JP6822572B2 - Soft error inspection method, soft error inspection device and soft error inspection system - Google Patents

Description

The present invention relates to a soft error inspection method, a soft error inspection apparatus, and a soft error inspection system.

It is known that semiconductor devices such as LSI (Large Scale Integration) cause malfunctions due to radiation, so-called soft errors. Such soft errors occur not only in mission-critical equipment such as infrastructure servers and spacons, but also in equipment that requires radiation resistance for space and medical use, FA (Factory Automation) equipment, communication base station equipment, etc. This is a problem for devices that cannot stop time.

In general, soft errors are difficult to inspect because no defects remain. Specifically, soft error inspection methods include running tests at high altitudes and tests using accelerators. The test using an accelerator is further divided into a method of irradiating the entire surface of the LSI chip with neutrons and ions and a method of locally irradiating condensing electrons and laser light. Of these methods, the laser evaluation method is superior in that it can be realized with small-scale equipment because it does not need to be evacuated, and that a specific memory area can be evaluated individually.

Japanese Unexamined Patent Publication No. 5-313000 Special Table 2012-512409

Dale McMorrow, William T. Lotshaw, Joseph S. Melinger, Stephen Buchner, John D. Davis, Reed K. Lawrence, James H. Bowman, Ron D. Brown, Dave Carlton, Joseph Pena, Juan Vasquez, Nadim Haddad, Kevin Warren, and Lloyd Massengill, "Single-Event Upset in Flip-Chip SRAM Induced by Through-Wafer, Two-Photon Absorption", IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 52, NO. 6, DECEMBER 2005, p.2421-2425

In the laser evaluation method, a laser beam having a wavelength in the infrared region (for example, 1500 nm) is used to generate an electric charge through a process called two-photon absorption. Specifically, transistors and the like are formed on the front surface side of a semiconductor device such as an LSI chip, and wiring and the like connected to these transistors and the like are formed of a metal material. Therefore, from the back surface side of the semiconductor device. The inspection is performed by irradiating a laser beam. Therefore, in order to irradiate the portion where the transistor is formed with the laser beam with a predetermined power, the distance from the back surface to the portion where the transistor is formed on the front surface side, that is, the thickness of the semiconductor device is predetermined. It is necessary to polish the back surface of the silicon substrate to a thickness.

However, it is extremely difficult to polish the semiconductor device so that the distance from the back surface to the portion where the transistor is formed on the front surface side, that is, the thickness of the semiconductor device is substantially constant over the entire surface. For this reason, when performing a soft error inspection of a semiconductor device by the laser evaluation method, an accurate inspection could not be performed. It should be noted that a person who is skilled in polishing may be able to polish the semiconductor device so that the thickness is substantially uniform by spending time and effort. However, in this case, a large amount of time and labor is required, which leads to a decrease in throughput, and it is almost impossible to make the thickness of the semiconductor device completely uniform by polishing. Therefore, strictly speaking, a soft error inspection is performed. It cannot be done exactly.

Therefore, in the soft error inspection method by the laser evaluation method, there is a demand for a method capable of accurately and quickly evaluating soft errors in semiconductor devices.

According to one aspect of the present embodiment, in the soft error inspection method for a semiconductor device, the step of polishing the back surface of the semiconductor device and the thickness of a plurality of portions in the soft error inspection region of the semiconductor device after the polishing. The step of measuring the size, the step of irradiating the back surface of the semiconductor device with a laser beam having a wavelength corresponding to the thickness of the semiconductor device, and the time of bit inversion of the portion of the semiconductor device irradiated with the laser beam. It is characterized by having a step of measuring.

According to the disclosed soft error inspection method, soft error evaluation of a semiconductor device can be performed accurately and quickly.

Explanatory drawing of back surface polishing of semiconductor device (1) Explanatory drawing of back surface polishing of semiconductor device (2) Correlation diagram between the wavelength of light irradiating silicon and the depth of transmission Structural diagram of the soft error inspection device according to this embodiment Flowchart of soft error inspection method in this embodiment Explanatory drawing of wavelength control condition table used in this embodiment Explanatory drawing of soft error inspection method in this embodiment (1) Explanatory drawing of soft error inspection method in this embodiment (2)

The embodiment for carrying out will be described below. The same members and the like are designated by the same reference numerals and the description thereof will be omitted.

First, the outline of the soft error and the soft error inspection method will be described. When a high-energy beam enters the inside of an LSI chip, it continues to travel while losing energy, and electron-hole pairs are generated along the track. When these charges move to the electrodes and flow into the storage node and reach a charge equal to or higher than the threshold value, the bit information recorded up to that point is inverted (0 → 1 or 1 → 0). This phenomenon is called a single event upset (SEU), and is the biggest cause of malfunction of the memory in the LSI chip.

