Photoelectric converting device and method for producing the same

Abstract: A method for producing a photoelectric converting device, an electrode is formed across an insulating layer on a control electrode domain of a semiconductor transistor. The method comprises forming a field insulating layer on a semiconductor substrate; forming a control electrode domain by a self-alignment process utilizing the field insulating layer as a mask; forming an insulating layer on the control electrode domain and forming an electrode thereon; and forming a main electrode domain by a self-alignment process utilizing the field insulating layer and the electrode as a mask.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of producing a photoelectric converting device, more particularly a photoelectric converting device obtained by forming an electrode across an insulating layer on a control electrode area of a semiconductor transistor. Related Background Art In conventional semiconductor image pickup devices, the cell size of the sensor portion constituting a separate device cannot be considered sufficiently small in comparison with the advanced degree of integration of the driving circuit portion. For example, in contrast to the usual cell size of about 70 µm² in a 256K DRAM, that of a semiconductor image pickup device for example of 500 x 500 pixels is as several times as large as 200 to 300 µm². An increase in the cell size increases the chip area and therefore reduces the number of chips per wafer. Also in comparison with usual digital logic circuit devices, semiconductor image pickup devices are associated with much lower production yields because they have to satisfy optical and analog characteristics. For these reasons the cost per chip of the sensor portion is inevitably higher than that of DRAM or similar devices. The reduction of cell size can be achieved either by a scale-down method, in which dimensions are proportionally reduced, or by a cell improvement method, in which the number of cell constituting components is reduced. The cell size has been reduced year after year with the progress of microintegration technology, but the rate of size reduction is in average in the range 0.88 times per year. Thus a drastic size reduction cannot be expected in the former method. More detailedly, though a size reduction results in a reduction in the chip area and thus an increase in the number of chips per wafer, the production yield is lowered due to increased difficulty of process control. Consequently the number of defectless chips per wafer will show a peak at a certain cell dimension. On the other hand, the latter method can achieve a drastic size reduction. For example, in case of a random access memory, a memory unit originally composed of 6 to 8 transistors in now simplified to a transistor and a capacitor. The latter cell improvement method is also effective in the-field of semiconductor image pickup devices. For example charge-coupled image pickup devices (CCD) are now superior to MOS image pickup devices in simplicity and integration, by isolating each carrier accumulating portion not by a mechanical structure but by an electrically formed potential well. Also the charge transfer portion does not require sources and drains in the MOS structure and can therefore achieve a higher degree of integration. Though CCD's can achieve a higher level of integration in comparison with MOS devices, they are disadvantageous in production yield, because, in CCD's, a defect in the charge transfer portion affects the image information of an entire corresponding line. Also defects in the substrate cannot be tolerated because the information transfer is conducted not by metal wirings but by the interior of the substrate itself, and these facts deteriorate the production yield and raise the production cost. Under such situation, CCD's and MOS devices are now competing each other in the field of semiconductor image pickup devices. Other photoelectric conversion devices are disclosed in Pat. Abs. of Japan, vol 9, no 305 (E-363)(2028) 3rd Dec 1985 and JP-A-60142561 and US-A-3624428. SUMMARY OF THE INVENTION According to the present invention there is provided a method of making a photoelectric conversion device as set out in claim 1. An embodiment of the present invention makes it possible to reduce the cell size of a photoelectric converting device through an improvement in the cell structure thereof. An embodiment of the present invention provides a method for producing a photoelectric converting device with an improved precision of the control electrode domain and the main electrode domain in the cells thereof. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1(a) is a plan view of a photoelectric converting device; Fig. 1(b) is a cross-sectional view along a line A - A' in Fig. 1(a); Fig. 2(a) is a schematic view of a sensor cell of a semiconductor image pickup device obtained according to a process for producing a photoelectric converting device embodying the present invention; and Fig. 2(b) is a cross-sectional view along a line A - A' in Fig. 2(a). