Photoelectric conversion device and method for manufacturing the same

Abstract: A photoelectric conversion device is caused to have a quick response against incident light by designing the photosensitive portion in such a way that photocurrent does flow only in the direction perpendicular to a semiconductor layer. The device is comprised of the semiconductor and a pair of electrodes which are formed on the opposed surfaces of the semiconductor layer. The semiconductor layer is etched together with the electrodes so that the layer and the electrodes have the same area and the same shape.

Description: The present invention relates to a method for manufacturing a photosensitive semiconductor device in accordance with the generic clause of Claim 1. Such a method is known from US-A-4 527 007, which also discloses a contact image sensor. The known contact image sensor for a facsimile machine is schematically illustrated in Fig.1. The sensor comprises a photosensitive semiconductor layer 3 provided with a pair of electrodes 2 and 4 on the opposed surfaces, which are fabricated by a lithography using three masks. The fabrication method of this prior art device is as follows. First, a conductive layer of chromium is formed on a substrate 1 and patterned with a first mask 1m to form a plurality of first electrodes 2. Over the patterned electrode 2, a photosensitive intrinsic semiconductor layer of amorphous silicon is deposited to a thickness of 1 micrometer by a glow discharge method and patterned with a second mask 2m of a metal to form a photoelectric conversion layer 3. As the semiconductor layer, the amorphous silicon layer is formed in such a way that the patterned layer covers the first electrode 2 sufficiently even with a possible displacement of the mask 2m. Then, a conductive layer of tin indium oxide is formed on the semiconductor layer 3 and patterned with a third mask 3m to form a second electrode 4. A rectifying diode can be formed from a junction (MI junction) between the second electrode 4 and the semiconductor layer 3, a Schottky barrier. When light incidents on the semiconductor layer 3 through the second electrode 4 as shown in Fig.1(C), electron-hole pairs are generated in proportion to the intensity of the incident light. The image sensor shown in Figs. 1(A) to 1(C), however, is constructed with the semiconductor layer 3 larger than the first electrodes 2. Because of this, there are generated electron-hole pairs also in the region that is disposed vertically between the opposed electrodes 2 and 4 and lateral drifts of the carriers generated in the region take place as shown in Fig.1(A) with arrows. The lateral drift imparts a lag to the response. On the other hand, the intrinsic semiconductor layer fabricated as in the above tends to form a parasitic channel on an exposed surface thereof which is easily transformed into an n-type, causing a large dark current and dispersion in the quality of products. Further, the use of three masks reduces the cost performance. It is the object of the present invention to provide a improved method for manufacturing a photosensitive semiconductor device being capable of converting incident light at the real time, having little time lag in response to the incident light, and having few parasitic channels in the photoelectric semiconductor of the device. This object is solved by a method having the features of Claim 1. Preferred embodiments are disclosed in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Figs.1(A), (B) and (C) show a prior art image sensor; Figs.2(A) through 2(J) are plan views and section views showing a linear contact image sensor in accordance with the present invention; Figs.3(A) through 3(d) are a plan view and section views showing another linear contact image sensor in accordance with the present invention; and Figs.4(A) through 4(J) are plan views and section views showing a two-dimensional contact image sensor in accordance with the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Figs. 2(A) to 2(J), an embodiment of the invention is illustrated according to the manufacturing process thereof. On a heat resisting transparent substrate 1 such as a quartz plate or a pyrex glass plate, a plurality of transparent electrode strips 19 of tin oxide by using a first photo mask 1m are formed. Further, a first conductive layer 2, a photosensitive semiconductor layer 3, and a second conductive layer 5 are formed in sequence. The first conductive layer is a tin indium oxide layer with 2·10⁻⁷m [2000 Å] in thickness. The semiconductor layer is composed of at least a substantially intrinsic semiconductor layer, such as an amorphous silicon