Semiconductor device

Abstract: Disclosed is a semiconductor device used as an electrically programmable read-only memory. The resistance of the device is irreversibly varied from a high value to a low value by applying an electric field. The device comprises a P-type silicon semiconductor substrate (101), an N-type diffused layer (103), an interlayer insulating film (104) formed on the diffused layer, an amorphous silicon layer (105) con­ taining a P-type impurity, and a metal film (106) forming conductive interconnections. The diffused layer (103) acts as lower electrodes. The metal film (106) serves as upper electrodes. The amorphous silicon layer (105) and the metal film (106) are stacked on the diffused layer.

Description: The present invention relates to a semiconductor device whose resistance value is varied from a high resistance to a low resistance irreversible by applying an electric field and, more particularly, to a semiconductor device usable for electrically programmable read-only memories. A so-called anti-fuse device having amorphous silicon layers inserted between successive electrodes has been used as a semiconductor device whose resistance value is varied from a high value to a low value irreversibly by applying an electric field. Especially, where anti-fuse devices are used in an electrically programmable read-only memory, it has been common practice to add a diode to each individual storage element, for making the ROM circuit configuration simpler. Such a diode is created by forming a Schottky junction on a lightly doped N-type semiconductor layer, the junction layer including platinum. A known semiconductor device is shown in Fig. 2, where a diffused layer formed in a silicon substrate is used as one electrode of a storage element. The silicon substrate, indicated by 201, is of the P type. Created on the substrate 201 are an oxidized film 202 forming a field-insulating film, an N⁺ -type diffused layer 203, an interlayer insulting film 204, an N⁻ -type diffused layer 205, a platinum silicide layer 206, an amorphous silicon film 207, and a film 208 for forming metal interconnections. The insulating film 204 is made from silicon oxide, for example. In this device, the N⁻ -type diffused layer 205 is in contact with the N⁺-type diffused layer 203. The platinum silicide layer 206, the amorphous silicon film 207, and the film 208 for forming metal interconnections are laminated on the diffused layer 205 which serves as lower electrodes. The film 208 acts as upper electrodes. In the operation, an electric field is applied between the upper and lower electrodes to cause an avalanche breakdown in the amorphous silicon film. At this time, Joule heat is evolved to change parts of the amorphous silicon film, thus forming low-resistance regions. The rectifying function of the Schottky diode between the platinum silicide layer and the N⁻ -type diffused layer is combined with the low-resistance regions of the amorphous silicon film to provide the known, electrically programmalbe read-only memory. Another known semiconductor device is disclosed in U.S. Pat. No. 4,442,507. This device has a polycrystalline silicon layer which is formed on a silicon substrate and acts as one electrode of each storage element. In particular, a polycrystalline silicon layer is heavily doped so as to exhibit N type. Another polycrystalline silicon layer formed on the heavily doped silicon layer is lightly doped and of the N type. A platinum silicide layer, an amorphous silicon film, and a film forming metal interconnections are deposited on the lightly doped polycrystalline silicon layer. The film forming the metal interconnections forms upper electrodes. Schottky diodes that are created between the platinum silicide layer and the lightly doped N-type polycrystalline silicon layer are employed. In this known device, the lightly doped N-type diffused layer is formed to create the Schottky junctions. This presents the following problems. (1) The resistance obtained after programming the device increases by an amount equal to the resistance of the lightly doped N-type diffused layer (examples 1 and 2). (2) Where programmable elements are formed in series on the same lower electrode, the resistance of the lower electrode further increases by an amount equal to the resistance of the lightly doped N-type diffused layer (example 1). (3) The resistance of the thin N-type diffused layer is easily modulated with voltage and so the resistance values of the programmed device easily vary during operation (example 1). (4) When the thin N-type diffused layer is formed to create the Schottky junctions, it is necessary to take account of allowance for alignment. This increases the area of patterning (example 1). The document International Electron Devices Meeting, December 6 - 9, 1987, Washington, DC, IEEE, pages 194-197, discloses a semiconductor device including an ion-implanted programmable element (IPEL). This known semiconductor device comprises in an n-type single crystal silicon wafer a p or n type diffusion