As will be described later, in the present embodiment, an energy beam is used for accelerated evaluation of SEU, but the mechanism of SEU generation differs between charged particles such as electron beams and laser light. In the case of charged particles such as electron beams, by irradiating the LSI chip with charged particles such as electron beams, electron-hole pairs are generated by ionization. On the other hand, in the case of laser light, when the LSI chip is irradiated with laser light, electron-hole pairs are generated by an excitation process called two-photon absorption, which absorbs two photons at the same time. Specifically, if the energy due to photons exceeds 1.12 eV, which is the bandgap energy of Si, electron-hole pairs are generated by indirect transition. As described above, the mechanism of electron-hole pair generation differs between charged particles such as electron beams and laser light, but the subsequent process of inducing SEU is the same.

As described above, when performing a soft error inspection of a semiconductor device by the laser evaluation method, polishing is performed from the back surface of the silicon substrate. Specifically, as shown in FIG. 1, a transistor forming layer 11 is formed on the surface 10a of the silicon substrate of the semiconductor device 10. The transistor forming layer 11 is formed with a transistor and wiring or the like formed of a metal connected to the transistor, and such wiring or the like may be an obstacle when performing a soft error inspection. Therefore, in order to irradiate infrared rays from the back surface 10b side of the silicon substrate of the semiconductor device 10, the back surface 10b of the silicon substrate of the semiconductor device 10 is polished.

In this case, as shown in FIG. 1A, the distance from the back surface 10b of the silicon substrate of the semiconductor device 10 to the transistor forming layer 11 is the same, that is, the back surface 10b and the front surface of the silicon substrate of the semiconductor device 10 are the same. It is necessary to polish so that it is parallel to 10a. However, such polishing is extremely difficult, and when the polishing is actually performed, the back surface 10b of the silicon substrate of the semiconductor device 10 is inclined with respect to the front surface 10a, as shown in FIG. 1 (b). Such a tendency becomes even more remarkable when performed by a person who is not accustomed to polishing.

As shown in FIG. 1B, when the back surface 10b of the silicon substrate of the semiconductor device 10 is inclined with respect to the front surface 10a, the distance from the back surface 10b of the silicon substrate of the semiconductor device 10 to the transistor forming layer 11 on the front surface side is reached. The distance varies from place to place. Therefore, when the laser beam in the infrared region is irradiated for the soft error inspection, the transmittance until reaching the transistor forming layer 11 is also different, so that the accurate soft error inspection cannot be performed.

In addition to the above, as a method for polishing the back surface 10b of the silicon substrate of the semiconductor device 10, there is local polishing such as dimple polishing for forming a dimple surface 10c on the back surface 10b of the silicon substrate of the semiconductor device 10. However, in such dimple polishing, it is necessary to accurately form the dimple surface 10c on the back surface 10b at the desired position to be inspected, but the dimple surface 10c is accurately formed on the back surface 10b at the desired position on the silicon substrate of the semiconductor device 10. Is extremely difficult to form. Further, in dimple polishing, the thickness of the dimple surface 10c differs for each portion.

As described above, when evaluating the soft error of a semiconductor device, it is necessary to polish from the back surface of the silicon substrate of the semiconductor device, but it is extremely difficult to polish so that the thickness of the silicon substrate is uniform. Have difficulty. Therefore, there is a demand for a soft error inspection method based on a laser evaluation method that can accurately, easily, and quickly inspect soft errors in semiconductor devices.

By the way, in a semiconductor device in which silicon is used, the wavelength of infrared light in which two-photon absorption occurs is 1.1 μm to 2.1 μm, but the transmittance in silicon depends on the wavelength (energy). When the wavelength of the irradiated infrared light changes, the transmittance also changes. FIG. 3 shows the relationship between the wavelength of the irradiated light and the transmission depth in silicon. As shown in FIG. 3, when the wavelength of the light applied to the silicon changes, the transmission depth in the silicon also changes.

FIG. 3 is based on the description of Non-Patent Document 1, and the permeation depth can be defined from the absorbance measurement. The ratio of the incident light to the emitted light is taken with respect to the known wavelength and sample thickness, and if it is completely absorbed, the absorbance becomes 1 and the transmittance becomes 0, and FIG. 3 is a plot of the thickness. FIG. 3 can also be said to be a depth at which light cannot penetrate any further. Regarding the relationship between absorbance and transmittance, when A is the absorbance, I ₀ is the incident light intensity, and I is the intensity of the light (emitted light) after passing through the sample, A = -log ₁₀ (I / I ₀ ). And the transmittance is I / I ₀ .

