Patent Application
20120080414
Embodiments of the present invention relate to patterning samples, and more particularly to laser patterning electrodes on semiconductor substrates.
Radiation detectors, capable of detecting X-rays and gamma rays, have been developed over the years for a variety of applications, such as, but not limited to, medical imaging and detection, non-destructive testing and security inspection. Cadmium Zinc Telluride CdZnTe (CZT), and particularly Cd(1-x)Zn(x)Te (where x is less than or equal 0.5), is a wide bandgap ternary II-VI compound semiconductor that finds application in radiation detectors because of its unique electronic properties. Particularly, CZT detectors are of great interest as these detectors are capable of providing high-resolution X-ray and gamma ray spectra at room temperature due to the high atomic numbers of Cd and Te and the favorable electronic band structure. Furthermore, CZT can operate in a direct-conversion mode at room temperature without liquid nitrogen cooling. Additionally, CZT can be formed into a wide variety of shapes for different radiation-detecting applications. A variety of electrode geometries, such as coplanar grids, have been developed to provide unipolar operation, thereby improving energy resolution. Moreover, CZT detectors are relatively low cost, high-resolution detectors.
Generally, for proper operation, CZT devices typically need two conductive electrodes applied to their surface to permit charge collection from the detector. Traditionally, commercial pixelated or segmented CZT devices have been patterned by conventional techniques such as photolithography and shadow masking. Patterning by photolithography involves numerous steps, including film deposition, photoresist (PR) coating, PR baking, exposure, developing, material etching and PR stripping. This complex process requires a large investment in equipment, high fabrication costs, high maintenance charges, large volumes of chemicals, and long fabrication times. Another drawback with photolithography is that it is poorly suited for patterning non-planar surfaces and provides no control over the chemistry of the surface.
Further, in shadow masking, a shadow mask is applied on a substrate, and then a film is deposited over the mask. The shadow mask is then removed to provide a pixelated device. Unfortunately, in this method of patterning, it is hard to maintain a small distance between the mask and the substrate without damaging the surface of the substrate. Additionally, shadow masks have relatively poor feature resolution.
Moreover, CZT is a soft and brittle crystalline material. Accordingly, it may be desirable to develop a robust technique and system for generating a pixelated CZT device that minimizes the risk of damage to the CZT substrate. In addition, there is also a need for a method that minimizes the number of processing steps by circumventing post treatment of the CZT substrate. There is a further need to develop a CZT device with a high interpixel resistivity and accurately placed electrode features.
In accordance with aspects of the present technique, a method for laser patterning a sample is presented. The method includes coating at least one side of a substrate to form a sample, where coating the at least one side of the substrate forms an interface between the coating and the at least one side of the substrate. Further, the method includes configuring a scanning pattern for patterning the sample. In addition, the method includes determining settings for one or more laser beams of a laser based on the configured scanning pattern. Moreover, the method includes focusing the one or more laser beams of the laser at or near a surface of the substrate by selecting a focal point of the one or more laser beams near the surface of the substrate and setting a scribe depth near the surface of the substrate. The method also includes patterning the sample based on the configured scanning pattern using the one or more laser beams to generate one or more pixelated devices from the sample.
In accordance with another aspect of the present technique, a system for laser patterning a sample is presented. The system includes a laser generator configured to generate one or more laser beams. Additionally, the system includes a motion controller configured to control a relative motion between the one or more laser beams and the sample, wherein the sample comprises a coating disposed on at least one side of a substrate, and wherein a portion of the coating forms an interface between the coating and the at least one side of the substrate. The system also includes an optical subsystem configured to direct the one or more laser beams at the sample, wherein the optical subsystem is operatively coupled to the laser generator, the motion controller, or both the optical subsystem and the motion controller. Furthermore, the system includes a central control unit operatively coupled to the motion controller and configured to configure a scanning pattern for patterning the sample, determine settings for the one or more laser beams of the laser based on the configured scanning pattern, focus the one or more laser beams of the laser at the surface of the substrate of the sample by selecting a focal point of the one or more laser beams near the surface of the substrate, and setting a scribe depth near the surface of the substrate. Furthermore, the central control unit is configured to pattern the sample based on the configured scanning pattern using the one or more laser beams to generate one or more pixelated devices from the sample.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
As will be described in detail hereinafter, a system and method for laser patterning a sample, such as semiconductor substrate are presented. By employing the method and system described hereinafter, a pixelated device having enhanced performance may be obtained. Furthermore, the method for laser patterning the sample to generate a pixelated device with enhanced performance described hereinafter presents a simplified process for pixellating a sample. Additionally, the method presents a simplified process and enhances flexibility while processing the sample.
