JP4759146B2 - Apparatus and method for secondary electron emission microscope with double beam - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates generally to an apparatus and method for using an electron beam to inspect a surface of an object microscopically, and more particularly to inspect a layer in a semiconductor device.
[0002]
[Prior art]
Various methods have been used to inspect the fine structure of a semiconductor. In the field of semiconductor chip manufacturing, these inspections occupy an important position in this field because whether or not the chip functions is determined by minute defects on the surface layer. For example, the intervening insulating layer holes or vias often provide a physical conduit to electrically connect the two outer conductive layers. If these holes or vias are not sufficiently etched or blocked by foreign material, an electrical connection cannot be established and the entire chip may not function. Inspection of minute defects on the surface of the semiconductor layer is necessary to ensure proper functioning of the chip.
[0003]
Electron beams have several advantages over other sample inspection mechanisms. In the light beam, the resolution limit is essentially about 100 nm to 200 nm, but the electron beam can inspect a shape size of several nanometers. Electron beams are manipulated fairly easily by electrostatic and electromagnetic elements, and are certainly easier to generate and manipulate than x-rays.
[0004]
In semiconductor defect inspection, the electron beam does not produce as many false positives as the optical beam. Optical beams are sensitive to color noise and grain structure problems, whereas electron beams are not sensitive to these problems. In particular, oxide trenches and polysilicon interconnects tend to be false positives in the optical beam due to the grain structure.
[0005]
Various approaches using electron beams have been made to inspect surface structures. In a low voltage scanning electron microscope (SEM), a narrow beam of primary electrons is raster scanned across the surface and secondary electrons are emitted. If the primary electrons in the scanning electron microscope beam are close to a certain known electron energy (denoted E2), the surface of the sample is relatively medium due to minimal charge accumulation problems for SEM. Remain sex. However, raster scanning with a scanning electron microscope is slow because each pixel on the surface is acquired sequentially. Furthermore, a complex and expensive electron beam manipulation system is required to control the beam pattern.
[0006]
Another approach is called a photoelectron emission microscope (PEM or PEEM). In that method, photons are applied to the surface of the sample of interest, and electrons are emitted from the surface by the photoelectric effect. However, on an insulating surface, there is a net electron flux from that surface, so the emission of these electrons generates a net positive charge on the sample surface. The sample continues to be positively charged until there are no emitted electrons, ie, electron decay occurs. This charge accumulation is particularly a problem when imaging insulating materials.
[0007]
Another method of inspecting a surface with an electron beam is known as a low energy electron microscope (LEEM). In that method, a relatively wide beam of low energy electrons is directed to enter the sample surface and electrons reflected from the sample are detected. However, LEEM has a similar charge storage problem because not all electrons hit the sample surface have enough energy to leave the surface, which causes additional electrons to the sample. The collision is prevented, and the surface image is distorted and shadows are formed.
[0008]
Several prior art publications discuss various approaches using an electron beam in a microscope, but none has established a way to use parallel imaging while reducing or eliminating the problem of charge accumulation. One of these approaches is described by Lee H. Veneklasen in “The Continuing Development of Low-Energy Electron Microscopy for Characterizing Surfaces”, Review of Scientific Instruments, 63 (12), December 1992, pages 5513-5532. is described. Vaneklasen generally states that the LEEM electron potential difference between the electron source and the sample can be adjusted between 0 and several keV, but is not aware of the charging problem and presents a solution. Not.
[0009]
Therefore, there is a need for a method and apparatus that uses an electron beam to inspect a sample surface that minimizes charge accumulation problems and increases the inspection speed of the sample surface.
[0010]
SUMMARY OF THE INVENTION
Accordingly, the present invention addresses the above-mentioned problems by providing an apparatus and method for parallel imaging. Generally speaking, the first beam is used to generate a relatively wide range image of the sample. Parallel imaging is achieved by using a relatively wide beam. The second beam, which has a lower collision energy than the first beam, can be used to reduce positive charge accumulation on the sample that can result from the first beam.
[0011]
In one embodiment, an apparatus for inspecting a sample is disclosed. The apparatus includes at least a first electron beam generator configured to direct a first electron beam having a first range of energy levels to a first region of the sample, and a second range of energy levels. And a second electron beam generator configured to direct a second electron beam having the second electron beam to a second region of the sample. The second region of the sample at least partially overlaps the first region, and the second range of energy levels is such that the charge accumulation caused by the first electron beam is suppressed. It is different from the range. The apparatus further comprises a detector configured to detect secondary electrons arising from the sample as a result of the interaction of the sample with the first and second electron beams. In a preferred embodiment, the first electron beam has a width suitable for parallel multi-pixel imaging, and the first and second electron beam generators are configured to generate the first and second beams simultaneously. ing.