zone a surface portion of which being amorphized by ion implantation. A metal film is provided over the wafer in contact with the ion-implanted amorphous film. This amorphous film changes to polycrystalline material when an applied voltage across the film reaches a critical field strength. Thus, the ion-implanted amorphous film is capable of undergoing a transition, upon exposure to electrical current, from a high resistance state to a low resistante state. It is an object of the invention to provice a semiconductor device which shows low and stable resistance values after having been programmed and has a small patterned region, and in which, when an electric field is applied, the resistance irreversibly varies from a high resistance to a low resistance and then the device exhibits rectification. The above object is achieved in accordance with the teaching of the invention by a semiconductor device as claimed. In this configuration, the first semiconductor layer is of the N type, for example. The second semiconductor layer formed on the N-type semiconductor layer is an amorphous semiconductor film, a polycrystalline semiconductor film, or a semiconductor film whose crystalline structure has been destroyed by ion implantation or an other technique, and each film is lightly doped with a P-type impurity. Before the device is programmed, a P-type impurity exists between lattices in the noncrystalline or polycrystalline semiconductor film. In this state, the film is electrically inactive and, therefore, it shows a high resistance irrespective of the direction of the applied electric field. Once a high electric field is applied to the film deposited on the N-type semiconductor layer as lower electrodes, an avalanche breakdown take place, producing excessive electric current. This gives rise to Joule heat, which changes partially the crystalline structure of the programmed film that is lightly doped and contains a P-type impurity. Also, the P-type impurity is activated. As a result, the deposited film which showed a high resistance acts as a diode that exhibits a low resistance in the forward direction. Ways of carrying out the invention are described below with reference to drawings which illustrate only specific embodiments and in which: Fig. 1 is a cross-sectional view of main portions of a semiconductor device according to the invention; Fig. 2 is a cross-sectional view of main portions of a known semiconductor device; Fig. 3(a) is a graph showing the electrical characteristic of a semiconductor device according to the invention, which has not yet been programmed; Fig. 3(b) is a graph similar to Fig. 3(a), but in which the device has been programmed; Fig. 4 is a cross-sectional view of main portions of another semiconductor device according to the invention; Fig. 5 is a cross-sectional view of main portions of a further semiconductor device according to the invention; and Fig. 6 is a cross-sectional view of main portions of still another semiconductor device according to the invention. Referring to Fig. 1, there is shown a semiconductor device embodying the concept of the invention. This device comprises a P-type semiconductor substrate 101 consisting of silicon or another material, an oxidized film 102 locally formed on the substrate and constituting a field-insulating film and consisting of silicon oxide, for example, an N⁺-type diffused layer 103, an interlayer insulating film 104 consisting of silicon oxide, for example, an amorphous silicon film 105 containing a P-type impurity, and a film 106 forming metal interconnections. The film 106 consists of one or more layers formed from Al, or a metal having a high melting point such as Mo, Ti, or W, or a silicide of such a high-melting point metal. The amorphous silicon film 105 having the P-type impurity and the upper electrode consisting of the film 106 are formed over the N⁺-type diffused layer 103 that forms the lower electrodes. By programming the programmable elements having a high resistance, they show rectification as described previously. This semiconductor device is fabricated in the manner described below. First, the oxidized film 102 is locally formed on the P-type semiconductor substrate 101. Then, ions of phosphorus of 4 x 10¹⁵ cm⁻² are injected into the silicon substrate 101 at 60 keV by an ion implantation technique to form the N⁺-type diffused layer 103 which becomes the lower electrodes of the programmable elements. Subsequently, a film of silicon oxide is deposited by a CVD process to form the interlayer insulating film 104. Openings necessary for direct connection with the N⁺-type diffused layer 103 are made. Also, openings required to make connection via the amorphous silicon film 105 containing the P-type impurity and constituting the main portions of the programmable elements are formed. Thereafter, SiH₄ is decomposed by a CVD process at a low temperature of 560°C to deposit an amorphous silicon film up to a thickness of 0.15µm. Boron fluoride of 1 x 10¹¹ to 1 x 10¹⁵ cm⁻² is introduced into the amorphous silicon film