The laser generator 12 generates one or more laser beams 24. These one or more laser beams 24 are employed to pattern the sample 20. In certain embodiments, the one or more laser beams 24 may include pulsed laser beams with a duty cycle less than 1%. By way of example, the one or more laser beams may include ultraviolet laser beams, nanosecond laser beams, picosecond laser beams, or femtosecond laser beams. In addition, the intensity profiles of the one or more laser beams 24 may include a Gaussian intensity profile, a knife intensity profile or a top-hat intensity profile. It may be noted that use of other types of laser beams is also contemplated in accordance with aspects of the present technique.
Moreover, the optical subsystem 14 is operatively coupled to the laser generator 12. Also, the optical subsystem 14 directs the one or more laser beams 24 generated by the laser generator 12 onto the sample 20. These one or more laser beams 24 that are directed towards the sample 20 by the optical subsystem 14 may generally be represented by reference numeral 26. In certain embodiments, the optical subsystem 14 may include a focusing unit (not shown in
In addition, the motion controller 16 is operatively coupled to the optical subsystem 14, as depicted in
Furthermore, as noted hereinabove, the optical subsystem 14 directs the one or more laser beams 24 generated by the laser generator 12 to the surface of the sample 20. In accordance with aspects of the present technique, the sample 20 includes a semiconductor substrate (not shown in
Once the surface is prepared, a coating (not shown in
The laser patterning system 10 also includes the central control unit 18 that is operationally associated with the laser generator 12. In some embodiments, the central control unit 18 is also operatively coupled to the motion controller 16. In one embodiment, the central control unit 18 controls the patterning of the sample 20. Particularly, in accordance with aspects of the present technique, the central control unit 18 facilitates patterning of the sample 20 using the one or more laser beams 26 by controlling the one or more laser beams 26 based on a determined scanning pattern and/or determined settings of the one or more laser beams 26. To that end, the central control unit 18 is configured to determine and execute a scanning pattern for patterning the sample 20 based on a desired configuration of a pixelated device that is formed subsequent to the laser patterning of the sample 20. As used herein, the term scanning pattern is used to refer to a desired pattern of one or more pixels in a pixelated device. In particular, the scanning pattern includes one or more pixels, one or more roads, a guard ring, or combinations thereof. It may also be noted that in accordance with aspects of the present technique, the sample 20 is patterned by the one or more laser beams 26 based on the configured scanning pattern. Specifically, the configured scanning pattern is used as a template for the one or more laser beams 26 for patterning the sample 20. Also, as used herein, the term one or more roads is used to refer to a separation between the one or more pixels. Moreover, the guard ring aids in capturing leakage current along edges of the device. Although the scanning pattern is described as including one or more pixels, one or more roads, a guard ring, or combinations thereof, it will be appreciated that use of other scanning patterns and/or geometries are also contemplated in accordance with aspects of the present technique.
The central control unit 18 determines settings corresponding to the one or more laser beams 26. Specifically, the settings of the one or more laser beams 26 are determined such that the configured scanning pattern may be patterned on the sample 20 to form the pixelated device. The settings of the one or more laser beams may include a desired overlap, a desired power, and/or a spot size, a pulse energy, a desired scan speed of the one or more laser beams 26 to pattern the sample 20 based on the configured scanning pattern.
Accordingly, the central control unit 18 determines the desired overlap of the one or more laser beams 26, the desired power of the one or more laser beams, and the spot size of the one or more laser beams 26. In certain embodiments, the desired power is determined by determining a pulse duration, a repetition rate, and individual pulse energy. Also, in one embodiment, the desired average power may be about 10 mW, in certain embodiments. Moreover, in some embodiments, the desired spot overlap of the one or more laser beams 26 may be in a range from about 50% to about 95%. The pulse energy of the one or more laser beams 26 may be in a range from about 10 micro joules to about 50 micro joules. The energy is determined by the power and pulse duration. The average power may be at least 10 mW, in certain embodiments. The one or more laser beams 26 may have a pulse duration of less than about 10 nanoseconds. The one or more laser beams 26 may also have a wavelength less than about 850 nm, focused laser intensity greater than about 0.2 GW/cm2, and a focused spot size less than about 60 microns.
Additionally, the central control unit 18 determines a repetition rate of the one or more laser beams 26 to aid in determining the desired the desired power. In one embodiment, the one or more laser beams 26 may have a repetition rate greater than about 1 kHz. By way of example, the repetition rate of the one or more laser beams 26 may be in a range from about 1 kHz to about 1 MHz. Additionally, determining the desired spot overlap includes determining a scan speed of the one or more laser beams for achieving the desired overlap. Particularly, the desired scan speed of the one or more laser beams for achieving the desired overlap is determined based on the desired spot size and the repetition rate. In one embodiment, the spot overlap ratio may have an overlap greater than about 50%. Furthermore, in some embodiments, the optical subsystem 14 modifies the one or more laser beams 24 generated by the laser generator 12 to generate the one or more laser beams 26 based on the determined settings.