[0012]
In another embodiment, a method for suppressing surface charge upon exposing the surface to a beam of charged particles is disclosed. The surface is exposed to a first group of electrons in the first beam. The first electron group has energy within the first range. The surface is exposed to a second group of electrons in the second beam. The second electron group has energy in a second range different from the first range. The second range of energy is predetermined to provide a second group of electrons that impact the surface and reduce the positive charge on the surface. In a preferred embodiment, the surface is exposed to the first group of electrons and the second group of electrons simultaneously.
[0013]
The present invention has several advantages. For example, the apparatus and method has the property of being faster and less noisy than conventional scanning electron microscopes and methods because it allows parallel imaging on a detector array for a large number of pixels. Furthermore, the need for an electron beam scanning system is not required and the density of the electron beam current is not so high, reducing the possibility of scratching sensitive samples.
[0014]
These and other features and advantages of the present invention are presented in more detail in the following detailed description of the invention and the accompanying drawings which exemplify the principles of the invention.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Reference will now be made in detail to specific embodiments of the invention. Examples of these specific embodiments are illustrated in the accompanying drawings. While the invention will be described in conjunction with these specific embodiments, it will be understood that it is not intended to limit the invention to the described embodiments. On the contrary, the intent is to cover alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims. In the following description, numerous details are set forth to provide a thorough understanding of the present invention. The present invention may be practiced without some or all of these details. In addition, in order to avoid unnecessarily obscuring the present invention, descriptions of well-known process operations are omitted.
[0016]
FIG. 1 is a diagram illustrating a basic configuration of a secondary electron emission microscope (SEEM) apparatus 100 having a dual beam according to an embodiment of the present invention. The SEEM 100 has primary electrons e along the trajectories 12 and 7, respectively. ₁ Are provided with two electron gun sources 10 and 8.
[0017]
Each electron beam is collimated by an electron lens 13 and then travels along tracks 12 and 7. The magnetic beam separator 14 then bends the path of the collimated electron beam so that the electron beam is incident along the electron optical axis OA perpendicular to the surface of the sample 9 to be examined. In other words, the two beams are directed to strike the same area of the sample almost simultaneously.
[0018]
The impact energy of one beam is selected to balance the charge on the sample surface. In one embodiment, one beam has a collision energy of the 1 keV level. The impact energy of the first beam may be selected such that the charge on the sample is neutral, but for certain materials (eg, insulators), the first beam has a net positive on the sample surface. Causes charge accumulation. That is, electrons are lost from the sample by the strong electric field generated on the surface by the high energy beam. Therefore, in order to reduce the positive charge accumulation on the sample resulting from the high energy beam, a second beam with low collision energy is used in conjunction with the first high energy beam. The low energy beam preferably has an energy of about 0 eV.
[0019]
Theoretically, the collision energy of the second beam is selected such that the surface voltage of the sample is fixed at a predetermined voltage value. When a low voltage beam is used, the surface of the sample is such that it is electrically connected to the electrode (ie, the second low beam electron source) by a wire. This voltage locking mechanism provides a way to automatically fix the voltage on the surface so that the surface is not charged even with a relatively wide range of substrate materials.
[0020]
In one embodiment, the low energy beam has a collision energy of about 0 eV. This value facilitates the reduction of positive charge accumulation resulting from the high energy beam. Theoretically, electrons in low energy do not have enough energy to cause the emission of secondary or backscattered electrons. These low energy electrons compensate for any surface charge caused by the high energy beam in the absence of them. Low energy electrons impact the surface when the surface is positively charged and eliminate this positive charge. If the surface is not sufficiently positively charged, these electrons will not collide.
[0021]
The objective electron lens 15 has primary electrons e. ₁ To produce two overlapping beams. The low energy beam preferably has a spot size wider than that of the high energy beam. That is, the low energy beam irradiates a spot that is equivalent to or includes the spot of the high energy beam. Therefore, a low energy beam can help reduce positive charge accumulation across the spot area of the high energy beam. Compensating for misfocusing of the beam focus system requires a wider spot size for low energy beams.
[0022]
The size of the electron beam spot from the two beams to the sample 9 is preferably in the range of 0.1 to 100 millimeters, more preferably about 1 to 2 millimeters. The size of the beam at the sample or imaging surface can be optionally varied by a zoom imaging system that controls the resolution and image acquisition rate. In any case, if it is desired to eliminate the peripheral effects, the beam width needs to be wider than the detector in the image plane and is preferably at least twice the intrinsic area of the detector. .
[0023]
Primary electron e ₁ Is incident on the sample 9, the electron e ₂ (Secondary electrons, backscattered electrons, or reflected electrons) are generated. E ₂ Returns to the objective lens 15 along an axis OA perpendicular to the inspection surface, where it is collimated again. The magnetic beam separator 14 moves along the image trajectory 16 with electrons e ₂ Bend. The electron beam traveling along the image trajectory 16 is focused on the imaging surface of the detector 20 by the projection electron lens 19.
