz wave optical system 151: THz wave generator,
151a: Condensing lens,
151b: THz wave generating element,
151c: lens,
153: THz wave projection optical system,
154: THz wave receiving optical system,
154A: light shielding diffuser,
154B: Mirror,
154C: Iris,
157: THz wave detector,
157a: a condenser lens,
157b: THz wave detection element,
157c: lens,
160: sample stage,
161: chuck mechanism;
162: rotation mechanism,
163: lifting mechanism,
164: horizontal mechanism,
172: Sample room adjustment management device terminal,
174: Sample chamber opening / closing mechanism,
200: Bonded wafer,
201: wafer for sealing,
202: MEMS wafer,
203: MEMS chip size package,
301, 302: Si wafer,
301a, 302a: thin oxide film on the surface of the Si wafer,
311: Hydroxyl group (OH group)
312: Hydrogen bond 313: Dehydration condensation reaction,
314: Bond of O and Si,
401: Curve representing the relationship between heat treatment temperature and tensile strength,
501: Time waveform of electric field intensity when transmitted through the wafer in the first bonding state (S1),
502: Time waveform of electric field intensity when transmitted through the wafer in the third bonding state (S3),
601: Power spectrum waveform obtained by Fourier transform of time waveform 501;
602: Power spectrum waveform obtained by Fourier transform of the time waveform 502,
701: a curve representing the relationship between the heat treatment temperature and the THz wave transmittance,
801: a curve representing the relationship between THz wave transmittance and bonding strength;
900: Bonded wafer 901: Wafer made of material through which THz wave is transmitted 902: Wafer made of material through which THz wave is transmitted 903, 904, 905: Interface
910: THz wave time waveform transmitted through the bonded wafer,
911: THz waves propagated through the optical path L1,
912: THz wave propagated through the optical path L2.
913: THz wave propagated through the optical path L3,
914: THz wave propagated through the optical path L4,
1000: Bonded wafer 1001: Wafer made of material through which THz wave is transmitted 1002: Wafer made of material through which THz wave is reflected 1003, 1004: Boundary surface,
1010: Time waveform of THz wave transmitted through the bonded wafer,
1011: THz wave propagated through the optical path L11,
1012: THz wave propagated through the optical path L12,
1013: THz wave propagated through the optical path L13,
1100: Bonded wafer 1101: Wafer of material through which THz wave is transmitted 1102: Wafer of material through which THz wave is reflected 1103: Interface
1106: Mirror,
1201: Example of horizontal line measurement result display,
1202: Vertical line measurement result display example,
1203: a junction site outside the normal range,
1300: Bond strength image,
1301: Site with strong bonding strength,
1302: a part where the bonding strength is slightly weak,
1303: a portion having low bonding strength,
1401: Void image by near infrared light,
1402: Example of void,
1403: Example of void,
1404: An arrow for confirming the details of the bonding strength.
S1: Immediately after bonding of the wafer or after low-temperature heat treatment,
S2: A state where the bonded wafer has been subjected to a medium temperature heat treatment,
S3: A state where the bonded wafer is subjected to a high-temperature heat treatment,
P2: Tensile strength equivalent to the bonding strength of the base material (Si),
P3: Bonding strength when the transmittance is TR3,
T2: the heat treatment temperature at which the entire surface becomes the third bonded state (S3),
E1: Peak value of the time waveform 501
E2: peak value of time waveform 502,
t1: Time when the optical path lengths of the pump light and the probe light coincide and the THz wave peaks,
t2: time to take the peak value 912 of the THz wave,
t3: time to take the peak value 913 of the THz wave,
t4: time to take the peak value 914 of the THz wave,
L1: Optical path of THz wave transmitted through the bonded wafer,
L2: the optical path of the THz wave that is reflected by the boundary surface 903 and then transmitted;
L3: the optical path of the THz wave reflected from the boundary surfaces 903 and 904 and then transmitted;
L4: the optical path of the THz wave that is reflected by the boundary surfaces 903, 904, and 905 and then transmitted;
L11: the optical path of the THz wave reflected by the boundary surface 1004,
L12: the optical path of the THz wave reflected by the boundary surface 1003,
L13: the optical path of the THz wave reflected by the boundary surfaces 1003 and 1004,
L15: optical path of THz wave when scanning the boundary surface with a line,
TR2: Si THz wave transmittance,
TR3: Arbitrary THz wave transmittance

Claims (20)

A sample stage for holding a bonded wafer;
A terahertz wave generator for generating a terahertz wave of a predetermined frequency band;
A terahertz wave detector for detecting a terahertz wave transmitted or reflected by the bonded wafer;
From the terahertz wave detected by the terahertz wave detector, an arithmetic unit that calculates the terahertz wave characteristic of the bonded wafer;
Have
The calculation unit calculates a bonding strength corresponding to the terahertz wave characteristic of the bonding wafer to be inspected from a relationship between the terahertz wave characteristic of the reference sample obtained in advance and the bonding strength. Inspection device. In the wafer bonding strength inspection apparatus according to claim 1,
The wafer bonding strength inspection apparatus, wherein the reference sample includes a plurality of bonded wafers heat-treated at different heat treatment temperatures. In the wafer bonding strength inspection apparatus according to claim 2,