Once the scanning pattern is configured and the settings corresponding to the one or more laser beams 26 are determined, in accordance with aspects of the present technique, the central control unit 18 focuses the one or more laser beams 26 to initiate patterning of the sample 20 based upon the configured scanning pattern. Particularly, the central control unit 18 selects a focal point for each of the one or more laser beams 26 at or near a surface of the sample 20. In certain embodiments, the focal point is selected to be in a range from about 0 μm to about 20 μm near the surface of the sample 20. By way of example, the focal point is selected to be in a range from about 0 μm to about 20 μm below the surface of the sample 20.
Moreover, the central control unit 18 also controls the scribing of the sample 20 using the one or more laser beams 26. It may be noted that the in one embodiment, the central control unit 18 controls the scribing of the sample 20 with our without the usage of the stage 22. In certain embodiments, scribing the sample 20 using the one or more laser beams 26 entails ablating the sample 20 based on the configured scanning pattern. The terms scribing and ablating may be used interchangeably. As will be appreciated, the surface of the sample 20 is typically brittle. Hence, scribing the surface of the sample 20 with a laser beam generally damages the brittle surface of the sample 20. In accordance with exemplary aspects of the present technique, damage to the surface of the sample 20 during the laser patterning process may be circumvented by setting a scribe depth near the surface of the sample 20. Particularly, the laser power that can still fully ablate the coating 60 and achieve high interpixel resistivity may be reduced by setting the scribe depth near the surface of the sample 20. As used herein, the term scribe depth is used to refer to a depth near the surface of the sample 20 at which the one or laser beams 26 are configured to scribe the sample 20 based on the configured scanning pattern. Accordingly, the central control unit 18 sets the scribe depth near the surface of the sample 20. In certain embodiments, the scribe depth is in a range from about 1 nm to about 20 μm below the interface layer of the sample 20. Additionally, in some embodiments, the scribe depth may be selected to be in a range from about 1 nm to about 1000 nm greater than a thickness of the interface layer.
The central control unit 18 also facilitates the patterning of the sample 20 based on the configured scanning pattern and the determined settings to generate one or more pixelated devices from the sample 20. It may further be noted that patterning the sample 20 may entail multiple passes with a position offset of the one or more laser beams 26 to obtain the desired width of the roads. The operation of the laser patterning system 10 will be explained in greater detail with reference to
Turning now to
Referring now to
Furthermore, as illustrated by step 62, the coating 60 is formed on the at least one side 56 of the semiconductor substrate 54 such that an interface layer 64 is formed between the coating 60 and the at least one side 56 of the semiconductor substrate 54. The semiconductor substrate 54 having the coating 60 disposed thereon may be referred to as a sample and may be generally be referenced by reference numeral 66. Particularly, a portion of the coating 60 formed on the least one side 56 of the semiconductor substrate 54 may diffuse into the semiconductor substrate 54, thereby creating the “interface” layer 64. It may be noted that the diffusion of the coating 60 into the semiconductor substrate 54 is dependent upon the material used in the coating 60. Also, this interface layer 64 may be conductive. Here again, the extent of conductivity of the interface layer 64 is dependent upon the material used in the coating 60. Moreover, the interface layer 64 may have a thickness in a range from about 0.1 nm to about 20 μm.
It may also be noted that in certain other embodiments, a diffusion barrier 84 may be formed between the coating 60 and the semiconductor substrate 54 to form the sample to be patterned.
With returning reference to
It may be noted that the pixel geometry is determined based on a desired spatial and energy resolution. Additionally, the pixel geometry is also dependent upon the quantity and complexity of electronics required to connect to each pixel. It may be noted that a device with a smaller ratio of pixel size to substrate thickness and a small gap between pixels is generally a better performing semiconductor device.
Additionally, it may be desirable to include a guard ring in the scanning pattern. The guard ring prevents degradation of the radiation detection performance of edge pixels. This degradation typically occurs due to surface leakage. In one embodiment, the guard ring may be disposed at a distance in a range from about 10 μm to about 100 μm from the edge of the pixelated device. Moreover, in accordance with aspects of the present technique, pixel dimensions, the width of the roads, and pixel distance to edge with guard ring are optimized for charge collection efficiency, including photopeak fraction. It may further be noted that in certain embodiments guard rings may be placed at both surfaces of the semiconductor surface 54. By way of example, the guard rings may be placed at a first surface (anode surface) and a second surface (cathode surface) of the semiconductor substrate 54.
