SILICON-ON-INSULATOR (SOI) TRENCH PHOTODIODE AND METHOD OF FORMING SAME

Abstract: A semiconductor device (and method for forming the device) includes a silicon-on-insulator (SOI) wafer formed on a substrate surface. An isolation trench in the wafer surface surrounds alternating p-type trenches and n-type trenches and electrically isolates the device from the substrate, thereby allowing the device to be effectively utilized as a differential detector in an optoelectronic circuit.

Description: SILICO -ON- INSULATOR (SOD RENCH PHOTODIODE AND METHOD OF FORMING SAME , The present invention generally relates to a photodiode, and more specifically, to a silicon on insulator (SOI) photodiode and a method of forming such a photodiode . Photodiode detectors are often used in integrated optoelectronic circuits for noise rejection. Commonly assigned US Patent Application No. 09/205,433 to Crow et al . (hereinafter "Crow"); discloses a photodiode structure for use as a detector which employs lateral PIN diodes (i.e., pn junction devices with lightly doped (near intrinsic) regions placed between the typical diode p- and n- type regions) grown in deep trenches in bulk silicon. Such structures are known as "bulk trench detectors." As shown in Fig. 1(a), the bulk trench photodiode detector 100 disclosed by Crow is composed of an array of long, deep trench structures 110 which alternate from p-type doping 110a to n-type doping 110b, thereby forming a lateral PIN diode structure. As shown in Fig. 1(b), this means that one terminal of the diode is electrically connected to the substrate 220, for example, the p-terminal (anode) for p-substrates, as used by typical analog and mixed signal processes. However, in the Crow photodiode, the terminal which is connected to the substrate 220 will be exposed to the substrate noise and low substrate impedance associated therewith (e.g., Rs in Figure 1(b) denotes the impedance to substrate, typically a few 10s of ohms) . Therefore, if this photodiode structure is employed as a differential detector 200, as shown in Figure 1(b), substrate noise will be injected directly into the preamplifier 210 from the diode terminal which is connected to the substrate 220 and the impedance of that substrate node will not be the same as that of the other terminal of the diode 230. An imbalanced impedance is, therefore, created at the input. Therefore, this photodiode can operate as a single-ended detector but not as a differential detector which is preferred. Another disadvantage of this photodiode structure is that it collects photoelectrons (i.e., carriers) 120 generated below the trench depth (i.e., beneath the volume occupied by the trench electrodes) where the diminishing electric fields will sweep the carriers to appropriate device terminals with less speed than the carriers generated between the trenches, resulting in a lower bandwidth than if the carriers are collected only from the region between the trenches. The present invention seeks to provide an improved photodiode structure . According to a first aspect of the present invention, a semiconductor device for photodetection is provided, comprising: a substrate; a wafer formed on said substrate, said wafer having a silicon layer and an insulating layer, the insulating layer being between the silicon layer and the substrate; and a plurality of alternating p-doped and n-doped trenches formed in said silicon layer of said wafer. In a second aspect of the present invention, a semiconductor photodiode is provided, including a substrate, a wafer formed on the substrate and having a silicon layer and an insulating layer, a plurality of alternating p-doped trenches and n-doped trenches formed in the silicon layer. The photodiode has a silicon epilayer which is preferably about 8 to 15 microns thick for 850 nm signals and, for reflection, an insulator layer which is about V, wave thick at the signal wavelength, or about 212 nm thick for 850 nm light. The photodiode further has a set of p-doped trenches and set of n-doped trenches which are interdigitated and alternated. These trenches are also as deep as the silicon epilayer and are filled with polysilicon. In another aspect, the photodiode has an isolation trench formed in the silicon layer and surrounding the p-doped and n-doped trenches. The trench is bounded at the bottom by the surface of the insulator layer (i.e., the trench is as deep as the silicon epilayer) . In addition, the trench may be doped with either p-type or n-type dopants and filled with polysilicon or another conducting substance. The trench isolates the photodiode from other devices so that the photodiode may be used effectively as a differential detector. In another aspect of the invention, the SOI photodiode described above is incorporated into an integrated circuit as a differential detector. In another aspect of the invention, the isolation trench is not doped but is filled with