[0024]
The detector 20 may have any shape as long as it is suitable for detecting secondary electrons. For example, the detector 20 may include an electronic imaging device (for example, a YAG screen 20a) for converting an electron beam into a light beam. The light beam may then be detected by a camera or preferably a time delay integrated (TDI) photodetector or electronic detector 20b. The operation of a TDI photodetector is disclosed in Chadwick et al. USP 4,877,326, which is incorporated herein by reference. Alternatively, the image information may be processed directly from a “back thinned” TDI electronic detector without using an electronic imaging device or YAG screen. By processing the signal collected from the TDI detector and comparing it with other data, the substrate can be inspected to identify and / or distinguish between possible defects and other shapes. Such processing and comparison can be performed in a die-to-die mode. In that mode, the nominally identical portion of the selected semiconductor die is compared with the other, and the difference represents a defect. Such processing and comparison can be performed in array mode instead or in addition. In that mode, the nominally identical portion of the repeating array on a single semiconductor die is compared to the other, and the difference represents a defect. Also, such processing and comparison can be performed in a die-to-database mode. In that mode, data (eg, an image) from the inspection portion of the substrate is compared with corresponding data derived from a reference database. The reference database may be derived from a database used for generating an image of the inspection portion of the substrate or may be the same as the database. Alternatively, the reference database can be derived from other rules that correspond to predicted images of structures such as partial repeats or arrays with designs that change as predicted, other repetitive structures, or test structures.
[0025]
FIG. 2 is a graph showing charge ratio versus primary electron energy characteristics of single incident electron beam inspection techniques such as LEEM, SEM, and SEEM. The production ratio η is the number of electrons emitted by the surface e ₂ The number of electrons incident on the surface e ₁ It is calculated by dividing by. Therefore, if η is not a constant value, net charge accumulation will occur, so the generation ratio η defines the amount of charge accumulation on the surface being examined. When the production ratio is greater than 1, more electrons are emitted than incident, which suggests that there is a net positive charge on the surface, conversely, the production ratio is greater than 1. If it is small, more electrons are incident on the surface than are emitted, suggesting that negative charges accumulate.
[0026]
The generation curve C is a mathematical function obtained experimentally, and defines generation ratios at various incident electron energies E with respect to a typical sample substrate. As shown in FIG. 2, the line segment L is a line segment indicating the charge balance η = 1, and the charge balance is realized, that is, e ₂ / E ₁ Only 3 points on the generation curve C are set to = 1. These three points are E ₀ = 0, E ₁ , E ₂ It is. (Energy E0 = 0 does not conform to the spirit of the present invention because it indicates a state in which electrons do not enter the sample). In the region I between the line segment L and the generation curve C, e ₁ Than e ₂ Is so large that there is an excessive negative charge. In region II between line segment L and generation curve C, e ₂ Than e ₁ There is an excessive positive charge. That is, more secondary electrons are emitted than incident primary electrons. In the region III between the line segment L and the generation curve C, the charge accumulation becomes negative again.
[0027]
From FIG. 2, the generation curve C has E ₁ And E ₂ It can be seen that there are only two singular points where charge balance can exist. The problem is that point E is actually stable. ₂ It is only that. That is, the energy E of primary electrons incident on the sample surface becomes E ₁ A small amount in either direction, ie ΔE ₁ If it only changes, the charge balance is lost immediately. The charge balance η is E ₁ Is + ΔE ₂ Or -ΔE ₂ As it approaches, gradually becomes negative or positive, and the beam energy is reduced to point E. ₂ Will return to. E ₁ And E ₂ The value of was experimentally determined from various substrates such as silicon dioxide, aluminum and polysilicon. Each substrate has its own generation curve, but the overall shape of these generation curves is as shown in the figure.
[0028]
FIG. 2 illustrates a problem with the prior art of electron beam inspection and illustrates why the SEEM technique described in the parent application provides an unexpected advantage. A low-energy electron microscope (LEEM) generally uses an electron energy of 100 eV or less to produce E ₁ Worked with less than. Point E ₁ Is unstable, and LEEM has a problem of charge accumulation. The scanning electron microscope (SEM) is stable E ₂ The SEM had no charge accumulation problem because it operated at a value just below, but it is just slow because it requires scanning. Prior to the present invention, a relatively high energy (eg, E ₂ It seems that no one thought about emitting a relatively wide beam of the LEEM parallel imaging system. Thus, the SEEM technology of the present invention was the first to recognize the advantage of combining LEEM parallel imaging and SEM charge balance.