semiconductor layer incorporating a p-i-n, n-i-n or m-i junction therein, each layers being deposited by a known CVD system. The silicon semiconductor layer apparently has a slight n- type conductivity, so that it may be doped with such a p-type impurity as boron to make the semiconductor substantially intrinsic. For instance, the semiconductor layer is composed of a 200 Å thick p-type semiconductor layer, a 9,5 · 10⁻⁷m [3500 Å] thick intrinsic semiconductor layer and a 3 · 10⁻⁸m [300 Å] thick n-type semiconductor layer, which are deposited by a multi-chamber plasma enhanced CVD system (as disclosed in Japanese Patent Application Nosho 54-104452 by the applicant). In this case, the second conductive layer 5 is formed of chronium with 1 · 10⁻⁷m [1000 Å] in thickness and a transparent conductive layer 4 of chronium silicide is formed as a by-product with (1 to 20) · 10⁻⁹m [10 to 200 Å] in thickness between the semiconductor layer 3 and the chromium layer 5. The chromium silicide layer 4 makes the contact between the semiconductor layer 3 and the electrode 5 ohmic as a buffer layer. Next, the laminate layers of the conductive layers 2, 4 and 5 and the semiconductor layer 3 are integrally etched by means of a second mask 2m to produce cell blocks 10 on each strip 19 as shown in Figs.2(C), (D) and (E). The cell blocks 10 constitute the sensitive portion of the contact image sensor, each being 150 micrometers in length and 100 micrometers in width with an interval of 30 micrometers, and are designed into a linear array. As shown in Fig.2(F), a photocurable organic resin layer 6, e.g., an aromatic polyimide precursor solution is formed with 1.6 micrometers in thickness by a coating method so as to completely enclose the cell blocks 10. Then, the transparent substrate 1 is pre-baked in a clean oven at 80 °C for an hour and irradiated from the back surface thereof with ultraviolet light by a known mask aligner but using no mask. The irradiation time is about two minutes when an aligner distributed from Cobilt Co is used. After curing, the uncured portion of the resin layer is removed using a suitable rinsing liquid so that the remaining resin layer is nearly flush with the top surface of the cell blocks which are exposed therefore. In this connection, the curing treatment reduces the volume of the resin approximately by a factor of 2, so that the thickness of the resin layer before curing is chosen about 1.6 micrometers for the cell blocks with 0.8 micrometer in thickness. With ultraviolet light having 300 to 400 nm wavelength(10mW/cm²), the irradiation time is sufficiently 15 to 30 seconds. Finally, the substrate 1 is heated in a nitrogen atmosphere at 180 °C for 30 minutes, at 300°C for 30 minutes and at 400°C for 30 minutes, and an aluminium layer 9 is formed to the thickness of 0.1 to 1.5 micrometer throughout the upper surface of the substrate 1 and etched by a third photomask 3m in such a way that the remaining aluminium layer traverses and surely covers over all the cell blocks in contact with the exposed surface of all the cell blocks 10. The width of the aluminium electrode 9 is bistriding the cell blocks 20 and extending beyond the perimeter of the blocks, and therefore light entering the cell blocks 20 through the back side of the device is certainly blocked. Then, the manufacturing of an image sensor with a smooth top surface is completed. For example, the cell blocks can be aligned 8 cell per 1 mm in a line. The inventor has been examined the vicinity of the contact between the cell blocks and the resin layer by an electron microscope in advance of forming the electrode 9. As a result, there have been observed slight unevenness but no crack which tends to be a cause of a short current between the opposed electrodes 2 and 4 by drawing the material of the electrode 2 or 4; furthermore no projection is observed which might lead to a disconnection of patterns arranged thereon, and the two lateral adjacent layers are recognized as smoothly connected with each other. By virtue of this configuration, it becomes possible to superimpose an ITO (indium tin oxide) layer as the electrode 9 over the layers without causing any possibility of disconnection at the position just above the interface. The cell block 20 thus formed is comprised of the first electrode 2 in contact with the line 19, the semiconductor layer 3, the second electrode 4 and the third electrode 5 in contact with the line 13. And the semiconductor layer 3 is completely enclosed with the electrodes on the top and bottom surfaces and the resin layer at the side surfaces so that no current leakage and no parasitic channel takes place on