at 60 keV by ion implantation to obtain desired diode characteristics. Thus, the amorphous silicon film 105 containing the P-type impurity is formed. This amorphous film 105 is photolithographically patterned. Then, a barrier metal and aluminum are sputtered to form a laminate, which is then photolithographically patterned to create the film 106 for forming conductive interconnections. The resistance of the thus formed device is irreversibly changed from a high value to a low value by the application of an electric field. The device shows rectification. The oxidized film 102 is formed by localized oxidation of silicon (LOCOS). It is also possible to form grooves in a semiconductor substrate and to insert an isulator in the grooves. The aforementioned barrier metal can be a metal having a high melting point such as Mo, Ti, or W, or a silicide or nitride of such a metal. Usually, the barrier metal is located under aluminum. The electrical characteristics of the semiconductor device are described next. As an example, ions of boron fluoride of 1 x 10¹⁵ cm⁻² are injected at 60 keV into the amorphous silicon film containing the P-type impurity. Openings required to make connection via the amorphous silicon film have a diameter of 1.2 µm. Fig. 3(a) shows the electrical characteristic of the device before it is programmed. Fig. 3(b) shows the electrical characteristic after the device has been programmed by applying an electric field to induce an avalanche breakdown and producing an electric current of 2 to 3 mA. Before the device is programmed, the electrical characteristic curve is substantially symmetrical with respect to the direction in which the voltage is applied. Electric current of only 10 nA flowed at 5 V, i.e. the resistance is high. Once a voltage is applied to program the device, if a positive voltage is applied to the upper electrode that is an aluminum film overlying the amorphous silicon film, a forward characteristic is exhibited. If a negative voltage is applied, a reverse characteristic is obtained. In the case of an undoped amorphous silicon film, after the device has been programmed, the characteristic curve is substantially symmetrical with respect to the direction in which the voltage is applied, and the resistance is low. These changes in the electrical characteristic are considered as being brought about by the mechanism described above. That is, the structure of the amorphous silicon film is partially changed by Joule heat produced by electric current. The P-type impurity is activated. PN junction diodes are created between the amorphous silicon film and the N⁺-type diffused layer that acts as the lower electrode. Referring next to Fig. 4, there is shown another semiconductor device according to the invention. This device is similar to the device shown in Fig. 1 except that the N⁺-type diffused layer 103 acting as the lower electrode and shown in Fig. 1 is replaced by an N⁺-type polycrystalline silicon film 403 on an oxide film 402. The device shown in Fig. 4 comprises a P-type semiconductor substrate 401, the locally formed oxidized film 402, the N⁺-type polycrystalline silicon film 403, an interlayer insulating film 404, an amorphous silicon film 405 containing a P-type impurity, and a film 406 forming metal interconnections. The film 406 consists of two layers, i.e., a barrier metal layer and an aluminum layer formed on it, in the same way as in the device already described in connection with Fig. 1. The use of the N⁺-type polycrystalline silicon film on the oxide film increases the degree of freedom to which the pattern is laid out. Further, the N⁺-type polycrystalline silicon acting as the lower electrode is surrounded by the insulating film consisting of the silicon oxide film. Therefore, the Joule heat generated on programming does not readily escape. Consequently, the device can be programmed with lower electric current. Referring to Fig. 5, there is shown a further semiconductor device according to the invention. This device comprises a P-type semiconductor substrate 501, an insulating film 502 consisting of a locally formed oxidized film, a low-resistance film 503, an interlayer insulating film 504, an N⁺-type polycrystalline silicon film 505, an amorphous silicon film 506 containing a P-type impurity, and a film 507 forming metal interconnections. The low-resistance film 503 which acts as a lower electrode is made from a metal of a low resistance, such as Mo, Ti, or W, or a silicide of such a metal, such as MoSi₂. Holes are formed in the interlayer insulating film 504. This device is fabricated in the manner described now. The substrate 501 is made from silicon. Silicon oxide is deposited on the substrate 501 to form the insulating film 502. Then, MoSi₂ is sputtered on the insulating film 502 up to a thickness of 0.2 µm and photolithographically patterned to form the metal film 503 that serves as the lower electrodes of programmable elements. A silicon oxide film is deposited by a CVD process to form the interlayer insulating film 504. Holes extending through