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Now, the present invention will be clarified in detail by an embodiment thereof shown in the attached drawings. A semiconductor image pickup device of a high level of integration can be formed with the photoelectric converting device disclosed in the Japanese Patent Laidopen No. 12759/1985, cf. EP-A-0132076. Fig. 1(a) is a plan view of said photoelectric converting device, and Fig. 1(b) is a cross-sectional view thereof along a line A - A'. In Figs. 1(a) and 1(b), photosensor cells are arranged on an n⁺-silicon substrate 12, and each photosensor cell is electrically insulated from neighboring photosensor cells by a separating area 13 composed of SiO₂, Si₃N₄ or polysilicon. Each photosensor cell is composed in the following manner. On an n⁻-domain 14 of a low impurity concentration formed for example by epitaxial growth, a p-domain 15 is formed by doping with a p-type impurity such as boron, and an n⁺-domain 16 is formed thereon by impurity diffusion or ion implantation. Said p-domain 15 and n⁺-domain 16 respectively constitute base and emitter of a bipolar transistor. On the p-domain 15 of the above-explained structure and on a separating area 13, there is formed an oxide layer 17 on which formed is an electrode 18 of a predetermined area. Said electrode 18 faces the p-domain 15 across the oxide layer 17 and receives a pulse voltage to control the potential of said p-domain 15 in floating state. There are further provided an emitter electrode 19 connected to the n⁺-domain 16, a wiring 20 for releasing a signal from the emitter electrode 19 to the outside, a wiring 21 connected to the electrode 18, an n⁺-domain 22 of a high impurity concentration formed on the rear face of the n⁺-silicon substrate 12, and an electrode 23 for biasing the collector of the bipolar transistor. The above-explained photoelectric converting device functions in the following manner. In response to incident light 24, electron-hole pairs are generated in the semiconductor in proportion to the intensity of incident light. The electrons drift to the positively biased n⁺-silicon substrate 12, but the positive holes are accumulated in the p-domain 15 (accumulating operation). The base potential is elevated by the accumulated positive holes. The base potential is further elevated by a voltage applied across the n⁺-domain 16 constituting the emitter and the n⁺-silicon substrate 12 constituting the collector, and by a positive voltage applied to the electrode 18. This potential change is read as the collector current, constituting an electric signal corresponding to the intensity of incident light (read-out operation). The charge accumulated in the p-domain 15 can be eliminated by grounding the emitter electrode 19 and applying a positive pulse to the electrode 18. Said positive voltage biases the p-domain 15 in forward direction with respect to the n⁺-domain 16, thereby discharging the accumulated charge (refreshing operation). The above-mentioned steps of accumulation, read-out and refreshing are repeated thereafter. However the above-explained photoelectric converting device, when applied to a semiconductor image pickup device of 500 x 500 pixels, requires a cell size of about 200 µm² and is not adequate for achieving a high level of integration. Fig. 2(a) is a plan view of a sensor cell of a semiconductor image pickup device, obtained by a method embodying the present invention for producing a photoelectric converting device not associated with the above-explained drawbacks, and Fig. 2(b) is a cross-sectional view along a line A - A'. In the present embodiment the semiconductor transistor is composed of a bipolar transistor, the control electrode is composed of the base and the main electrode is composed of the emitter thereof. In Fig. 2(a), 1 indicates a sensor cell. On a CZN(100) 10 Ω-cm silicon substrate 2, there is formed a field oxide layer 3 of a thickness of 1 µm by a LOCOS process. Under said field oxide layer 3 there may be formed an n⁺-channel stopper 5 if necessary, for preventing the transfer of positive holes from the neighboring channel. Then a p-domain 4, constituting a base, of an impurity concentration of 10¹⁴/cm³ and a diffusion depth of 0.8 µm is formed by a self-aligning process utilizing said field oxide layer 3 as a mask. The basic dimension (minimum rule) is equal to the length of a side of a contact hole to be explained later, and said p-domain 4 constituting an active area is 3 x 2 times as large as said basic dimension. Then an insulating layer 7 of a thickness of 500 Å (10 Å = 1nm) is formed, and a polysilicon electrode 8 of a thickness of 4,000 Å is formed thereon, constituting a gate electrode for the p-domain 4. Said electrode 8 is partially placed on the field oxide layer 3 and has a margin for misalignment. Then formed is an n⁺-domain 6, constituting an emitter, of an impurity concentration of 10¹⁹/cm³ and a diffusion depth of 0.3 µm is formed by self-aligned ion implantation utilizing the field oxide layer 3 and the electrode 8. The silicon substrate 2, p-domain 4 and n⁺-domain 