Fabrication process for a silicon photosensitive element

Abstract: A groove (4) is formed on the surface of a silicon substrate (1) by way of etching. A silicon device (3) for a driver of a photosensitive element is formed on the surface of the substrate (1) where the groove (4) is not formed. With the groove, a super lattice structure (6) of Si and Si 1-x Ge x is buried to form a photosensing portion (2). The photosensing portion is formed with an avalanche photodiode or a PIN diode. The photosensing portion is formed to have no step with the surface of the substrate. On the other hand, SOI silicon oxide layer is provided on the back side of the substrate to form the structure of SOI substrate. By this, a photo reflection layer (7) of SiO₂ layer is provided below the photosensing portion (2). Thus, a silicon type photosensing element and the silicon device for driver can be formed on a common chip simultaneously for reducing production cost and for improving sensitivity and photo converting efficiency of the photosensing element.

Description: The present invention relates generally to fabrication process for a silicon photosensitive element. More specifically, the invention relates to a fabrication process for a silicon photosensitive element for an optical communication of an OEIC structure, which photosensitive element has a superlattice structure of Si and Si 1-x Ge x mixed crystal at a photosensing portion and a high speed silicon device for driving the photosensing portion at the circumference thereof. An optical communication network in a 1.3 micron band holds a market which will be completed toward the future as a data communication means for multi-media for family use. As photosensitive element for the 1.3 micron band, compound type material is currently employed. However, such compound type material holds a problem in the viewpoint of reliability. On the other hand, a drive is typically a Si device. Therefore, both elements cannot be formed within a common chip to cause rising of a production cost and to border down-sizing. As a photosensitive element for the 1.3 micron band employing a Si type material, an avalanche photodiode type element employing Si and SiGe has been reported in IEEE, Electron Device Letters, Vol. EDL-7 No. 5, May 1986, pages 330 to 332 and Appl.Phys. Lett. 49(13), September 29, 1986, pages 809 to 811. The report proposes a photosensitive element which is fabricated by forming PN junction and a superlattice structure of Si/Si 1-x Ge x (x = 0.6) by employing epitaxial growth technology on a Si substrate having a surface, on which nothing is formed, forming a photosensing portion by way of mesa etching, and introducing a light through a cleavage surface by means of an optical fiber. However, in this structure, while the photosensing portion of the avalanche photodiode can be formed, a large step may be formed on the silicon substrate by performing mesa etching. By the step, a conductor wiring layer to be formed on the surface of the silicon substrate becomes discontinuous to make it difficult to form the driver of the Si device on the common chip. Even when this problem could be avoided, it becomes necessary to perform a Si transistor fabrication process after formation of the mesa structure. Since the transistor fabrication process requires high temperature heat treatment, it is possible to break the superlattice structure of Si 1-x Ge x /Si. Furthermore, in such mesa structure, since the PN junction of the avalanche diode is exposed, it becomes necessary to perform passivation. Then, the passivation has to be performed at low temperature so as not to break of superlattice structure of Si/Si 1-x Ge x . Therefore, it is difficult to satisfactorily perform passivation. Accordingly, in the above-mentioned prior art, a leak current flowing on the mesa etching surface, and thus a dark current in the photosensing portion becomes large to cause difficulty in rising sensitivity. A means and method for forming an optical sensor with an integrated circuit structure is described in US-A-4847210. Therein an epi-coated semiconductor wafer is masked and a cavity is etched through the epi-layer to the substrate. A dielectric sidewall is formed on the cavity sidewall and a substantially intrinsic semiconductor region, preferably grown by selective epitaxy, to refill the cavity. The upper surface of the intrinsic region is then heavily doped and contacted by a low resistance polysilicon layer which is substantially transparent to incoming light. The top of the so formed optical