[0029]
In theory, E ₂ Can operate a relatively wide beam, but with stable collision energy (eg, E ₂ ) Would be difficult to maintain a wide beam. E ₂ The value of depends on the composition of the material and therefore inevitably changes as the beam moves over the sample and crosses various structures with different material compositions. Therefore, even if the SEEM device is initially configured to operate at E2 with respect to the first beam position on the first material of the sample, the collision energy will be as the beam traverses different materials on the sample. E ₂ It will be out of place. Accordingly, the present invention provides a second beam of low collision energy to compensate for possible charge accumulation.
[0030]
Since the second beam compensates for positive charge accumulation, the first beam is referred to as E for the purpose of minimizing charge accumulation. ₂ There is an advantage that it is not necessary to operate with. The high energy beam is E ₁ And E ₂ Preferably between E and E ₁ And E ₂ It is further preferred to operate at a production peak between. Local charge balance may be automatically established by appropriate selection of the beam energy and flux such that the surface voltage is fixed to a selected range. The energy of the high energy beam may be selected such that a positively charged effect occurs on the sample surface (eg, the production ratio is greater than 1). Thus, the high energy beam causes more secondary and backscattered electrons to be emitted from the surface than is absorbed from the incident high energy beam. For example, for most materials, collision energies in the range of about 100 eV to about 2000 eV may be used. The energy of the low energy beam is selected so that the beam is attracted to the positively charged sample surface and cancels such positive charge. A low energy beam with a current density at least twice that of a high energy beam appears to work well to maintain a voltage balance on the sample surface. Such a low energy beam is therefore preferred. Several embodiments for configuring and using a high energy beam in combination with a low energy beam are described in the US provisional application filed May 15, 2000 referenced above. That application is fully incorporated herein.
[0031]
It should be noted that for the purposes described above, the energy of the primary electrons needs to be measured at the surface of the sample. The energy of the electron focused by the objective electron lens 15 is generally different from the energy of the electron on the sample, called the collision energy. This collision energy is often not easy to predict. The collision energy depends on factors such as the current density of the beam, the material of the sample, and the electric field on the surface.
[0032]
FIG. 3 is a table summarizing the differences and advantages of the four technologies, PEEM, LEEM, SEM, and SEEM. PEEM emits secondary electrons using photons instead of primary electrons. The problem with PEEM is that secondary electrons are ejected from the sample surface by photons, resulting in the problem of positive charge accumulation on the target material of the insulating sample, but there are no negatively charged particles that replace these secondary electrons. Since the inspection photon beam of PEEM may be wide, parallel imaging can be realized.
[0033]
In a low energy electron microscope (LEEM), a wide beam of primary electrons is launched onto the surface to be inspected and parallel imaging can be achieved. These primary electrons are relatively low energy and the imaging method includes a step of reflecting these low energy electrons from the surface. Since only low-energy electrons are incident, primary electrons are reflected, but secondary electrons are hardly emitted. Also, these electrons do not have enough energy to escape from the sample surface, so low energy is indicative of negative charge accumulation.
[0034]
In a scanning electron microscope (SEM), the focus of the electron beam is adjusted to a narrow spot size, so it is necessary to use relatively slow raster scanning imaging. However, the SEM is the stable point E of the generation curve. ₂ The operation of charge neutralization is realized by generating high-energy primary source electrons. High energy primary electrons generate secondary electrons in the SEM.
[0035]
In the secondary electron emission microscope (SEEM) technique of the present invention, a beam of high energy primary electrons is directed at a sample surface with a certain energy. Due to the introduction of a relatively wide beam of primary electrons, parallel imaging can be performed much faster than SEM imaging. In addition, these primary electrons can be incident with energy E2 and can be incident with much lower energy electrons so that the charge of the sample remains neutral. Thus, SEEM has the most preferable characteristics of LEEM and SEM.
[0036]
4 and 5 compare the imaging methods of the scanning electron microscope and the secondary electron emission microscope, respectively. In FIG. 4, a scanning electron microscope generates a beam of electrons 41 and directs it to the surface of a sample 42 having a specific area D. Beam 41 has a width “w” in the range of 5 to 100 nanometers (50 to 1000 angstroms). This beam 41 is raster scanned in a pattern indicated by path 43 across the surface of sample 42. (The number of scan lines has been greatly reduced for illustration). In order to control the beam 41 to move along the raster path 43, the inspection system preferably comprises an electron beam manipulator for electromagnetically deflecting the electron beam 41. Electrostatic deflection may be used instead or in combination.