Further, when the silicon is doped with an impurity element, the value of the transmission depth also changes according to the concentration of the doped impurity element. Specifically, as shown in FIG. 3, in silicon doped with an impurity element, the transmittance of infrared light having a wavelength of 1.1 μm to 1.2 μm is the highest, and the concentration of the doped impurity is high. The transmission depth changes depending on the above, for example, from several tens of μm to several hundreds of μm. In addition, N = 2.4 × 10 ¹⁹ cm ^-3 is silicon coated with an N-type impurity element at a concentration of 2.4 × 10 ¹⁹ cm ^-3 . Further, N = 6 × 10 ¹⁸ cm ^-3 is obtained by doping silicon with an N-type impurity element at a concentration of 6 × 10 ¹⁸ cm ^-3 .

Therefore, when inspecting a semiconductor device in which an impurity element is doped in a silicon substrate, as shown in FIG. 3, when the relationship between the wavelength of the irradiated light and the transmission depth is known, , The relationship can be used. If it is not known, prepare a silicon substrate in which the concentration of the doped impurity element is known, and measure the transmittance and transmission depth when the wavelength is changed on the silicon substrate to measure the wavelength. You may create a graph, table, etc. of the relationship between and the transparency depth.

By the way, the thickness of an LSI chip, which is a semiconductor device, varies depending on the specifications. For example, when the chip is made into a chip, the thickness is back-grinded to about 100 μm, or the thickness is about 1 mm. is there. In the soft error inspection method by the laser evaluation method, it is necessary to irradiate the laser beam from the back surface of the silicon substrate of the LSI chip and transmit the laser light to the transistor forming layer on which the transistor is formed. Therefore, when the silicon substrate of the LSI chip is thick, it is necessary to polish it from the back surface to make it thinner. However, the silicon substrate is polished from the back surface so that the overall thickness is about several μm. It is difficult to do. If the thickness of the silicon substrate is about 50 μm, the silicon substrate can be polished relatively easily without cracking or breaking even without know-how regarding polishing.

By the way, the place where the electric charge that causes the soft error is generated is the transistor forming layer on which the transistor on the surface side of the silicon substrate is formed. Therefore, in the present embodiment, the thickness of each portion in the region to be inspected of the silicon substrate whose back surface is polished is measured, and infrared light is emitted so that the transmittance in each portion becomes substantially constant. Set the wavelength and irradiate. That is, the silicon substrate is irradiated by changing the wavelength of infrared light so that the transmittance is substantially constant according to the thickness of the silicon substrate. As a result, the intensity of infrared light incident on the transistor cambium formed on the surface side of the silicon substrate can be made uniform, and even if the thickness of the silicon substrate varies from portion to portion, it is accurate. It is possible to inspect for soft errors.

The thickness of the region to be inspected on the silicon substrate can be measured by using an interference microscope such as a confocal microscope or an optical interferometer. In the present embodiment, the thickness of the silicon substrate of the semiconductor device is measured for each portion of the region to be inspected for soft errors, and laser light is irradiated while changing the wavelength based on the measured thickness. , Check for soft errors. As a result, soft error inspection can be performed accurately and quickly.

(Soft error inspection device and soft error inspection system)
Next, the soft error inspection device according to the present embodiment will be described. FIG. 4 shows a soft error inspection device and a soft error inspection system according to the present embodiment. The soft error inspection device according to the present embodiment includes a scanning stage 20, a scanning control unit 21, an irradiation source 30, an irradiation control unit 31, a substrate thickness measuring unit 40, a substrate thickness measuring control unit 41, a measuring instrument 50, a control unit 60, and the like. have. The control unit 60 has an information processing unit 61 and a storage unit 62, and the display unit 63 and the input unit 64 are connected to each other.

The semiconductor device 10 which is a sample to be inspected is an LSI chip or the like, and the semiconductor device 10 is installed on the scanning stage 20 and moved in two dimensions by the scanning stage 20, that is, in the X-axis direction and the Y-axis direction. Can be done. The scanning stage 20 is controlled by the scanning control unit 21. In the present embodiment, the semiconductor device 10 is shown in FIG. 1A or FIG. 1B.

The irradiation source 30 emits a laser beam to be irradiated to the semiconductor device 10, is a tunable laser capable of changing the wavelength, and is controlled by the irradiation control unit 31. In the present embodiment, the case where the laser beam is emitted from the irradiation source 30 will be described, but charged particles such as electron beams may be emitted. When the semiconductor device 10 is a semiconductor device formed of silicon, the wavelength region in which two-photon absorption occurs in silicon is 1100 nm to 2100 nm. Therefore, for example, the wavelength of the laser light emitted from the irradiation source 30 is about. It is 1500 nm.