an oxide which further isolates the photodiode. In another aspect of the invention, the insulator layer of the SOI wafer has a dielectric reflecting stack to reflect light that has penetrated the silicon epilayer back into the trenches thereby increasing responsivity. In another aspect of the invention, the photodiode is tilted at an angle with respect to an incoming light beam. The tilting increases the path length along which light enters the photodiode, increasing responsivity. In another aspect of the invention, incoming light is refracted into the photodiode by placing a prismatic cover on top of the photodiode which also increases the path length along which light enters the photodiode, increasing responsivity. In another aspect of the invention, the surface of the silicon epilayer of the SOI wafer is etched, using e.g. KOH, to form either prismatic or tetrahedral features which increases the path length along which light enters the photodiode, increasing responsivity. In another aspect of the invention, the insulating layer on the SOI wafer is made thicker than M wavelength of the signal light and is used as a slab waveguide. Light is coupled into the insulating layer from the edge of the device, and a diffraction grating directs light upward into the active photodiode layer. In another aspect of the invention, the photodiode device is provided without an isolation trench. This particular structure cannot operate as a differential detector but provides a high speed single-ended detector. In yet another aspect of the invention, a method is provided for forming the SOI trench photodiode according to its several embodiments . For a better understanding of the present invention, reference will now be made to the accompanying in which, by way of example: Figure 1 (a) shows the lateral PIN diode structure of a conventional bulk trench photodiode; Figure 1 (b) shows a symbolic equivalent of the conventional photodiode ; Figure 2 (a) shows the base insulating layer structure of a photodiode according to the present invention; Figure 2 (b) shows a symbolic equivalent of the photodiode according to an embodiment of the present invention; Figure 3 shows a top view of the circumferential trench isolation structure of a photodiode according to an embodiment of the present invention; Figure 4 shows an SOI trench photodiode with a prismatic cover according to an embodiment of the present invention; Figure 5 shows an SOI trench photodiode with an etched silicon epilayer according to an embodiment of the present invention; Figure 6 shows an SOI trench photodiode having a thick insulating layer for a waveguide according to an embodiment of the present invention; Figure 7 shows a flow diagram illustrating a preferred method for forming an SOI trench photodiode according to the present invention; and Figure 8 shows n-type and p-type trenches as formed according to a preferred method for forming an SOI trench photodiode according to the present invention. Referring now to the drawings, Figure 2(a) illustrates an SOI trench photodiode 300 according to a preferred embodiment of the present invention. Generally, as shown in Figure 2 (a) the trench photodiode according to the preferred embodiment of the present invention includes an SOI wafer 310 formed on a silicon substrate 320. The wafer 310 includes a silicon epilayer 330 which is preferably about 8 microns thick (the top-surface active layer of the bonded SOI wafer will be referred to as an "epilayer" herein) . Beneath the epilayer is an insulator layer 340 which is preferably about A of the signal light wavelength thick (e.g., roughly 212 nm thick for 850 nm signal wavelength) . The insulating layer 340 on the silicon wafer 310 may be, e.g., silicon oxide, sapphire or any other insulating material on which silicon may be bonded or grown and which is commonly used in the art. Other dimensions of the wafer such as thickness, length and width, are standard wafer dimensions in the semiconductor industry. The SOI wafer 310 may be either a bonded wafer (i.e., formed by bonding the oxide layers of two separate silicon wafers together by annealing) or a Silicon-Implanted with Oxygen (SIMOX) wafer (i.e., formed by implanting oxygen molecules into a few thousand angstroms of silicon on top of an insulator and then growing the silicon on top thicker) . As shown in Figure 2 (a) , an isolation trench 350 may be formed in the silicon epilayer 330 of the wafer 310 and around the perimeter of the detector. The depth of this isolation trench 350 is generally within a range of 3 to 12 microns, and preferably, about 8 microns deep which is the standard for deep trench processing and which has reasonably good responsivity at 850 nm which is the standard wavelength for short wavelength fibreoptic links. The isolation trench 350 surrounds and acts to electronically isolate the photodiode 300, and needs only be wide enough to provide this electrical