sensor is substantially flush with the surface of the silicon body. The European application EP-A-0663699 which falls within the terms of Article 54(3) EPC shows an manufacturing method of an opto-electric semiconductor device which allows to selectively grow a P type silicon layer and a Si/Ge x Si 1-x superlattice layer under low temperature conditions in an area encircled with a groove, of which at least the sidewalls consist of silicon oxide film, which is formed in the silicon substrate. Thereby, the leak at the side of the superlattice layer can be reduced. Furthermore, by burying a metal film in the groove, the loss of light at the side of the superlattice layer can be suppressed. In a superlattice structure of Si 1-x Ge x /Si serving as a light absorbing layer, large distortion is present since Ge has greater lattice constant. Therefore, when several thousands Å of thickness is provided for the superlattice structure, defect may be caused to increase dark current. Accordingly, the thickness of the light absorbing layer becomes quite thin in comparison with the diameter of the optical fiber core. Therefore, even when light is introduced through the cleavage surface to provide greater thickness in a direction parallel to the wafer surface, efficiency becomes low due to small cross sectional area of the light absorbing layer. Therefore, as proposed in A. Splett et al., IEEE Photonic Technology letters, Vol 6, No. 1, January, 1994, p 59 to 61, efficiency can be improved by employing a structure fabricated through the process of forming 2 µm of Si 1-x Ge x containing 2% of Ge on a Si substrate and subsequently forming the avalanche photodiode having the superlattice structure of Si 1-x Ge x /Si thereon and thus by forming a wave guide of Si 1-x Ge x to enclose the light due to difference of refraction indexes between the Si 1-x Ge x layer and the Si of the substrate. However, the difference of the refraction indexes between the Si 1-x Ge x layer containing 2% of Ge and the Si substrate is too small to provide sufficient light enclosing effect. In order to increase the difference of the refraction indexes to completely enclose the light, the mixing rate of Ge has to be increased. However, increasing of the Ge mixing rate inherently cause the problem of distortion as set forth above. Therefore, it is an object of the present invention to provide a fabrication process of a silicon photosensitive element in which the occurance of defects in the superlattice structure is suppressed. This object is achieved with the fabrication process of claim 1. Claims 2 and 3 are useful developments of the fabrication process of claim 1. The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken to be limitative to the present invention, but are for explanation and understanding only. In the drawings: Figs. 1A to 1C are general views showing a silicon photosensitive element made by employing the fabrication process according to the invention; Figs. 2A to 2D are general views showing a fabrication process of a silicon photosensitive element according to the invention; Fig. 3 is a chart showing a relationship between a reverse bias voltage and a dark current in a silicon type photosensitive element made by employing the fabrication process according to the invention; and the conventional mesa type element; and Fig. 4 is a chart showing a relationship between a reverse bias voltage and a photo current of a silicon type photosensitive element made by employing the fabrication process according to the invention. A silicon type photosensitive element made by employing the fabrication process according to the invention will be discussed hereinafter in detail with reference to the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of silicon photosensitive element made by employing the fabrication process according to the invention. It will be obvious, however, to those skilled in the art that the silicon photosensitive element made by employing the fabrication process according to the invention may be practiced without these specific details. In other instance, well-known structures are not shown in detail in order to unnecessary obscure the present invention. As shown in Fig. 1A, on the surface of a silicon (Si) substrate 1, a photosensing portion 2 and a silicon device 3 are formed. In the photosensing portion 2, a groove 4 is formed by selectively etching the surface of the silicon substrate 1. The entire surface including the groove 4 and the silicon device 3 are covered with a silicon oxide layer 5 or a silicon