The reference sample is formed by bonding two wafers by direct bonding, and the chemical bonding state of the bonding portion between the two wafers is such that hydrogen bonding is generated at the bonding portion by hydrophilization treatment. Bonding strength inspection characterized in that it includes three states: a state in which dehydration condensation is generated from hydrogen bonds at the junction by intermediate temperature heating, and a state in which covalent bonds are generated at the junction by high temperature heating. apparatus. In the wafer bonding strength inspection apparatus according to claim 1,
The operation unit determines whether or not the bonding strength of the bonded wafer to be inspected is within a predetermined normal range from the terahertz wave characteristic of the bonded wafer to be inspected. . In the wafer bonding strength inspection apparatus according to claim 1,
The terahertz wave characteristics include terahertz wave transmittance, terahertz wave reflectance, terahertz wave absorption spectrum, terahertz wave absorbance, terahertz wave polarization state, terahertz wave phase, terahertz wave complex refractive index, terahertz wave A wafer bond strength inspection apparatus comprising at least one of a complex dielectric constant, a terahertz wave complex conductivity, a terahertz wave scattering intensity, and a terahertz wave scattering range. In the wafer bonding strength inspection apparatus according to claim 1,
A display unit for displaying a distribution image of the bonding strength of the inspection target bonded wafer generated by superimposing the bonding strength of the inspection target bonding wafer on the image of the inspection target bonding wafer; A wafer bonding strength inspection device characterized by that. In the wafer bonding strength inspection apparatus according to claim 6,
The display unit displays a bonding strength distribution image of the bonded wafer to be inspected by contour lines or by color. In the wafer bonding strength inspection apparatus according to claim 6,
In the distribution image of the bonding strength of the bonding wafer to be inspected, when the user designates a portion that does not reach the predetermined bonding strength via the input device, the calculation unit is based on terahertz time domain spectroscopy. A wafer bonding strength inspection apparatus characterized by obtaining a terahertz wave characteristic of the part. In the wafer bonding strength inspection apparatus according to claim 1,
A mirror is disposed on the bonded wafer, and the terahertz wave generated by the terahertz wave generator is configured to be sequentially reflected by the bonded wafer and the mirror and detected by the terahertz wave detector,
The irradiation point of the terahertz wave on the bonded wafer is arranged along a straight line, whereby the bonding strength of the bonded wafer is measured along the straight line. A sample stage for holding a bonded wafer;
A femtosecond laser that generates femtosecond laser light;
A beam splitter that branches the femtosecond laser light into pump light and probe light;
A terahertz wave generator for generating a terahertz wave of a predetermined frequency band by inputting the pump light;
An optical system for projecting terahertz waves for guiding terahertz waves generated by the terahertz wave generator to the bonded wafer;
A terahertz wave detector that samples the terahertz wave transmitted or reflected by the bonded wafer with the probe light; and
A terahertz wave receiving optical system for guiding the terahertz wave transmitted or reflected by the bonded wafer to the terahertz wave detector;
From the terahertz wave sampled by the terahertz wave detector, an arithmetic unit that calculates the terahertz wave characteristics of the bonded wafer;
Have
The calculation unit calculates a bonding strength corresponding to the terahertz wave characteristic of the bonding wafer to be inspected from a relationship between the terahertz wave characteristic of the reference sample obtained in advance and the bonding strength. Inspection system. The wafer bonding strength inspection system according to claim 10,
The reference sample includes a plurality of bonded wafers that have been heat-treated at different heat treatment temperatures, and the plurality of bonded wafers have different chemical bonding states at a bonded portion depending on the heat treatment temperature. system. The wafer bonding strength inspection system according to claim 10,
Furthermore, a time delay stage for adjusting an optical path difference between the optical path of the pump light and the probe light is provided,
When the optical path of the pump light and the optical path of the probe light are set to the same length by the time delay stage, the peak value of each pulse of the terahertz wave is sampled by the terahertz wave detector,
By year time delay stage, when the difference in optical path of the optical path of the pump light and the probe light is continuously changed, the phase of the sampling points continuously change in response to each pulse of the terahertz wave, a terahertz wave A wafer bonding strength inspection system characterized in that a plurality of data with different sampling point phases are obtained for each pulse, and a time waveform representing each terahertz wave pulse is obtained. The wafer bonding strength inspection system according to claim 10,