isolation, typically 0.1 to 0.5 um wide. Figure 2 (b) shows a symbolic equivalent of the inventive photodiode and specifically indicates the electrical isolation of the device from other devices caused by the isolation trench 350. This isolation allows the inventive photodiode to be used as a differential detector 410 in an optoelectrical circuit 400. It is preferred that the isolation trench 350 is formed as deep as the silicon epilayer 330. In other words, the bottom of the isolation trench is the top of the insulator layer. This helps to ensure that the carriers injected into the epilayer by other electronic devices formed in regions adjacent to the photodiode do not reach the sensitive photodiode device. Therefore, the device is less susceptible to coupled noise from other circuits fabricated along with the photodetector. In fact, even if the isolation trenches are not as deep as the epilayer, isolation is improved over the standard bulk trench detector design 100. In addition, the surface of the isolation trench 350 is either p-doped or n-doped, depending on whether the substrate onto which the wafer is bonded is p-type or n-type. For example, if the substrate is p-type, the surface of the isolation trench 350 is n-doped and the isolation trench 350 would be connected to a quiet Vdd to isolate the photodiode. Conversely, for an n-type substrate, the surface of the isolation trench 350 is p-doped, in which case the isolation trench 350 would be connected to a quiet ground. In the embodiment shown in Figure 3, the wafer epilayer 330 has a set of p-type trenches 520 and n-type trenches 530 which are surrounded by the isolation trench 350. As shown, the two sets of trenches 520, 530 are alternating and interdigitated to form a lateral diode with PINIPIN doping, where I is the intrinsic substrate doping. The trenches 520, 530 are preferably formed about 8 microns deep and about 1/2 of a micron wide. The 1/e absorption depth for 850 nm light in silicon to about 15 microns. Thus, for a trench depth of 8 microns, about 40% of the light contacting the photodiode detector will generate carriers below the trenches . The alternating trenches 520, 530 are preferably filled with polysilicon, and metallization is then deposited to contact the polysilicon at the top of the trenches. This allows the metallization to contact the entire length of the trench, and is most compatible with existing processes. (Note that the metallization could be deposited into the alternating trenches 520, 530 to contact the doped semiconductor. Although this would reduce the parasitic series resistance presented by the doped polysilicon in the trenches and increase the speed of the photodiode, this process is costly and, therefore, polysilicon is used in the preferred embodiment) . In addition, the inventors have found that the inventive photodiode can be formed without an isolation trench so as to operate not as a differential detector, but as a high speed single-ended detector. This unique and unobvious design results in a photodiode 300 which is electrically isolated from the bulk substrate 320 and, therefore, does not inject substrate noise into the circuit. In addition, the inventive photodiode does not collect carriers 360 below the trenches 520, 530 and is, therefore, faster than conventional photodiodes . It is noted that, in a modification of the above, the isolation trench 350 could be left undoped and still provide greater isolation than conventional devices. Further, an oxide could be grown in the undoped isolation trench 350 which would isolate the photodiode 300 even more effectively than a doped trench. Furthermore, the responsivity of the photodiode 300 can be increased using any of several alternative methods which seek to guide or deflect light away from simple normal incidence to a more oblique angle, preferably in a direction parallel to the long axis of the trench electrodes. This has the effect of increasing the optical path length available to generate carriers without requiring significantly greater electrode area. Larger electrode areas would increase detector capacitance with adverse impact on electrical performance. The photodiode 300 could be tilted at an angle with respect to the incoming light beam. Depending on the degree to which the light is collimated, the path length is increased by 1 / (cos q) where q is the tilt angle. There will be some "edge" effects at the perimeter of the photodiode where this increase will not be effective, but this will usually be a small fraction of the entire area for most geometries. Additionally, a prismatic cover 610 as shown in Figure 4 could be affixed to the top of the finished photodiode 300. The cover refracts normally incident incoming light 620 (according to Snell's law) to create the oblique light path 630 into the photodiode 300. Moreover, instead of using the prismatic cover 610, the inventive photodiode could have