nitride layer. After formation of the silicon oxide layer 5, a superlattice structure 6 of Si and Si 1-x Ge x is buriedly formed by selective epitaxial growth within the groove 4 to form the photosensing portion 2. The photosensing portion 2 is constructed as an avalanche photodiode or a PIN diode. When the photosensing portion 2 is buried by selective epitaxial growth, the silicon substrate is etched in a substantial depth, an optical fiber is fixed to be placed at the same height to the photosensing portion to form a construction to introduce the light in parallel along the surface, and electrodes at respective portion are formed, the step on the surface of the substrate can be eliminated. By this, breakage due to step of the wiring can be avoided. Therefore, the photosensing portion and the Si device to be the driver can be formed on the common chip. On the other hand, since the Si 1-x Ge x layer as the photosensing portion can be grown at the final step of the device fabrication process, defect to cause dark current may not be formed by the high temperature heat treatment in the formation process of the silicon device portion to be the driver. Furthermore, since PN junction is completely covered by the side wall oxide layer during the process of epitaxial growth, dark current due to junction leakage can be minimized. Next, as shown in Fig. 1B, by employing lamination type or SIMOX type SIO substrate as a substrate 1, a SOI silicon oxide layer 7 is provided on the back side of the silicon substrate 1. Therefore, the silicon oxide layer 7 is provided below the photosensing portion 2 constructed by the avalanche photodiode or PIN diode which is formed at the surface of the silicon substrate 1. Then, the SiO 2 oxide layer 7 is arranged below the photosensing portion 2. Then, the light discharged from the optical fiber core portion can be enclosed within the photosensing portion 2 without causing diffusion in the silicon substrate for difference of refraction index between the SiO 2 layer and the Si 1-x Ge x layer. Thus, light conversion efficiency can be improved. Also, as shown in Fig. 1C, when a several µm of silicon layer is formed below the avalanche photodiode or the PIN diode by controlling thickness of an upper silicon layer of the lamination type or SIMOX type SOI substrate, the silicon layer may serve as an optical wave guide 8. Then, the light introduced into the silicon layer as the optical wave guide is sequentially introduced into the photo absorbing layer of Si/Si 1-x Ge x and converted therein into electric signals. By providing such optical wave guide, the problem that the cross-sectional area of the photo absorbing layer is quite small in comparison with the cross-sectional area of the core portion of the optical fiber, can be solved to further improve the light conversion efficiency. In order to realize the structures shown in Figs. 1A to 1C, it becomes necessary to selectively grow the superlattice structure of several thousands Å (10 Å = 1 nm) of Si and Si 1-x Ge x forming the avalanche photodiode or the PIN diode without causing a facet and defect from the side wall. Thus, when a thick layer is selectively grown, it is inherent to simultaneous supply halogen type gas. However, this may cause a problem of occurrence of facet. The inventors of the present invention have found that occurrence of facet or the defect from the side wall can suppressed by flowing a silane type gas onto the silicon surface at such a growth temperature, at which coverage of the silicon surface by hydrogen becomes sufficiently high, for performing selective epitaxial growth. It is considered that this effect is attained since the hydrogen on the surface restricts diffusion of the atom on the surface. However, under such condition, it is not possible to grow a thick selectively grown layer. When Si 2 H 6 gas is directed onto SiO 2 , the molecule of Si 2 H 6 gas is once trapped on the surface of the SiO 2 in metastable state, and then released again. At this time, in a certainty determined by the substrate temperature, a small number of Si 2 H 6 molecule causes decomposition to deposit on the SiO 2 as Si atom. When the amount of Si atom deposited on the SiO 2 reaches a critical amount, it causes nucleation to form a polycrystalline silicon island on the SiO 2 . Once the polycrystalline silicon island is formed, since the growth speed of Si becomes equal to the growth speed at the Si opening portion, the polycrystalline silicon island grows abruptly. In case of growth employing the silane type, selective