the semiconductor layer. Further, the opposed electrodes 2 and 4 can have the same area and the same shape as the semiconductor layer 3 regardless of the precision of the patterning. An example of the photoelectric characteristics obtained by an experiment using a photoelectric device with p-i-n junctions according to the invention will be described hereinbelow. Namely, the dark current with an applied voltage of 3 V is 1.8 x 10⁻¹³ A (the cell area; 100 micrometers x 150 micrometers and the photo current with an irradiation of 100 Lx is 3.5 x 10⁻⁸ A. The response characteristics of the device was such that incident light switching over per one microsecond could be followed up. A prior art device as described above could not follow up such a frequent change. Another experiment was made for a photoelectric device incorporating n-i-n junctions therein. Characteristics of the current passing through the junction versus the applied bias is symmetrical about the zero bias. The dark current was 4 x 10⁻¹³ A and the photo conductivity with reference light of 100 Lx. The device could follow satisfactorily up an intermittent irradiation at intervals of 0.2 microsecond. Although the embodiment described in the above is constructed for a device which receives incident light through the substrate, the invention is also applicable to the device which recieves incident light from the reverse side. Another embodiment of this type will be described with reference to Figs.3(A) through 3(D). In the manufacturing process for a device of this kind, the same steps as in the process for the previous embodiment can be employed until the step illustrated in Fig.2(G), so that the process will be described only after said step. After depositing the resin layer 6 in Fig.2(F), the resin layer 6 and the conductive layer 5 are removed in such a way that the surface of the remaining resin layer is flush with the chromium silicide layer 4 as shown in Fig.3(A). The removal of the resin layer is carried out in the same manner as in the former embodiment. Finally, the substrate 1 is heated in a nitrogen atmosphere at 180°C for 30 minutes, at 300°C for 30 minutes and at 400°C for 30 minutes, and an aluminium layer 9 is formed to the thickness of 0.1 to 1.5 micrometers throughout the upper surface of the substrate 1 and etched by a third photomask 3m in such a way that the remaining aluminium layer traverse and surely covers over all the cell blocks in contact with the exposed surface of all the cell blocks 10 as shown in Figs.3(B), 3(C) and 3(D). The width of the aluminium electrode 9 is bistriding the cell blocks 20 and extending beyond the perimeter of the blocks, and therefore light enterring the cell blocks 20 through the back side of the device is blocked certainly. Then, the manufacturing of an image sensor with a smooth top surface is completed. For example, the cell blocks can be aligned eight cells per 1 mm of a line. Figs.4(A) to 4(J) show a further embodiment of the invention in which cell blocks are arranged in a matrix. The embodiment demonstrates a two-dimensional contact image sensor. On a heat resisting transparent substrate 1 such as a quartz plate or a pyrex glass plate, a plurality of transparent electrode strips 19 of tin oxide by using a first photo mask 1m are formed. Further, a first conductive layer 2, a photosensitive semiconductor layer 3, and a second conductive layer 5 are formed in sequence. The first conductive layer is a tin indium oxide layer with 2·10⁻⁷m [2000 Å] in thickness. The semiconductor layer is composed of at least a substantially intrinsic semiconductor layer, such as an amorphous silicon semiconductor layer incorporating a p-i-n, n-i-n or m-i junction therein, each layers being deposited by a known CVD system. The silicon semiconductor layer apparently has a slight n-type conductivity, so that it may be doped with such a p-type impurity as boron to make the semiconductor substantially intrinsic. For instance, the semiconductor layer is composed of a 200 Å thick p-type semiconductor layer, a 13,5·10⁻⁷m [3500 Å] thick intrinsic semiconductor layer and a 3·10⁻⁸m [300 Å] thick n-type semiconductor layer, which are deposited by a multi-chamber plasma enhanced CVD system (as disclosed in Japanese Patent Application No.sho 54-104452 by the applicant). In this case, the second conductive layer 5 is formed of chromium with 10⁻⁷ m [1000 Å] in thickness and a transparent conductive layer 4 of chromium silicide is formed as a by-product with (1 to 20)·10⁻⁹ m [10 to 200 Å] in thickness between the semiconductor layer 3 and the chromium layer 5. The chromiun silicide layer 4 makes the contact between the