6 constitute a vertical bipolar transistor with h ff in the order of 1,000. Then a PSG interlayer insulator 9 of a thickness of 7,000 Å is formed, a contact-hole 11 is opened, and a metal wiring 10 of a thickness of 8,000 Å is electrically connected to said n⁺-domain 6. In the present embodiment, the cell dimension for a basic dimension of 2 µm is 10 x 8 = 80 µm², which is comparable to the cell area of 70 µm² of 256K DRAM of 2 µm design rule, and which is almost 1/3 of the cell area of 200 to 300 µm² in the known semiconductor image pickup devices. Also the percentage of aperture excluding the electrode 8 and the metal wiring 10 is 51 % and is acceptable in practice. In the present embodiment, the use of LOCOS process in the formation of the field oxide layer 3 provides a flat surface, thus facilitating fine pattern formation in the following steps. Also said field oxide layer 3 is used in the self alignment in the formation of the p-domain 4 and n⁺-domain 6 to avoid alignment error, thereby obtaining stable characteristics of the aforementioned bipolar transistor. Also since the n⁺-domain 6 is formed by the self-alignment process utilizing the electrode 8, the p-domain 4 constituting the active area can be formed with a minimum necessary area, without unnecessary gate-emitter parasite capacity. Also said electrode 8 can be formed very close to the p-domain 4 constituting the base domain, or the active domain of the transistor, so that various parasite effects resulting from excessive base resistance can be avoided. Also, since the active area, or p-domain 4, can be made minimum, it is possible to reduce the base-collector capacity and to improve the device characteristics. In another embodiment of the present invention, a conventional photoetching process may be employed, instead of the LOCOS process, for forming the field insulating layer 3. Though such process is disadvantageous in obtaining a flat surface, it is free from bird's beaks associated in the LOGOS process and can form the base and emitter with exact dimensions. Thus such embodiment allows to achieve further stabilization of the transistor performance. Also as another embodiment, another substrate such as an epitaxial substrate is employed as the silicon substrate 2 for simultaneous formation of an NMOS device etc. A further increase in the percentage of aperture can be achieved, in the foregoing embodiments, by forming the sensor cell larger than 5 x 4 times of the basic dimension. In this case, the p-domain 4 should be preferably positioned close to the corner of the sensor cell for achieving a higher percentage of aperture. The electrode 8 and the metal wiring 10 can be composed of any usual material, such as polysilicon aluminum, an aluminum alloy, a high-melting metal or a silicide. Also the insulating layer 7 may be composed of any usual material such as an oxide or a nitride. As explained in the foregoing, the method of the foregoing embodiments for producing the photoelectric converting device allows to improve the positional precision of the control electrode area and the main electrode area almost without unnecessary margins through formation thereof by a self-aligning process, thereby reducing the cell size of the sensor cell and providing the photoelectric converting device inexpensively with an improved production yield. The foregoing embodiments are advantageously applicable for example to semiconductor image pickup devices requiring a high level of integration.

Claims: A method of making a photoelectric conversion device having first and second semiconductor regions (2,4), an insulating layer (7) on the second semiconductor region (4), and an electrode (8) on the insulating layer, comprising the steps of: forming a field insulating layer (3) on a semiconductor substrate (2) providing the first semiconductor region; forming the second semiconductor region (4); forming the insulating layer (7) on the second semiconductor region (4); and forming the electrode (8) on the insulating layer to be capacitively coupled to the second semiconductor region, characterised in that: said method includes the step of forming a third semiconductor region (6), having a different conductivity type from that of said second semiconductor region, within said second semiconductor region, whereby the third semiconductor region (6) is an emitter region, the second semiconductor region (4) is a base region and the first semiconductor region (2) is a collector region; said second semiconductor region (4) being formed by a self-aligned process using the field insulating layer (3) as a mask; and said third semiconductor region (6) being formed by a self-aligned process using the field insulating layer (3) and the electrode (8) as a mask. A method according to claim 1 which comprises the step of forming a further electrode (10) in contact with the third semiconductor region (6).

Inventors: ARIKAWA SHIRO, HOSHI JUNICHI, SATOH TAMOTSU, ARIKAWA, SHIRO, HOSHI, JUNICHI, SATOH, TAMOTSU
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1987
Main classification: H01L27
IPC codes: H01L27