In certain other embodiments, the scanning pattern may not include a guard ring. Particularly, the scanning pattern may include one or more pixels to the edge of the semiconductor substrate 54. Once the size of the one or more pixels, the size of the one or more roads, and the inclusion or exclusion of the guard ring are determined, the scanning pattern is configured. The scanning pattern so configured may then be employed to scribe the sample using the one or more laser beams.
Furthermore, a second embodiment 100 of a scanning pattern is depicted in
Referring again to
Accordingly, to facilitate qualified machining, settings corresponding to the one or more laser beams 26 may be determined. As previously noted, the settings of the one or more laser beams 26 are typically determined based on the scanning pattern configured at step 36. Also, in certain embodiments, the settings of the one or more laser beams 26 may include a power, a pulse rate, a scan speed, laser pulse width, laser pulse energy, repetition rate, a focal spot size, an overlap ratio of the one or more laser beams, or other factors that affect machining quality. In one embodiment, ultrashort pulsed lasers may be employed. As used herein, the term ultrashort pulsed lasers is used to refer to lasers that are typically less than 1 ns. In certain embodiments, a laser having a duration in a range from about 50 femtoseconds to about 350 femtoseconds and having a wavelength of about 800 nm laser may be employed.
Furthermore, the laser pulse energy may be lowered to a value near 50 micro joules. In one embodiment, the laser pulse energy may be lowered to a value between about 5 micro joules and about 50 micro joules. The laser pulse energy may be lowered using an external laser energy modulator (not shown), where the external energy modulator includes a quarter waveplate and a polarizer that can change laser light energy without affecting the laser cavity stability. Moreover, laser focus may be set to a value near 50 microns. Also, an overlap ratio of the one or more laser beams may be selected in a range from about 50% to about 95%. Although the present technique is described using a laser having a pulse width less than about 1 ns, other nanosecond lasers may also be employed to pattern the sample 66. In accordance with further aspects of the present technique, a top-hat laser beam may be optionally employed to achieve a flat machining contour.
Moreover, the scan speed of the one or more laser beams 26 is determined based on an overlap percentage of the one or more laser beams 26, a focal spot size of the one or more laser beams 26 and a laser repetition rate. The focal spot size of the one or more laser beams 26 is representative of a scribe width of the one or more laser beams. By way of example, in one embodiment, the scan speed v may be determined using the following relation:
v=f*(1−r)*D (1)
where f is the repetition rate, r is the overlap percentage, and D is the diameter of the focal spot size.
By way of example, if the focal spot size has a diameter of 30 μm, the repetition rate is 1000 Hz, and the overlap percentage is 60%, then the scan speed is 12 mm/second. It may be noted that based on the desired overlap percentage, the scan speed of the one or more laser beams may be varied.
As previously noted, focusing the laser beams on the surface of the semiconductor substrate damages the brittle semiconductor substrate. In accordance with exemplary aspects of the present technique, the one or more laser beams are focused away from the brittle surface of the semiconductor substrate, thereby circumventing damage to the surface of the semiconductor substrate 54 due to intense machining at the center of the semiconductor substrate 54. Accordingly, at step 40, the one or more laser beams are focused away from the surface of the semiconductor substrate 54 and instead focused near the brittle surface of the semiconductor substrate 54. Particularly, the one or more laser beams, such as the one or more laser beams 26 (see
Turning now to
By way of example, in the embodiment of the sample 80 (see
Referring once again to
As depicted in
As illustrated in
Referring now to
In accordance with aspects of the present technique, the patterning of the sample 66 entails translation of the sample 66. Particularly, the translation of the stage 22 supporting the sample 66 is initiated before energizing the laser generator. As depicted in
The one or more laser beams 26 are focused at the determined focal point and are configured to scribe the sample 66 at the determined scribe depth as the sample 66 is translated in a first direction 146. Scribing the sample 66 at the determined scribe depth entails ablating the coating 60. The scribing process may also entail ablating a portion of the semiconductor substrate 54 of the sample 66. Reference numeral 148 is generally representative of a road scribed by the one or more laser beams 26 on the sample 66 in the first direction 146. The translation of the sample 66 is continued until a determined position, where the determined position is located at a determined distance from an edge of the sample 66. This determined position may generally be represented by reference numeral 150. It may be noted that in accordance with aspects of the present technique, machined material generated during the scribing of the sample 66 may be extracted. In one embodiment, a side vacuum may be used to extract the machined material. In other embodiments, an air knife may be used to blow off debris from the path of the one or more laser beams 26. Furthermore, the extracted machined material may be analyzed to check for presence of substrate matter. If substrate matter is detected in the extracted material, it may be confirmed that the desired scribing depth has been reached. This information may subsequently be used to adjust scribing parameters in real-time to obtain an optimal scribing profile.