semiconductor layer 3 and the electrode 5 ohmic as a buffer layer. Next, the laminate layers of the conductive layers 2, 4 and and the semiconductor layer 3 are integrally etched by means of a second mask 2m to produce a plurality of cell blocks 10 on each strip 19 as shown in Figs.4(C), 4(D) and 4(E). The cell blocks 10 are arranged in the form of a matrix, in order to constitute a two-dimmensional contact image sensor. The cell blocks 10 are the sensitive portion of the contact image sensor, each being 150 micrometers long and 100 micrometers wide with an interval of 30 micrometers. As shown in Fig.4(F), a photocurable organic resin layer 6, e.g., an aromatic polyimide precursor solution is formed with 1.6 micrometers in thickness by a coating method so as to completely enclose the cell blocks 10. Then, the transparent substrate 1 is pre-baked in a clean oven at 80 °C for an hour and irradiated from the back surface thereof with ultraviolet light by a known mask aligner but using no mask. The irradiation time is about two minutes when an aligner distributed from Cobilt Co is used. After curing, the uncured portion of the resin layer is removed using a suitable rinsing liquid so that the remaining resin layer 6 is nearly flush with the top surface of the cell blocks which, therefore, are exposed. In this connection, the curing treatment reduces the volume of the resin approximately by a factor of 2, so that the thickness of the resin layer before curing is chosen about 1.6 micrometers for the cell blocks with 0.8 micrometer in thuickness. With ultraviolet light having 300 to 400 nm wavelength(10mW/cm²), the irradiation time is sufficiently 15 to 30 seconds. Finally, the substrate 1 is heated in a nitrogen atmosphere at 180°C for 30 minutes, at 300 °C for 30 minutes and at 400 °C for 30 minutes, and a plurality of separate aluminium layers 9 are formed to the thickness of 0.1 to 1.5 micrometers throughout the upper surface of the substrate 1 and etched by a third photomask 3m in such a way that the blocks on each row of the matrix are covered completely by one of aluminium layers which remain separately extending along rows of the matrix. The width of the aluminium electrode 9 is bistriding the cell blocks 20 and extending in the column direction beyond the perimeter of the blocks, and therefore light enterring the cell blocks 20 through the back side of the device is blocked certainly. Then, the manufacturing of an image sensor with a smooth top surface is completed. For example, the cell blocks can be aligned 64 cell per square milimeter. The cell block 20 thus formed is comprised of the first electrode 2 in contact with the line 19, the semiconductor layer 3, the second electrode 4 and the third electrode 5 in contact with the line 13. And the semiconductor layer 3 is completely enclosed with the electrodes at the top and bottom surfaces and the resin layer at the side surfaces so that no current leakage and no parasitic channel take place on the semiconductor layer. Further, the opposed electrodes 2 and 4 can have the same area and the same shape as the semiconductor layer 3 regardless of the precision of the patterning. As described in the above, a device according to the present invention is comprised of laminate layers of which the peripheries are surrounded respectively by a resin layer. The upper surface of the layer is exposed as if the resin layer is not provided. Although the surfaces on the layer and the resin layer are configured preferably flush with each other, the resin layer may be formed with its upper surface positioned lower than that of the member. The present invention is useful especially for an image sensor because a number of sensor elements can be finely disposed on a substrate without causing cross talkings between adjacent ones and because the sensor elements maintain high reliablity for a long time and have a high abrasion resistance against external impacts. It should be understood that the present invention is not limited to the specified embodiment but only to the appending claims, and a variety of modifications and changes of the embodiments may occur to those skilled in the art. Although the embodiments have been described as to contact image sensors, the present invention can be applied also to photoelectric conversion devices of other types which have only one or a few conversion element. A photoelectric conversion semiconductor layer can incorporate p-i, n-i, m-i(Schotokky contact), n-i-p or m-i-n junctions, besides p-i-n or n-i-n junctions. The semiconductor substance to be used for the invention may be SixGe 1-x ( 0<x≦1), SixC 1-x (0<x≦1) or SixSn 1-x (0<x≦1).