The substrate thickness measuring unit 40 is an interference microscope such as a confocal microscope or an optical interferometer, and is controlled by the substrate thickness measuring control unit 41. The measuring instrument 50 is a tester or the like, and is connected to the terminal of the semiconductor device 10, and can measure a change in information stored in the semiconductor device 10.

The control unit 60 controls the entire soft error inspection device and performs information processing operations related to the soft error inspection method according to the present embodiment, and is a scanning control unit 21, an irradiation control unit 31, a substrate thickness measurement control unit 41, and a measurement. The vessel 50 and the like are connected. Further, the information processing unit 61 is connected to a storage unit 62 for storing information, a display unit 63 for displaying necessary information, an input unit 64 for inputting information to the information processing unit 61, and the like. In the present embodiment, the soft error inspection system is capable of executing the soft error inspection method described below under the control of the control unit 60.

(Soft error inspection method)
Next, the soft error inspection method in the present embodiment will be described with reference to FIG. The semiconductor device 10 to be inspected by the soft error inspection method in the present embodiment is a SRAM (Static Random Access Memory), a flash memory, an FPGA (field-programmable gate array), an ASIC (application specific integrated circuit), or the like. .. The soft error inspection method in the present embodiment is performed based on the control in the control unit 60. In the present embodiment, it is assumed that the relationship between the wavelength of the infrared light irradiating the silicon and the transmission depth has been measured in advance or obtained from literature or the like, and this information is stored in the storage unit 62. It is assumed that it is remembered in.

First, in step 102 (S102), the back surface of the silicon substrate in the semiconductor device 10 to be inspected for soft errors is polished. Specifically, the silicon substrate of the semiconductor device 10 is polished so that the thickness is about 50 μm. This is because the silicon substrate does not crack or break at this thickness. In the present embodiment, the thickness of the silicon substrate after polishing may vary.

Next, in steps 104 (S104) to 108 (S108), the thicknesses of a plurality of portions in the region (soft error inspection region) to be inspected for soft errors of the semiconductor device 10 are two-dimensionally measured for each portion. Measure. As a result, two-dimensional information on the thickness in the soft error inspection area is obtained.

Specifically, in step 104, the thickness of a portion of the semiconductor device 10 to be inspected for soft errors is measured by the substrate thickness measuring unit 40, and the process proceeds to step 106 (S106). In step 106, scanning for measuring whether or not the measurement of the thickness of each portion of the region to be inspected for soft errors of the semiconductor device 10 is completed, that is, to measure the thickness of each portion of the region to be inspected. Determine if is finished. When the scanning for measuring the thickness of each part of the area to be inspected for soft errors is completed, the process proceeds to step 110, and the thickness of each part of the area to be inspected for soft errors is measured. If the scanning for this is not completed, the process proceeds to step 108. In step 108, the position is moved to the portion where the thickness is measured next within the region to be inspected for soft errors of the semiconductor device 10, and the process proceeds to step 104. After that, in step 104, the thickness of the semiconductor device 10 at the portion where the thickness is measured next after the position is moved is measured. By repeating the steps from step 104 to step 108 in this way, a map of the thickness distribution in the region to be inspected for soft errors of the semiconductor device 10 is created. This information is stored in the storage unit 62 in the control unit 60.

Next, in step 110 (S110), the value of the impurity concentration in the semiconductor device 10 is input. Specifically, if the value of the impurity concentration in the semiconductor device 10 is known, the value is input from the input unit 64. If it is not known, the value of the impurity concentration in the semiconductor device 10 may be measured and input by various methods.

Next, in step 112 (S112), the thickness of the semiconductor device 10 measured in steps 104 to 108 so that the irradiated transmittance becomes uniform based on the input value of the impurity concentration of the semiconductor device 10. The wavelength of infrared light corresponding to this is extracted.

Next, in step 114 (S114), a wavelength control condition table is created based on the wavelength extracted in step 112. In the wavelength control condition table, for example, as shown in FIG. 6, the wavelengths of infrared light corresponding to the thickness of the semiconductor device 10 are arranged two-dimensionally in M rows and N columns, and the storage unit. It is stored in 62.

Next, in step 116 (S116), the semiconductor device 10 to be inspected is irradiated with the laser beam for a predetermined time while changing the wavelength of the laser beam based on the wavelength control condition table shown in FIG. Checks for soft errors. Specifically, as shown in FIG. 7, the laser beam is irradiated to a predetermined position of the semiconductor device 10 for a predetermined time, and the time series data of the bit information of the position where the laser beam is irradiated is measured and stored. It is stored in the part 62 and the like.