a silicon epilayer 330 of the SOI wafer 310 that is etched (by using, for example, potassium hydroxide) to form either prismatic 710 or tetrahedral 720 features as shown in Figure 5. Like the above-mentioned prismatic cover 610, this etching creates an oblique light path 630, increasing responsivity. However, this structure has the disadvantage of having a non-planar surface for subsequent processing, and would require additional care to maintain surface passivation and metal line integrity for connections to the doped trenches. In all of these configurations, the insulating layer 340 can be enhanced to act as a backside reflector. This can result in a photogeneration region bounded on top and bottom by reflective layers, acting either as Bragg reflectors or by total internal reflection respectively. Therefore, the quantum efficiency of the photodiode 300 is greatly enhanced without increasing the electrode capacitance. Furthermore, as shown in Figure 6, the SOI wafer 310 could include a thicker insulating layer 340 and a diffraction grating 830. Specifically, the insulating layer 340 could be formed sufficiently thick (about 10 microns) to accept light from the core 810 of a butt-coupled fiber 805, or from an optical lensing system to focus the light into the thick insulating layer 340, so that the light is guided using the insulating layer 340 as a waveguide. The diffraction grating 830 could be formed in the insulating layer 340 by ion implantation or other methods. The diffraction grating 830 diffracts the light in the waveguide up to the photodetector 300. This particular configuration can easily be used in photodetector arrays that are integrated with silicon circuitry. Figure 7 is a flow chart illustrating a preferred method 900 for forming the inventive photodiode 300. The method includes forming 910 the SOI wafer on a substrate. Further, the isolation trench is etched 920 into the surface and around the perimeter of the SOI wafer using a conventional etching method. The p-type trenches are then etched 930 in the SOI wafer, as indicated in Figure 8 (a) . The trenches are formed preferably about 8 microns deep with a width as narrow as feasible compatible with the etching technology, typically 0.5 microns wide. As shown in Figure 8 (b) , the trenches are then filled 940 with a p-type polysilicon. As an alternative, the walls of the trenches may be doped p-type by ion implantation, or diffusion of a p-type material deposited in the trenches and the p-type trenches are then filled 945 with a conductive material which acts as one of the electrodes of the photodiode . The surface of the wafer is then planarized 950 by polishing. As shown in Figure 8(c), in step 960 through 980, steps 930 through 950 are repeated to deposit n-type material into the other trenches so as to form the n-type trenches. Metallization is then bonded 990 to the polysilicon or other conductive material at the top of the trenches. Alternatively, both the p-type and n-type trenches could be etched in the same step, and a photoresist used to block alternate trenches. In that case, the open trenches would be processed as in steps 940 to 950 with a p-type material. The photoresist would then be removed and a second photoresist layer would be applied to cover the trenches already processed. An n-type material would then be deposited in the empty set of alternating trenches as in steps 970 through 980. The photoresist would be removed and, after the wafer is polished if needed, metallization would be bonded to the polysilicon or other conductive material at the top of the trenches. During this process, the isolation trench 350 would be doped with n-type material or a p-type material, depending on whether the substrate was p- substrate or n+ substrate, as explained above. Alternatively, the isolation trench 350 could be left undoped and empty or, a thin thermal oxide layer may be formed on the walls of the isolation trench 350. Any of the above measures may provide adequate isolation of the photodiode 300 from circuits in the surrounding epilayer. Its unique and unobvious features electrically isolate the inventive photodiode from the bulk substrate and, therefore, prevent the photodiode from receiving substrate noise injected from the rest of the circuit. In addition, these features prevent the inventive photodiode from collecting carriers below the trenches, thereby making it faster than conventional photodiodes. Moreover, as shown above, if the insulating layer thickness is properly designed to act as a reflector for the wavelength of interest, and/or if the surface of the diode is modified to redirect normally incident light at a more oblique angle, the responsivity of the diode will be increased even further with no increase in device capacitance. While the invention has been described in terms of preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the scope of the appended claims .