growth is broken through the foregoing process. The inventors has further found that selectivity is not broken even by growing a thick layer and the selectively grown silicon layer is almost not etched by inserting a steps of interrupting the growth of the polycrystalline silicon core on the SiO 2 before the core becomes large, maintaining the temperature of the substrate within a given range and dosing only Cl 2 or F 2 molecule to the substrate. The effect is based on the following principle. When the Si 2 H 6 gas is dosed on the SiO 2 , Si atom density on the SiO 2 is increased progressively. When Cl 2 or F 2 is directed onto SiO 2 before formation of polycrystalline silicon is caused, the Si atom on the SiO 2 reacts with Cl 2 or F 2 to cause evaporation as SiCl 2 or SiF 2 having high vapor pressure. At this time, while halogen gas is also directed onto the Si epitaxial layer above the Si opening portion, no etching is caused on Si crystal. It is considered that for etching the Si crystal, it is required to cut a high covalency back bond, and whereas Si on the SiO 2 has relatively weak back bond to cause such difference. Accordingly, it becomes possible to continue growth again. However, once the polycrystalline silicon core is formed, Si in the core cannot react with Cl 2 or F 2 . Therefore, once the polycrystalline silicon core, it becomes impossible to make Si evaporated. In case of Si 3 N 4 , the condition of the selective growth can be extended on the basis of the same principle. By repeating the foregoing process, the thick Si and Si 1-x Ge x layer can be grown without causing facet or dislocation. Thus, the structures as shown in Figs. 1A to 1C can be realized. Next, discussion will be given for concrete example of a fabrication process according to the invention to form a photosensitive element as illustrated in Fig. 1A. Figs. 2A to 2D are diagrammatic illustration showing the shown photosensitive element fabrication process in order of the process steps. At first, as shown in Fig. 2A, on the surface of the Si substrate 1 which has (100) face on the surface, the device 3 for driver, such as pre-amplifier, identification circuit and so forth is formed through normal silicon process. Also, the entire surface of the Si substrate 1 is covered with the oxide layer 8. Next, as shown in Fig. 2B, after masking the portion of the device 3 for the driver with a resist layer, dry etching is performed to form a groove 4 in a dimension of 1 µm in depth, 30 µm in width and 500 µm in length, for example, is formed on the surface of the portion of the substrate 1 where the device 3 is not formed. Subsequently, as shown in Fig. 2C, after performing thermal oxidation for entire surface, the side wall of the oxide layer 5 is formed on the side surface of the groove 4 by etching back. At this time, on the bottom of the groove 4, a silicon region of the substrate 1 is exposed. Also, arsenic is injected to the bottom of the groove 4 by way of ion implantation to form a n-type layer in a doping rate of about 2 x 10 19 cm -3 . Next, by chemical washing, contaminant on the bottom of the groove 4 is removed. Thereafter, the superlattice structure 6 of Si and Si 1-x Ge x is formed by selective epitaxial growth in the groove 4. By this, the avalanche photodiode as the photosensitive element can be formed. In the growth of the superlattice structure 6, UHV-CVD device for silicon was employed. As a primary discharge pump, a turbo molecular pump having a discharge rate of 1000 ℓ/s is employed. Si 2 H 6 gas, GeH 4 gas and Cl 2 gas are mass flow controller, to supply the gas obliquely from 100 mm lower position through a stainless steel nozzles which are independent of each other. At a substrate temperature of 660 °C, 1 SCCM of Si 2 H 6 gas is supplied for 400 seconds. Subsequently, the substrate temperature is risen to 775 °C, Cl 2 gas is supplied for 30 seconds. By repeating the foregoing processes (supply of Si 2 H 6 gas at 660 °C and supply of Cl 2 gas at 775 °C) respectively three times, 3000Å of non-doped silicon layer was formed. Subsequently, at the substrate temperature of 660 °C, 1 SCCM of Si 2 H 6 gas is supplied for 400 seconds and 1 SCCM of B 2 H 6 gas (diluted to have 1% of H 2 ) is supplied for 400 seconds. Thereafter, rising the substrate temperature to 775 °C, Cl 2 gas is supplied for 30 seconds. By repeating this process twice, B-doped silicon layer in a thickness of 2000 Å and in a doping rate of 2 x 10 17 cm -3 is formed. Furthermore, at the substrate temperature of 480 °C, 1 SCCM of Si 2 H 6 