the insulating film 504 are formed at locations at which the N-type polycrystalline silicon film 505 and the amorphous silicon film 506 containing the P-type impurity are to be formed. That is, the holes are formed in the insulating film 504 at locations at which direct connections with the metal film 503 forming the lower electrodes are made. Subsequently, N-type polycrystalline silicon is deposited up to a thickness of 0.2 µm. The polycrystalline silicon film 505 is etched into a desired pattern. Ions of phosphorus of 4 x 10¹⁵ cm⁻² are injected into the polycrystalline silicon at 60 keV by ion implantation, and then the laminate is annealed in an electric furnace to activate the impurities, thus forming the N-type polycrystalline silicon film 505. Then, SiH₄ is decomposed at a low temperature of 560°C by a CVD process to deposit amorphous silicon as a layer up to a thickness of 0.15 µm. Ions of boron fluoride of 1 x 10¹¹ to 1 x 10¹⁵ cm⁻² are implanted at 80 keV to obtain desired diode characteristics. Thus, the amorphous silicon film 506 containing a P-type impurity is formed. Then, the doped amorphous silicon film 506 is etched into a desired pattern covering the N-type polycrystalline silicon film 505. Subsequently, a barrier metal and aluminum are successively sputtered to form the film 507 for forming the metal interconnections. Finally, this film 507 is patterned by photolithographic techniques. As a result, a read-only memory is created which can be programmed by dielectric breakdown. Referring next to Fig. 6, there is shown still another semiconductor device according to the invention. This device comprises a P-type semiconductor substrate 601, an insulating film 602 that is a locally formed oxidized film, a low-resistance film 603 made from a low-resistance metal or metal silicide, such as MoSi₂, an N⁺-type polycrystalline silicon film 604 formed on the low-resistance film 603, an interlayer insulating film 605, an amorphous silicon film 606 containing a P-type impurity, and a film 607 forming metal interconnections. The low-resistance film 603 acts as a lower electrode. This semiconductor device is manufactured in the manner described below. First, the insulating film 602 consisting of a film of silicon oxide is formed on the semiconductor substrate 601 made from silicon. Then, MoSi₂ is sputtered on the laminate up to a thickness of 0.2 µm to form the metal film 603 which creates the lower electrodes of programmable elements. Then, polycrystalline silicon is deposited on the laminate to a thickness of 0.2 µm. Subsequently, ions of phosphorus of 4 x 10¹⁵ cm⁻² are implanted at 60 keV, followed by annealing of the laminate inside an electric furnace to form the N-type polycrystalline silicon film 604. The metal film 603 and the N-type polycrystalline silicon film 604 are etched into a desired pattern by a photoetching technique. Then, silicon oxide is deposited as a film by chemical vapor deposition to form the interlayer insulating film 605. Holes extending through the insulating film are formed at locations at which the amorphous silicon film 606 containing a P-type impurity are to be formed, namely at locations at which direct connection with the N-type polysilicon film 604 forming a part of the lower electrode is to be made. Then, SiH₄ is decomposed at a low temperature of 560°C by a CVD process to form the amorphous silicon layer of 0.15 µm thick. Ions of boron fluoride of 1 x 10¹¹ to 1 x 10¹⁵ cm⁻² are implanted at 80 keV to derive desired diode characteristics. Thus, the amorphous silicon film 606 containing a P-type impurity is formed and etched into a desired pattern. A barrier metal and aluminum are successively sputtered as films to form the film 607 creating metal interconnections. Finally, the laminate is etched into a desired pattern. In this way, a read-only memory which can be programmed by dielectric breakdown is made. In the examples of Figs. 5 and 6, chemical vapor deposition (CVD) and ion implantation are employed to fabricate the P-type semiconductor film. Instead, sputtering can be used. During chemical vapor deposition or sputtering, a dopant may be added. The P-type semiconductor film can be P-type polycrystalline silicon. Also, it can be polycrystalline silicon or crystalline silicon whose crystalline structure has been destroyed by ion implantation or another technique. The P-type semiconductor film can be replaced with an N-type semiconductor film where the lower electrode is a P⁺-type diffused layer. In the above examples, the metal film forming the lower electrode is made from MoSi₂. It is also possible to fabricate the metal film from TiSi₂ or WSi₂. Further, the metal film can be polycide. Furthermore, the semiconductor substrate can be either P-type silicon or N-type silicon. In the examples shown in Figs. 5 and 6, the lower electrodes are made from a metal such as Mo, Ti, or W, or silicide of such a metal. This lowers the resistance of the lower electrodes. Therefore, where programmed regions are arranged in series, they can be programmed with