Claims: CLAIMS 1. A semiconductor device for photodetection comprising: a substrate; a wafer formed on said substrate, said wafer having a silicon layer and an insulating layer, the insulating layer being between the silicon layer and the substrate; and a plurality of alternating p-doped and n-doped trenches formed in said silicon layer of said wafer. 2. A semiconductor device according to claim 1, wherein the a p-doped trenches and n-doped trenches are each formed by respective p-doped and n-doped polysilicon layers formed in said trenches and metallization layers are formed on the polysilicon layers. 3. A semiconductor device according to claim 2, wherein said polysilicon layers fill their respective trenches. 4. A semiconductor device according to claim 1, wherein said p-doped and n-doped trenches are formed by respective p and n doping of the silicon layer walls of the trenches and metallization or conductive material layers are formed on the wall surfaces of the trenches or the trenches are filled with metallization or other conductive material. 5. A semiconductor device according to any preceding claim, wherein said substrate comprises silicon. 6. A semiconductor device according to any preceding claim, wherein the thickness of said silicon layer is substantially 8 microns. 7. A semiconductor device according to any preceding claim, wherein an isolation trench is formed in said silicon layer surrounding said plurality of alternating p-doped trenches and n-doped trenches to isolate said p-doped and n-doped trenches. 8. A semiconductor device according to claim 7, further comprising: a silicon oxide layer grown on the surface of said isolation trench. 9. A semiconductor device according to claim 7 or 8 , wherein said substrate comprises an n-type substrate, and wherein said isolation trench comprises a p-doped trench and is coupled to receive a predetermined voltage . 10. A semiconductor device according to any preceding claim, wherein, in operation, said semiconductor device is tilted to a predetermined angle about an axis lying in a direction at right angles to the long axes of said p-doped trenches and said n-doped trenches, whereby a light beam is directed into said semiconductor at an angle which is more oblique than normal incidence. 11. .A semiconductor device according to any preceding claim, further comprising: a prismatic cover which covers said silicon layer for refracting an incoming light beam. 12. A semiconductor device according to any of claims 1 to 10, wherein prismatic or tetrahedral features are anisotropically etched on a surface of said silicon layer for refracting an incoming light beam. 13. A semiconductor device according to any preceding claim, further comprising: a dielectric stack formed in or at the surface of said insulating layer for selectively reflecting light back into said alternating p-doped and n-doped trench region and designed to reflect unabsorbed light of the signal wavelength. 14. A semiconductor device according to any of claims 1 to 10, further comprising: a suitably thick insulating layer to act as a light waveguide; and a diffraction grating formed in said insulating layer to diffract light at the signal wavelength into the semiconductor device. 15. A semiconductor device according to claim 7 or any of claims 8 to 14 appended to claim 7, wherein the depth of said isolation trench corresponds to the thickness of said silicon layer. 16. A method of forming a semiconductor device for photodetection, comprising the steps of: forming a wafer having a silicon layer and an insulating layer on a substrate, with the insulating layer being between the silicon layer and the substrate ; and forming a plurality of alternating p-doped and n-doped trenches in said silicon layer of said wafer. 17. A method according to claim 16, wherein the a p-doped trenches and n-doped trenches are each formed by forming respective p-doped and n-doped polysilicon layers in said trenches and wherein metallization layers are formed on the polysilicon layers. 18. A method according to claim 17, wherein said polysilicon layers are formed so as to fill their respective trenches. 19. A method according to claim 17, wherein said p-doped and n-doped trenches are formed by respective p and n doping of the silicon layer walls of the trenches and wherein metallization or conductive material layers are formed on the wall surfaces of the trenches or the trenches are filled with metallization or other conductive material. 20. A method of forming a semiconductor device for photodetection, comprising: forming a wafer having a silicon layer and an insulating layer on a substrate; forming a first set of trenches and a second set of trenches in said silicon layer; alternately implanting first impurity ions in said first set of trenches and second impurity ions in said second set of trenches ; filling said first set of trenches and said second set of trenches with polysilicon and conductive material; and bonding metallization to said polysilicon in first set of trenches and said second set of trenches . 21. A method as claimed in claim 20, further comprising: forming an isolation trench in said silicon layer, wherein, during said implanting, one of said first impurity ions or said second impurity ions are implanted in said isolation trench. 22. A semiconductor device for photodetection comprising: a substrate; a wafer formed on said substrate, said wafer having a silicon layer and an insulating layer; a plurality of alternating p-doped trenches and n-doped trenches formed in said silicon layer of said wafer; and a polysilicon layer formed in said alternating p-doped trenches and n-doped trenches for connecting to metallization. 23. A semiconductor device according to claim 22, further comprising: a p-doped or n-doped isolation trench formed in said silicon layer of said wafer and surrounding said alternating p-doped trenches and n-doped renches.

Inventors: KWARK YOUNG HOON, MOY DAN, RITTER MARK, ROGERS DENNIS, WELSER JEFFREY JOHN, KWARK, YOUNG, HOON, MOY, DAN, RITTER, MARK, ROGERS, DENNIS, WELSER, JEFFREY, JOHN
Applicants: IBM, IBM UK, IBM UNITED KINGDOM LIMITED, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 2001
Main classification: H01L31
IPC codes: H01L21, H01L27, H01L29, H01L31