gas and 20 SCCM of GeH 4 gas are supplied for 50 seconds, and at the substrate temperature of 610 °C, 1 SCCM of Si 2 H 6 gas for 30 seconds. By repeating this for 10 times, 10 cycle of superlattice structure constituted of 50Å of Si 0.4 Ge 0.6 layer and the silicon layer of 100Å. At this time, by rising the temperature to 775 °C once per two cycles and supplying Cl 2 for 30 seconds, selectivity is assured. Subsequently, at the substrate temperature of 660 °C, 1 SCCM of Si 2 H 6 gas and 10 SCCM of B 2 H 6 gas (diluted to contain 1% of H 2 ) are supplied for 400 seconds. Thereafter, by rising temperature to 775 °C, Cl 2 is supplied for 30 seconds. By repeating these processes twice, a silicon layer of 2000Å in the thickness and 2 x 10 19 cm -3 , doped with B is formed. Subsequently, as shown in Fig. 2D, the silicon substrate 1 at the portion where the optical fiber 10 should be fixed is etched in a depth of 63 µm and in a width of 125 µm, for example. Then, the optical fiber 10 is fixed so that the core portion 11 may be placed at the same height to the photosensing portion 2. By this, the light from the optical fiber 10 can be introduced into the photosensing portion 2 in parallel along the surface of the substrate 1. When electrodes at respective portion are formed, since no step is present on the surface, the wiring will never cut by the step. Accordingly, the photosensing portion 2 and the Si device 3 to be the driver can be connected with the wiring which has no step and is not possible to be broken. Therefore, the photosensing portion 2 and the driver device 3 can be formed on a common chip. Also, since the Si 1-x Ge x layer of the photosensing portion 2 can be grown at the final step in the device fabrication process, no defect will be created in the superlattice structure by high temperature heat treatment in the process step of forming the silicon device 3. Therefore, dark current will never be caused. Furthermore, since the PN junction is completely covered with the side wall oxide layer 5 during the process of selective epitaxial growth, generation of dark current due to leakage at the junction can be minimized. At this time, the capacity of the avalanche photodiode was 0.3 pF/µ 2 upon application of 10V. Fig. 3 shows a relationship between a reverse bias voltage of the avalanche photodiode and the dark current. In addition, a reverse bias-dark current curve of the mesa type photosensitive element reported in Appl. Phys. Lett. Vol. 49, p809 is shown in Fig. 3 for comparison. As can be clear from Fig. 3, the dark current of the avalanche photodiode buried by way of selective growth is small. Therefore, sensitivity of this avalanche photodiode is high. When 140 ps pf pulse of 1.3 µ wavelength of Nd:YAG laser is fed, sensitivity in reception of the photosensitive element of the present invention was -36 dBm. While the foregoing silicon photosensitive element made by employing the fabrication process according to the invention has been discussed in terms of employment of the avalanche photodiode in the photosensing portion, it has been confirmed that similar effect may be attained even when the PIN diode is employed. Next, concrete discussion will be given for characteristics of the photosensitive element shown in Fig. 1B. The construction of Fig. 1B can be obtained by employing SIMOX or laminated type SOI substrate as the Si substrate. In the silicon photosensitive element made by employing the fabrication process according to the invention, the lamination type SOI substrate constituted of a SiO 2 layer in thickness of 0.5 µm and an upper silicon layer in thickness of 1.5 µm. On the substrate, the silicon device 3 for driver and the avalanche photodiode forming the photosensing portion 2 are formed through the process as shown in Figs. 2A to 2D. Fig. 4 shows a relationship between the reverse bias voltage and a photo current upon introduction of Nd:YAG layer of 1.3 microns of wavelength (second embodiment). For comparison, the result in the case where SiO 2 layer 7 is not present as the photosensitive element shown in Fig. 1A is shown, in addition. As can be appreciated from Fig. 4, in the photosensitive element shown in Fig. 1B having the SiO 2 layer 7 in the lower layer, the photo current is large and thus the photosensitive element shown in Fig. 1B has higher sensitivity in comparison with the photosensitive element shown in Fig. 1A. When the pulse of 140 ps of the Nd:YAG laser having 1.3 microns of wavelength is applied, the sensitivity in reception of the shown photosensitive element