[0037]
FIG. 5 shows parallel imaging of the secondary electron emission microscopy technique of the present invention. The beam 54 is emitted from an electron gun source and has a width “W” typically about 1-2 millimeters above the surface of the sample 55. Sample 55 has a unique area D, which is much larger than the width W of the electron beam. In SEEM, the width of the electron beam 54 is much larger than in the SEM, but it may still be possible and necessary to move the sample 55 relative to the beam to scan the sample 55. However, in the preferred embodiment, the SEEM need only mechanically move the stage of the sample 55 relative to the beam 54 and does not require an electron beam deflection system that electromagnetically manipulates the beam 41. Of course, electromagnetic and / or electrostatic deflection may be used and may be useful when compensating for table vibrations or manipulating a wide beam along a curved trajectory. Since the SEEM inspection system of the present invention images thousands to millions of pixels in parallel, it can operate much faster than the SEM inspection system.
[0038]
FIG. 5 further shows an enlarged view of the imaging portion of the beam 54 on the sample 55 to illustrate the parallel multi-pixel imaging region 56 within the beam 54. A rectangular detector array region 56 occupies the central portion of the beam 54 and defines an imaging aperture. (The detector array is either a time delay integrated (TDI) or non-integrated type. The detector array 56 images approximately 500,000 to 1 million pixels in parallel.
[0039]
Thus, SEEM is 500,000 to 1 million times faster than SEM, depending on the number of pixels in the detector array. If the SEEM spends 1 millisecond to see a pixel, the SEM can only spend 1 to 2 nanoseconds for that pixel to capture the same data frame at 100 MHz. Therefore, the current density on the sample surface in the SEEM is 10 times that of the SEM. ⁶ Double (ie, 1 million times) smaller, resulting in less sample damage. For example, if 10,000 electrons are required per pixel to obtain a good image, the SEM needs to apply a larger amount of electrons to the pixel spot per unit time. In SEEM, one million pixels are imaged at the same time, so the same number of electrons are emitted over a longer period of time.
[0040]
Furthermore, SEEM has better noise reduction characteristics than SEM. At 100 MHz, the SEM sees each pixel in 1 nanosecond, while the SEEM sees each pixel in 1 millisecond. Thus, SEEM averages noise above 1 kHz, whereas SEM can only average noise above 100 MHz. This represents fewer false positives and better signal-to-noise ratio for defect detection applications.
[0041]
In addition to the preferred method of suppressing charge accumulation using a second low energy beam in conjunction with a first high energy beam, the SEEM floods the sample 55 with the beam 54, eliminating peripheral effects. An additional advantage in charge suppression can be obtained by imaging only the central portion of the beam 54 to do so. Typically, if the surface charge to be imaged is non-uniform, the image will be distorted by beam deflection. Since there is no electron flux outside the beam diameter boundary, the sample surface around the beam 54 is not as uniformly distributed in charge as the surface inside the beam. There is a further peripheral effect since the charge remains in the area where the beam has already been scanned. By irradiating the beam over an area 54 wider than the imaging area of the detector array area 56, these imaging distortions are avoided. In SEM, the peripheral effect cannot be eliminated by this method because the diameter of the beam is too small to open further. Additional methods for reducing the effects of surface charge accumulation are disclosed in Monahan USP 5,302,828, which is incorporated herein by reference.
[0042]
The present invention may optionally include further additional means for maintaining the charge balance of the sample. One possibility is to attach an electrode to the sample and apply an auxiliary electric field. Voltage control provides more freedom for charge balance stability by supplying current to the electrodes. Another possibility is to introduce a low pressure gas such as argon into the vacuum chamber containing the sample to control the charge balance. The low pressure gas serves to prevent excessive charge accumulation in the sample. The above technique is illustrative of additional suppression means for maintaining the charge stability of the sample and is not exhaustive and there may be other such techniques for controlling charge suppression. It may be discovered later.
[0043]
These additional charge suppression means may optionally be used in conjunction with Monahan's flooding method. Using an electron beam of a certain energy relative to the E2 value of the material is one way to maintain a stable charge balance. By using additional or alternative charge suppression means such as secondary low energy beams, over-irradiation, electrodes, and / or low pressure gases, another way to maintain this charge balance is provided. These primary and secondary charge suppression mechanisms may optionally be combined with the charge suppression device.
[0044]
It is useful to compare the limitations imposed by the SEEM and SEM maximum scan rates. The advantages of SEEM over SEM are summarized below.
There is little noise. Longer image integration times are obtained for a given sample range. Noise is reduced due to averaging over longer sampling times.
There is little distortion of image. By over-illuminating the area on the wider sample than is imaged, a more uniform charge distribution is maintained in the area to be imaged and distortions due to peripheral effects are eliminated.
Lower current density. Lower current density is possible due to parallel imaging and longer dwell times, which means that the potential for sample damage is reduced. Faster. Parallel imaging means that a large number (eg 1 million) of pixels are imaged simultaneously in SEEM. In SEM, only one pixel is imaged at a time.
No need for fast scanning electronics. These scanning systems are complex and expensive but do not require SEEM because they provide faster parallel imaging.