lower electric current. In the aforementioned four examples, the semiconductor films having a P-type impurity between lattices are formed by chemical vapor deposition and ion implantation. These films can also be created by sputtering. The semiconductor films may be made from polycrystalline silicon into which ions of a P-type impurity are injected. Also, they may be polycrystalline silicon or crystalline silicon whose crystalline structure has been destroyed by ion implantation. In the above examples, the P-type semiconductor film has an impurity between lattices. Where the lower electrode is a P⁺ -type diffused layer, the film may be a semiconductor film containing an N-type impurity between lattices. In the examples shown in Figs. 5 and 6, the metal films forming the lower electrodes are made from MoSi₂. These films may also made from TiSi₂, WSi₂, W, or Mo. In the above-described four examples, the film 106, 406, 507, and 607 which create metal interconnections to form upper electrodes may be one or more layers of aluminum, or a high-melting point metal such as W, Ti, Mo, or silicide thereof. As described thus far, the present invention is not limited to the aforementioned four examples, but various changes and modifications may be made. As described, the resistance of the first semiconductor film that is located just under a programmed film and has the first conductivity type is low. Also, it is not modulated with voltage. Since the structure is simple, the area of the patterned regions is small. In addition, the device is fabricated by simple processes. By forming the lower electrode over the substrate, the generated Joule heat is effectively utilized and so the device can be programmed with lower electric current. The resistance of the lower electrode is lowered by forming the lower electrode from a high-melting point metal such as Mo, Ti, or W, or silicide of such a metal, such as MoSi₂, TiSi₂, or WSi₂. Hence, a read-only memory capable of operating at a high speed can be obtained.

Claims: A semiconductor device comprising: a first semiconductor layer (103; 403; 505; 604) having a first conductivity type; a second semiconductor layer (105; 405; 506; 606) which is formed on the first semiconductor layer and made of an amorphous semiconductor, a polycrystalline semiconductor, or a semiconductor having a destroyed crystal structure, said second semiconductor layer containing impurities having a second conductivity type different from the first conductivity type; and an upper electrode layer (106; 406; 507; 607) overlying and conductively connected to the second layer; wherein said second semiconductor layer is capable of undergoing an irreversible transition, upon exposure to electrical current from a voltage applied between said first semiconductor layer and upper electrode, from a high resistance state to a low resistance state, and wherein a diode is formed between said first and second semiconductor layers once the second semiconductor layer has assumed said low resistance state. The semiconductor device of claim 1, wherein said first semiconductur layer (103) is a diffused layer, and wherein said second semiconductor layer (105) is made from polycrystalline silicon, amorphous silicon, or silicon having a destroyed crystal structure. The semiconductor device of claim 1, wherein said first semiconductor layer (403) is a doped layer of polycrystalline silicon, and wherein said second semiconductor layer (405) is made from polycrystalline silicon, amorphous silicon, or silicon having a destroyed crystal structure. The semiconductor device of claim 1, wherein said first semiconductor layer (505) is made from doped polycrystalline silicon and connected with an underlying layer (503) of a metal or metal compound via holes through an interlayer insulating film (504), and wherein said second semiconductor layer (506) is made from polycrystalline silicon, amorphous silicon, or silicon having a destroyed crystal stucture. The semiconductor device of claim 1, wherein said first semiconductor layer (604) is a doped polycrystalline silicon layer formed on a layer (603) of a metal or metal compound and wherein said second semiconductor layer (606) is connected with the first semiconductor layer via holes through an interlayer insulating film (605), and made from polycrystalline silicon, amorphous silicon, or silicon having a destroyed crystal structure. The semiconductor device of claim 1, 2, 3, 4, or 5, wherein said upper electrode layer (106; 406; 507; 607) is formed of a metal or metal compound. The semiconductor device of claim 6, wherein said metal or metal compound consists of Al, Mo, Ti, or W, or silicide of such a metal. The semiconductor device of claim 4 or 5, wherein said layer of metal or metal compound consists of one or more layers formed from Mo, Ti, or W, or silicide of such a metal.

Inventors: HIRAKAWA KAZUKI C O SEIKO EPSO, HIRAKAWA, KAZUKI C/O SEIKO EPSON CORPORATION
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1989
Main classification: H01L29
IPC codes: H01L29