Once the sample 66 is patterned till position 150 in the first direction 146, the scribing of the sample 66 is stopped. Subsequently, the translation of the sample 66 in a second direction 154 is initiated as indicated by reference numeral 152. Here again, the laser generator 12 is energized at the determined position 150, thereby avoiding cutting through the continuous guard ring that is formed around one or more pixels. Furthermore, in accordance with exemplary aspects of the present technique, starting and stopping the laser patterning at the numerous determined positions advantageously circumvents any overlap in the patterning of the sample 66. Specifically, no region on the sample 66 is scribed more than once, thereby avoiding scribing of the sample 66 beyond a desired depth. Accordingly, at any given position on the sample 66, if a road has previously been scribed, the laser generator 12 may be de-energized to prevent scribing the sample beyond the desired depth. The patterning of the sample 66 is continued along the second direction 154 until a determined position 156 is reached, where the scribing of the sample 66 is stopped. Reference numeral 158 is generally representative of the pattern (road) scribed by the one or more laser beams 26 on the sample 66 in the second direction 154.
Subsequently, the translation of the sample 66 in a third direction 162 is initiated and is generally represented by reference numeral 160. The laser generator 12 is energized at the position 156, thereby avoiding scribing of the guard ring. The patterning of the sample 66 is continued along the third direction 162 until a determined position 164 is reached, where the scribing of the sample 66 is stopped. Reference numeral 166 is generally representative of a pattern (road) scribed by the one or more laser beams 26 on the sample 66 in the third direction 162.
The patterning of the sample 66 is further continued by translating the sample 66 in a fourth direction 170. Reference numeral 168 is generally indicative of a translation of the sample in the fourth direction 170. Particularly, the sample 66 is patterned until the determined position 144 is reached. Reference numeral 172 is generally representative of a pattern (road) scribed by the one or more laser beams 26 on the sample 66 in the fourth direction 170.
Once the sample 66 is patterned along a periphery of the sample 66, the sample 66 may be further patterned based on the configured scanning pattern. Particularly, the sample 66 may be further patterned to form one or more pixels. Accordingly, the sample 66 is once again translated along the first direction 146 until a position on the sample 66 that is dependent upon the pixel size is reached. This distance may generally be represented by reference numeral 178. Subsequently, the sample 66 is translated in the second direction 154 until a determined position 176 is reached. This translation is generally represented by reference numeral 174. The laser generator 12 is energized at the determined position 176 to facilitate scribing a desired pattern 180 (road) along the second direction 154. The scribing is stopped at a determined position 182 to ensure that the road 166 is not scribed beyond a desired depth. By scribing the pattern 180, pixels 184 are formed. This process may be continued until all the roads have been scribed to form one or more pixels 184 in accordance with the scanning pattern configured at step 36.
With returning reference to
Turning now to
The method and system described hereinabove advantageously facilitate higher accuracy, reproducibility, and flexibility for patterning semiconductor devices with desired road widths. Furthermore, the method for laser patterning allows high productivity and low labor intensity as well as low to moderate capital investment depending on the laser employed. Additionally, the exemplary method for laser patterning may be employed to pattern different materials as the method for laser patterning a sample is not material dependent.
Moreover, the exemplary method for laser patterning entails fewer processing steps compared to conventional photolithography patterning thereby substantially enhancing the speed of forming pixelated devices with enhanced performance. Additionally, interpixel isolation is formed as the one or more laser beams scribe the road, thereby circumventing the need for subsequent processing of the CZT substrate. Furthermore, the method and system for laser patterning presented hereinabove aids in generating a pixelated CZT device that minimizes the damage to the CZT substrate. The pixelated CZT device so formed exhibits a high interpixel resistivity and a high charge collection efficiency while minimizing pixel to pixel space.
Additionally, this method for laser patterning facilitates the manufacturing of devices with pixels of different shapes. By way of example, using this method, a device having square pixels, rectangular pixels, circular pixels, hexagonal pixels, or pixels of other shapes may be manufactured. Furthermore, devices including other electrode structures, such as interpixel grids, may also be manufactured. In certain embodiments, the interpixel grids are generally representative of a thin line between the pixels and are typically maintained at a slightly higher potential than the pixels. These interpixel grids provide “steering” of a charge cloud to a pixel.