[0045]
FIG. 6A illustrates a method in which defects in vias between semiconductor device layers are detected in accordance with one embodiment of the present invention. The stages in the process of manufacturing the semiconductor device 60 are shown. In this example, the semiconductor device 60 includes a substrate 61, a metal layer 62 deposited on the substrate 61, and an insulating layer 63 formed on the entire metal layer 62. Vias or holes 64, 65 are shown through the insulating layer 63 to reach the metal layer 62. In the next stage of manufacture, a second metal layer 66 is formed throughout the insulating layer 63 and the vias 64, 65 are filled with a conductive material to form an electrical connection between the metal layers 62 and 66. However, at this stage of manufacture, the metal layer 66 has not yet been deposited and is shown in dotted lines. In general, the vias 64 and 65 are formed by etching the insulating layer 63. However, in the figure, the via 64 is shown closed and the via 65 is not closed. For example, the via 64 may be plugged with foreign matter or may be plugged due to an incomplete etching process. In any case, via 64 is a defective via and via 65 is a complete via.
[0046]
FIG. 6A further shows a beam 67 of primary electrons incident on the insulating layer 63 perpendicular to the surface of the semiconductor device 60. This beam 63 is a combination of a first beam having high energy electrons and a second beam having low energy electrons. Since the layer 63 is an insulating material, the mobility of electrons on the layer 63 is limited. Therefore, the insulating layer 63 tends to collect charge on its surface, which creates charge accumulation problems associated with conventional inspection techniques such as LEEM. However, in the secondary electron emission microscope (SEEM) technique of the present invention, as long as the low energy beam has an energy suitable for suppressing charging of the layer 63, the energy of the high energy electron in the beam 67 is , E of the material of the insulating layer 63 ₂ A value close enough to the value may be selected, or a different energy may be used. Therefore, upon irradiation with the primary electron beam 67, the secondary electron beam 68 is generated by the insulating material 63 with minimal charge accumulation on the surface 63 of the material. The energy of the low energy electrons in beam 67 is selected to minimize charge accumulation on the sample surface resulting from the high energy electrons.
[0047]
A returning electron beam 68 is emitted in a direction perpendicular to the surface of the insulating layer 63 and opposite to the primary electron beam 67. The reflected electron beam 68 contains information about the defect vias and complete vias 64, 65 that are detected back through the optical system and then the operator can determine whether the semiconductor device 60 is defective. Processed so that it can be determined.
[0048]
FIG. 6B shows an electron beam inspection of the semiconductor device 60 of FIG. 6A in the next manufacturing stage. The metal wirings 66a and 66b extend in a direction perpendicular to the paper surface and connect to the metal layer 62 via the vias 64 and 65, thereby providing an electrical connection between the wirings 66a and 66b and the layer 62. The primary electron beam 67 is incident on the semiconductor device 60, specifically, the metal wirings 66 a and 66 b and the insulating layer 63. Inspection imaging of the surfaces of the metal wirings 66 a and 66 b and the insulating layer 63 can be realized by voltage contrast information obtained from the reflected electron beam 68.
[0049]
Thus, process control monitoring for the semiconductor industry is improved compared to optical beam inspection by reducing or eliminating grain structure and color noise using the electron beam inspection of the present invention. Once a defect is recognized, if the defect is significant, it may be corrected using means such as focused ion beam implantation. Information obtained from the inspection of the substrate can also be used to improve the production amount of semiconductor devices to be subsequently manufactured. The SEEM system of the present invention can be used in the field as part of a semiconductor wafer manufacturing apparatus, if necessary. In the apparatus, it is possible to inspect a wafer before and after processing or during processing, and use a common robot for handling the wafer and moving between the inspection and processing stations. The SEEM can also be placed in a vacuum chamber that is further used for other manufacturing processes such as ion implantation or evaporation. The data obtained from the inspection can be sent to the next processing station or returned to the previous processing station to improve automatic process control.
[0050]
The images from the high and low energy irradiation beams contain various types of information and are preferably separated. One new means of separating low energy mirror images and high energy scattered images in dual beam SEEMs utilizes the principle of synchronous detection. In this model, the low energy beam and the high energy beam are switched on and off alternately without being superimposed. The image is recorded only when the desired reflection or scattering image is present. During the undesired period of the switching cycle, the image beam is also interrupted.
[0051]
In order to maintain the desired charge balance from the low energy beam, the switching speed between the two images can be selected to meet certain conditions. Surface potential change (charge accumulation) rate dV / dt is determined by absorption (J _a ) And scattering (J _s ) Difference J between current densities _a -J _s (Charge / second / square centimeter) and the capacitance C of the surface layer per square centimeter of the surface area, dV / dt = (J _a -J _s ) / C. If ΔV is an acceptable increase in surface potential (approximately 0.1 V) in one cycle, the beam is Δt <C · ΔV / (J _a -J _s ) Needs to be switched in less than a period. If the switching period Δt for the image element is sufficiently short, the image will behave as if the two beams are actually superimposed, even if an image derived from only one beam is observed. Compared to the image subtraction method described above, this method has the advantage that it can reject not only unwanted contrast but also noise.