Specifically, as shown in FIG. 7, the time-series data of the bit information of the position where the laser beam is irradiated by the measuring instrument 50 such as a tester in the state where the laser beam is irradiated, that is, the bit. The temporal change of the information is measured and stored in the storage unit 62 or the like. FIG. 8 shows time-series data of bit information of the position where the laser beam is irradiated, which is detected by the measuring instrument 50. FIG. 8A shows a state in which the bit information is inverted from 0 to ₁ when the time t1 elapses from the start of the irradiation of the laser beam. FIG. 8B shows how the bit information is inverted from 0 to 1 when the time t ₂ has elapsed from the start of the laser beam irradiation. FIG. 8 (c), from the start of the irradiation of the laser beam, at the time of the lapse of time t _3, the bit information is reversed in the 0 → 1, further, when the time has elapsed t _4, the bit information is 1 → 0 Shows how it is reversed again. In the present application, inversion of bit information may be described as bit inversion.

In the region to be inspected for soft errors in the semiconductor device 10, such measurement is performed by scanning the laser beam by changing the measurement location and changing the wavelength of the laser beam.

As described above, the soft error inspection method in the present embodiment is completed.

According to the soft error inspection method in the present embodiment, even when the thickness of the semiconductor device 10 is not uniform, accurate inspection can be performed in the area to be inspected for soft errors.

Although the embodiments have been described in detail above, the embodiments are not limited to the specific embodiments, and various modifications and changes can be made within the scope of the claims.

10 Semiconductor device 20 Scanning stage 21 Scanning control unit 30 Irradiation source 31 Irradiation control unit 40 Board thickness measurement unit 41 Board thickness measurement control unit 50 Measuring instrument 60 Control unit 61 Information processing unit 62 Storage unit 63 Display unit 64 Input unit

Claims (9)

In the soft error inspection method for semiconductor devices
The process of polishing the back surface of the semiconductor device and
After the polishing, a step of measuring the thickness of a plurality of portions in the soft error inspection region of the semiconductor device, and
A step of irradiating the back surface of the semiconductor device with a laser beam having a wavelength corresponding to the thickness of the semiconductor device,
A step of measuring the bit inversion time of the portion of the semiconductor device irradiated with the laser beam, and
A soft error inspection method characterized by having. The wavelength of the laser beam corresponding to the thickness of the semiconductor device is set so that the transmittance of the laser beam in a plurality of portions of the soft error inspection region of the semiconductor device is uniform. The soft error inspection method according to claim 1. The measurement of the thickness of the semiconductor device is performed two-dimensionally in the soft error inspection region of the semiconductor device.
The soft error inspection method according to claim 1 or 2, wherein the irradiation of the laser beam to the semiconductor device is performed while scanning the laser beam in the soft error inspection region. It has a step of creating a wavelength control condition table in which information on the wavelength distribution of the laser beam irradiated in the soft error inspection region is arranged based on the thickness of the semiconductor device measured two-dimensionally.
The soft error inspection method according to claim 3, wherein when scanning the laser beam, the laser beam having a wavelength in the wavelength control condition table is irradiated. The soft error inspection method according to any one of claims 1 to 4, wherein the thickness of the semiconductor device is measured by a confocal microscope or an interference microscope. The soft error inspection method according to any one of claims 1 to 5, wherein the soft error inspection method is performed by absorbing two photons of the irradiated laser beam. The stage where the semiconductor device is installed and
An irradiation source that emits laser light that irradiates the semiconductor device,
A measuring instrument connected to the semiconductor device and measuring bit inversion in the region irradiated with the laser beam.
A substrate thickness measuring unit that measures the thickness of a plurality of parts in the soft error inspection region of the semiconductor device, and a substrate thickness measuring unit.
A storage unit that stores the bit inversion time for each portion of the semiconductor device that is irradiated with the laser beam measured by the measuring instrument.
A control unit that controls to change the wavelength of the laser beam emitted from the irradiation source according to the thickness of the semiconductor device.
A soft error inspection device characterized by having. The storage unit stores the wavelength of the laser beam corresponding to the thickness of the semiconductor device.
The soft error inspection according to claim 7, wherein the wavelength of the laser beam is set so that the transmittance of the laser beam becomes uniform even if the thickness of the semiconductor device changes. apparatus. A soft error inspection system including the soft error inspection apparatus according to claim 7 or 8, wherein the control is performed by the control unit.

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