The terahertz wave receiving optical system is configured to guide only the scattered light or the terahertz wave other than the scattered light among the terahertz waves transmitted through the bonded wafer to the terahertz wave detector. Wafer bonding strength inspection system. The wafer bonding strength inspection system according to claim 10,
The terahertz wave receiving optical system is configured to guide only the scattered light or terahertz waves other than the scattered light out of the terahertz waves reflected from the bonded wafer to the terahertz wave detector. Wafer bonding strength inspection system. In the inspection method of the bonding strength of the wafer,
Preparing as a reference sample a plurality of bonded wafers composed of two or more wafers including a wafer formed of a material that transmits terahertz waves on the outside;
A heat treatment step of heat-treating the plurality of bonded wafers at different heat treatment temperatures;
A bonding strength measuring step for measuring the bonding strength of the plurality of bonded wafers;
A vibration electric field measurement step of irradiating the bonded wafer with a terahertz wave of a predetermined frequency band and detecting a vibration electric field obtained thereby,
From oscillating electric field detected by the oscillating electric field measuring step, and the terahertz wave characteristic data calculating step of calculating a terahertz wave characteristics of the bonding wafer,
From the result obtained by the bonding strength measurement step and the terahertz wave characteristic data calculation step, a step of obtaining a relationship between the terahertz wave characteristic of the reference sample and the bonding strength;
A sample vibration electric field measurement step of irradiating a terahertz wave to a bonded wafer of a sample to be inspected of the same material as the reference sample, and detecting a vibration electric field obtained thereby,
From the vibration electric field detected by the sample vibration electric field measurement step, a sample terahertz wave characteristic data calculation step for calculating the terahertz wave characteristic of the sample to be inspected,
From the relationship between the terahertz wave characteristics of the reference sample and the bonding strength, a sample bonding strength calculation step for calculating the bonding strength corresponding to the terahertz wave characteristics of the bonding portion of the sample to be inspected,
A method for inspecting the bonding strength of a wafer, comprising: In the bonding strength inspection method according to claim 15,
The reference sample is formed by bonding two wafers by direct bonding,
The method for inspecting bonding strength, wherein the reference sample includes a plurality of bonded wafers having different chemical bonding states at a bonded portion depending on the heat treatment temperature. In the bonding strength inspection method according to claim 16,
The chemical bond state includes three states: a hydrogen bond state due to a hydrophilization treatment of the joint, a hydroxyl group distribution state, and a state where the bond is changed from a hydrogen bond to a covalent bond by heating. Inspection method for bonding strength. In the bonding strength inspection method according to claim 15,
Determining the relationship between the oscillating electric field and the bonding strength,
A terahertz wave transmittance calculating step for obtaining the terahertz wave transmittance of the bonded wafer from the oscillating electric field;
From the terahertz wave transmittance of the bonded wafer, the step of determining the relationship between the heat treatment temperature of the bonded wafer and the terahertz wave transmittance of the bonded wafer;
A tensile strength measurement step of measuring the tensile strength of the bonded wafer by a tensile test;
From the tensile strength of the bonded wafer, the step of obtaining the relationship between the heat treatment temperature of the bonded wafer and the tensile strength of the bonded wafer;
From the relationship between the heat treatment temperature of the bonded wafer and the terahertz wave transmittance of the bonded wafer, and the relationship between the heat treatment temperature of the bonded wafer and the tensile strength of the bonded wafer, the tensile strength and terahertz wave transmittance of the bonded wafer A process of seeking a relationship;
A method for inspecting bonding strength, comprising: In the bonding strength inspection method according to claim 15,
The terahertz wave characteristic data includes at least one of terahertz wave transmittance, reflectance, absorption spectrum, absorbance, polarization state, phase, complex refractive index, complex dielectric constant, complex conductivity, scattering intensity, and scattering range. An inspection method for bonding strength, comprising: In the bonding strength inspection method according to claim 15,
The vibration electric field measurement step includes
A branching step of branching laser light oscillated by the femtosecond laser into pump light and probe light;
A step of generating a terahertz wave by guiding the pump light to a terahertz wave generator;
Guiding terahertz waves from the terahertz wave generator to a terahertz wave detector;
Guiding the probe light to the terahertz wave detector;
A sampling step of sampling the terahertz pulse wave that has reached the terahertz wave detector with the probe light;
Detecting a pulse wave of an oscillating electric field generated by the terahertz wave detector in synchronization with the sampling;
A method for inspecting bonding strength, comprising:

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