Claims: A method for manufacturing a photosensitive semiconductor device, said method comprising: a step of preparing a substrate (1); a step of forming a first electrode (2) on said substrate (1); a step of forming a photoelectric semiconductor layer (3) on said first electrode (2); a step of forming a second electrode (5) on said semiconductor layer (3); characterized by a step of forming cell blocks (10,20) by integrally etching said first and second electrodes (2,5) and said semiconductor layer (3) at once with the aid of a plurality of masks applied on said second electrode (5) wherein said masks are located in positions corresponding to said cell blocks (10,20) respectively, and a step of disposing a resin layer (6) completely enclosing said cell blocks (10,20), whereby said resin layer (6) is formed from a negative photoresist by photoetching with said cell blocks (10,20) as masks and a curing process is carried out after which the remaining resin layer (7) is nearly flush with the top surface of said cell blocks (10,20). The method of claim 1 characterized in that said semiconductor layer (3) contains silicon and said second electrode (5) comprises chromium and that a chromium silicide layer (4) is formed as a by-product between said semiconductor layer (3) and said chromium layer (5). The method of claim 1 or 2 characaterized by a step of forming a plurality of electrode strips (19) on said substrate (1) connecting said first electrodes (2) of a row of said cell blocks (10,20). A photosensitive semiconductor device obtainable by the method of claim 1, having a plurality of cell blocks (10,20) thereon, each cell block (10,20) comprising: a first electrode (2) on said substrate (1), a photosensitive semiconductor layer (3) on said first electrode (2); and a second electrode on said photosensitive semiconductor layer (3); characterized in that a resin layer (7) fills the spaces between said cell blocks, said resin layer (7) being flush with the top surface of said cell blocks (10,20) and said semicoductor layer (3) being completely enclosed by the first and second electrodes (2,5) and by the resin layer (7). The photosensitive semiconductor device of claim 4 characterized in that said semiconductor layer (3) contains silicon and said second electrode (5) comprises chromium and that a transparent chromium silicide layer (4) is formed between said semiconductor (3) and said second electrode (5). The photosensitive device of claim 5 characterized in that said photosensitive semiconductor layer (3) comprises amorphous silicon. The photosensitive device of claim 5 or 6 characterized in that said photosensitive semiconductor layer (3) incorporates a pi, ni, mi, pin, or nin junction. The photosensitive device of any of the claims 5 to 7 characterized in that said chromium silicide layer (4) is (1-20)x10⁻⁹m (10 to 200 Å) thick. The photosensitive device of any of the claims 5 to 8 characterized in that said substrate (1) is transparent. The photosensitive device of any of the claims 5 to 9 characterized in that said device is an image sensor.

Inventors: YAMAZAKI SHUNPEI, YAMAZAKI, SHUNPEI
Applicants: SEMICONDUCTOR ENERGY LAB, SEMICONDUCTOR ENERGY LABORATORY CO., LTD.
Application year: 1986
Main classification: H01L31
IPC codes: H01L27, H01L31