was -38 dBm. While the foregoing photosensitive element has been discussed in terms of employment of the avalanche photodiode in the photosensing portion, it has been confirmed that similar effect may be attained even when the PIN diode is employed. Next, the characteristics of the photosensitive element shown in Fig. 1C will be discussed concretely. The construction of Fig. 1C can be obtained by employing the SIMOX or lamination type SOI substrate as the Si substrate and by providing sufficiently thicker thickness for the upper silicon layer in comparison with the thickness of the avalanche photodiode portion. In the photosensitive element made by employing the fabrication process according to the invention, the SOI substrate constituted of a SiO 2 layer 7 in a thickness of 0.5 microns and an upper silicon layer in a thickness of 3.5 microns was employed. On this substrate, the device 3 for driver and the avalanche photodiode forming the photosensitive element 2 are formed through the process as set forth above. By employing such process, approximately 2 µm of Si layer 8 is formed between the avalanche photodiode and the SiO 2 layer. The Si layer 8 serves as a waveguide. Therefore, the light introduced into this range is sequentially introduced into the Si/Si 1-x Ge x photo absorbing layer and converted into the electric signal. By providing the waveguide 8, the problem that the cross-sectional area of the photo absorbing layer is much smaller than the cross-sectional area of the core portion of the optical fiber and thus conversion efficiency of the light can be improved. Fig. 4 shows the relationship between the reverse bias voltage and the photo current of the shown avalanche photodiode upon introduction of Nd:YAG laser of 1.3 microns wavelength. The results when the SiO 2 layer 7 is not present as in the photosensitive element shown in Fig 1A and when SiO 2 layer is formed immediately below the avalanche photodiode is formed, are shown in addition. The photosensitive element shown in Fig 1C having the Si layer 8 between the avalanche photodiode and the SiO 2 layer 7, has the largest photo current. Therefore, the photosensitive element shown in Fig 1C has the highest sensitivity. The sensitivity in reception of the shown photosensitive element when 140 ps of pulse of the Nd:YAG laser having 1.3 microns wavelength was applied, was -40 dBm. While the foregoing photosensitive element has been discussed in terms of employment of the avalanche photodiode in the photosensing portion, it has been confirmed that similar effect may be attained even when the PIN diode is employed. While the shown photosensitive elements have been discussed in terms of examples employing Si 2 H 6 and german gas (GeH 4 ) for selective epitaxial growth for forming the photosensitive portion in the shown photosensitive elements, it has been confirmed that the completely identical phenomenon will be arisen by silane gas (SiH 4 ), trisilane gas (Si 3 H 8 ), digerman gas (Ge 2 H 6 ) . Also, while the selectivity of the SiO 2 has been discussed in the shown photosensitive elements, the identical phenomenon has been observed in the case of Si 3 N 4 .

Claims: A fabrication process of a silicon photosensitive element comprising the steps of: forming a silicon device (3) serving as a driver on a surface of a silicon substrate (1); forming a groove (4) by etching the surface of said silicon substrate; coating said groove except the bottom surface thereof with a silicon oxide layer (5) or a silicon nitride layer; forming a photosensitive portion in said groove having no step with the surface of said substrate (1) by selectively forming a Si layer within said groove by alternately supplying Si 2 H 6 and Cl 2 ; selectively forming a Si 1-x Ge x layer serving as photosensing portion in said groove by alternately supplying Si 2 H 6 , and GeH 4 and Cl 2 ; repeating a plurality of times the last two steps whereby a photosensitive portion (2) consisting of a superlattice structure (6) of Si and Si 1-x Ge x layers is formed, and selectively forming an electrode layer in said groove by alternately supplying Si 2 H 6 and B 2 H 6 or Si 2 H 6 , and PH 3 and Cl 2 . A fabrication process as set forth in claim 1, wherein said silicon substrate is a SOI substrate. A fabrication process as set forth in claim 1, wherein said silicon photosensitive element is an element for optical communication.

Inventors: TATSUMI TORU, TATSUMI, TORU
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1995
Main classification: H01L31
IPC codes: H01L31, H01L27