[0052]
In this mode of operation, both the illumination beam and the image beam need to be electro-optically turned on and off (blocked). In one embodiment, the irradiation beam is interrupted by switching the control grid of the electron gun between two voltages. The beam is turned off when the negative bias applied to the grid is sufficient to prevent the detachment of electrons from the cathode. When the grid is more positively biased, the beam is turned on. In another embodiment, the image plane is deflected (deflecting is accomplished magnetically, electrostatically, or both) so that the beam only passes through the aperture if it needs to be unobstructed. Both of these blocking methods may be used for dual beam SEEM to separate the images.
[0053]
In another embodiment of the present invention, both the reflection mode and the second order / backscatter mode are used sequentially to produce an image with different characteristics. In this embodiment, each type of image is continuously generated as in the previous embodiment. However, the image beam is not blocked because there are no unnecessary parts in the switching cycle. Instead, each type of image is used to determine the shape of the substrate under observation. For example, in "array mode" or "die-to-die mode" for semiconductor inspection, the reflection mode image of the first part of the die is the reflection mode (mirror mode) of the corresponding part of the array from the same die (for array mode). ) Compared to image or corresponding part of another die (for die-to-die mode). A similar die-to-die comparison or array comparison can then be performed on the secondary / backscatter image. Information obtained from both types of comparisons can be used to better identify and characterize defects in the semiconductor substrate. A detailed description of the die-to-die mode comparison can be found in commonly assigned USP 5,502,306.
[0054]
More generally, the secondary electron emission microscope of the present invention is used for defect inspection of any semiconductor device, thin film magnetic head, semiconductor manufacturing reticle, flat panel (eg, liquid crystal or field effect) display. Insulating material quantities, semiconductor materials, conductive materials, or even superconductors and plasmas can be imaged using SEEM. A typical semiconductor manufacturing process involves UV reduction projection of a reticle pattern generated for a wafer design, followed by chemical etching of each device layer. Alternatively, the semiconductor device is patterned using an ion beam, etching, or a CMP process. Process inspection and monitoring of the intermediate and final products are then performed with the method of the present invention. The SEEM system of the present invention can also be used for defect re-inspection. In re-inspection, a previously inspected wafer may be re-inspected and the defects located there may be characterized.
[0055]
FIG. 7 shows a method of applying the secondary electron emission microscope (SEEM) of the present invention to the inspection of the biological sample 70 on the stage carrier 77. The biological sample 70 has various shapes 71, 72, 73, 74. For example, the sample 70 may be a cell including a cell wall 71, a cell nucleus 72, a protoplasm 73, and a mitochondrion 74. Alternatively, the sample 70 may be human tissue including muscle tissue 71, bone tissue 72, body fluid 73, and malignant cells 74. The primary electron beam 75 enters the sample 70 perpendicularly. The reflected electron beam 76 is generated by irradiation of the cell 70 with the beam 75 and returns vertically through the electron optical system. Information about the cell 70 is encoded into the beam 76, detected and processed to obtain information about the cell 70.
[0056]
While the invention has been described in detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be made within the scope of the appended claims. It should also be noted that there are many alternative ways of implementing the process and apparatus according to the present invention. Accordingly, the embodiments are considered to be illustrative and not restrictive, and the invention is not limited to the details shown herein, but the appended claims and equivalents. It can be corrected within the range.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a basic configuration of a secondary electron emission microscope (SEEM) apparatus having a double beam according to an embodiment of the present invention.
FIG. 2 is a graph showing the relationship between charge balance generation ratio and primary electron energy in a system with a single beam.
FIG. 3 is a table comparing the SEEM technology of the present invention and the conventional electron beam inspection technology.
FIG. 4 is a diagram illustrating an SEM imaging method.
FIG. 5 is a diagram showing a SEEM imaging method for comparison with FIG. 4;
FIG. 6A is a diagram showing a method in which a SEEM electron beam detects a defect (occlusion) in a via of an insulating layer.
FIG. 6B is a diagram showing a method of inspecting a metal wiring connected to a via by an electron beam of SEEM.
FIG. 7 shows how a SEEM electron beam is used to study biological samples.
[Explanation of symbols]
7 orbit
8 Electron gun source
9 samples
10 Electron gun source
12 orbit
13 Electron lens
14 Magnetic beam separator
15 Objective electron lens
16 Image trajectory
19 Projection electron lens
20 Detector
20a YAG screen
20b Time delay integrated photodetector or electronic detector
41 beam
42 samples
43 Route
54 Beam
55 samples
56 Parallel multi-pixel imaging area, detector array area
60 Semiconductor devices
61 substrates
62 Metal layer
63 Insulation layer
64 Bahia
65 Bahia
66 Second metal layer
67 Primary electron beam
68 Secondary electron beam, reflected electron beam
70 biological samples
71 Shape, cell wall, muscle tissue
72 Shape, cell nucleus, bone tissue
73 Shape, protoplasm, body fluid
74 Shape, mitochondria, malignant cells
75 Primary electron beam
76 Backscattered electron beam
77 Stage carrier
100 Secondary electron emission microscope

Claims (20)

A device for inspecting a sample,
A first electron beam generator configured to direct a first electron beam having an energy level in a first range to a first region of the sample;
A second electron beam generator configured to direct a second electron beam having an energy level in a second range to a second region of the sample;
The second region of the sample overlaps at least partially with the first region, and the second range of energy levels is different from the first range, so that the first electrons The charge accumulation caused by the beam is suppressed,
Furthermore, example Bei a detector configured to detect secondary electrons generated from the sample as a result of the interaction between the sample and the first and second electron beams,
The second range of energy levels of the second beam is lower than the first range of energy levels of the first beam, and electrons are accumulated on the surface of the sample, so Selected to reduce the accumulation of positive charge caused by the electron beam
apparatus.   The apparatus of claim 1, wherein the first electron beam has a width suitable for parallel multi-pixel imaging.   3. An apparatus according to claim 1 or claim 2, wherein the first and second electron beam generators are configured to generate the first and second beams simultaneously.   4. An apparatus according to any preceding claim, wherein the first electron beam is adapted to generate a spot size on the sample in the range of about 0.1 to 100 millimeters. A device with a width.   4. An apparatus as claimed in any preceding claim, wherein the first electron beam has a width adapted to produce a spot size on the sample in the range of about 1-2 millimeters. Have a device. 6. The apparatus according to any of claims 1-5, wherein the first range of energy levels is selected such that a collision energy value for the first beam is about 1 keV. The second range is selected such that the collision energy for the second beam is about 0 eV.   7. The apparatus according to claim 1, wherein the second region of the sample that receives the second beam is the first region of the sample that receives the first beam. Completely encompassing the device. The apparatus according to any one of claims 1 to 7, wherein the first and second electron beam generators include:
A first electron gun source configured to generate the first electron beam;
A second electron gun source configured to generate the second electron beam;
A magnetic beam separator configured to direct the first and second electron beams to the sample;
An apparatus that takes the form of an objective lens configured to focus the first and second beams on the sample. 9. The apparatus according to any one of claims 1 to 8, wherein the detector is
A projection electron lens for focusing the secondary electrons on the image plane;
An electronic imaging device configured in the image plane to receive secondary electrons and convert the secondary electrons into photons;
An apparatus that takes the form of an optical detector configured to receive the photons and generate an image of the sample.   10. Apparatus according to claim 9, wherein the optical detector is in the form of a camera or a time delay integrated detector. 9. The apparatus according to any one of claims 1 to 8, wherein the detector is
A projection electron lens for focusing the secondary electrons on the image plane;
An apparatus that takes the form of a backside thin delay integrated detector configured in the image plane to receive and detect the secondary electrons.   12. The apparatus according to claim 1, wherein the second range of the energy level of the second beam is obtained by fixing the surface of the sample at a predetermined voltage value. An apparatus wherein the accumulation is selected to be released from the surface of the sample. A method for suppressing charge of a surface upon exposure of the surface to a beam of charged particles, comprising:
Exposing the surface to a first group of electrons in a first beam;
The first group of electrons has an energy in a first range;
Further comprising exposing the surface to a second group of electrons in the second beam;
The second electronic group, is the energy within the second lower than the first range, from the second group of electrons impinging on said surface, the child conductive for reducing the positive charges of the surface so as to provide, Ru determined Teisu the energy of the second range of said second group of electrons impinging on the surface in advance, method. 14. The method of claim 13 , wherein the surface is alternately exposed to the first group of electrons and the second group of electrons. 15. A method according to claim 13 or claim 14 , further comprising detecting secondary electrons from the surface derived from the first beam or the second beam only. 16. A method according to any of claims 13 to 15 , wherein the first beam is an incident beam of an electron beam microscope. 17. A method as claimed in any of claims 13 to 16 , wherein the second group of electrons is provided in the form of an unfocused beam. 18. The method of claim 17 , wherein the unfocused beam is incident on the substrate over a region that is wider than the region of the substrate on which the first beam is incident. 19. A method according to any of claims 13 to 18 , further comprising exposing the surface to an inert gas. 20. The method of claim 19 , wherein the inert gas comprises a cation, and the ion impacts the surface and carries away excess negative charge from the surface.

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