Static Random Access Memory Having Structure of First-, Second- And Third-Level Conductive Films

The present invention relates to a semiconductor device and, more particularly, to a semiconductor device such as a static random access memory which has a high-resistance element having a high resistance and which is suitable for realizing larger scale integration.

FIG. 1 is a circuit diagram of a static random-access memory cell which is composed of a flip-flop circuit consisting of driver MOS transistors Tr.sub.1 and Tr.sub.2 and load resistors R.sub.1 and R.sub.2, and transfer MOS transistors Tr.sub.3 and Tr.sub.4. The memory cell is selected through a word line 13 and data lines 12 to read out data. The load resistors R.sub.1 and R.sub.2 have a high resistance, i.e., 10.sup.10 to 12.sup.12 .OMEGA.. Such load resistors R.sub.1 and R.sub.2 are provided for the purpose of compensating for leakage current which may flow when the MOS transistors Tr.sub.1 and Tr.sub.2 constituting the flip-flop circuit are at "high" level, thereby supplying a current sufficient to enable a static operation to the drain of each of the MOS transistors Tr.sub.1 and Tr.sub.2, and also for the purpose of reducing the stand-by current in the memory. High-resistance elements having the above-described characteristics have heretofore been formed by employing a polycrystalline silicon film. However, to form high-resistance elements suitable for large scale integration, it is an effective practice to reduce the width of such high-resistance polycrystalline silicon film or reduce the thickness of the film. Reduction in the width of the film owes much to the microfabrication technology, and reduction in the film thickness leads to problems such as an increase in resistance of the interconnection area other than the high-resistance area and undesirable etching of the polycrystalline silicon film in the step of etching contact holes for connection with electrodes of aluminum or the like.

One type of method of reducing the thickness of a polycrystalline silicon film is disclosed in the specification of Japanese Patent Laid-Open No. 210658/1984. More specifically, as shown in FIG. 2 herein, a thick insulator 2 is provided on a P-type semiconductor substrate 1, and a thin polycrystalline silicon film 3 is provided all over the surface of the insulator 2 and formed into a desired pattern by a photolithographic technique. Further, a thick polycrystalline silicon film 4 is provided in a similar manner and covered with an insulator 5, and openings are formed to provide electrodes 6 of aluminum or the like. The present inventors produced a resistor having the structure shown in FIG. 2 and have found the following problems. Namely, although a high resistance is obtained by means of the thin polycrystalline silicon film 3 in the prior art structure shown in FIG. 2, when the thick polycrystalline silicon film 4 at the connection with the aluminum electrodes 6 is etched, the thin polycrystalline silicon film 3, which is an underlayer, is undesirably etched by overetching, which overetching needs to be carried out due to nonuniformity in thickness of the thick polycrystalline silicon film 4 and due to nonuniformity in etching rate within the silicon wafer. More specifically, if it is assumed that the thickness of the thin polycrystalline silicon film 3 is represented by t.sub.1, the thickness of the thick polycrystalline silicon film 4 by t.sub.2, the rate of nonuniformity in film thickness by .alpha., and the rate of nonuniformity in etching rate by .beta., the thin polycrystalline silicon film 3 is completely etched away when the following condition is met:

Even when the following condition is satisfied, i.e.,

if the thin polycrystalline silicon film 3 is etched, the nonuniformity in the film thickness further increases to make it difficult to control the film thickness, resulting, disadvantageously, in an increase in variation of resistance.

In view of the above-described circumstances, it is an object of the present invention to provide a resistance elements such as load resistors of a static random access memory, which has a reduced necessary area and is readily produced, and a method for forming such resistance element.

It is another object of the present invention to provide a resistance element which has a small device size and whose resistance is readily controlled.

It is still another object of the present invention to provide a static memory including a load element having a reduced occupied area and a high resistance.

Generally, the present invention provides a resistance element having a structure in which a first conductor film is formed over a substrate, with a second conductor film formed so as to extend over at least part of the first conductor film, so that part of the second conductor film is stacked on the first conductor film. The second conductor film (e.g., a second level film) has a relatively high resistivity as compared to the resistivity of the first conductor film (e.g., a first level film), and is relatively thin as compared to the thickness of the first conductor film. Such second conductor film constitutes a high-resistance area of the resistance element. More specifically, the present invention provides a resistance element having a structure in which an interconnection area is formed using a relatively thick first polycrystalline silicon film, and a high-resistance area is formed using a relatively thin second polycrystalline silicon film, the thin second polycrystalline silicon film being connected to the upper side of the thick first polycrystalline silicon film. Even more specifically, the present invention provides a static random access memory having MOS transistors, a power supply line and load resistors, wherein gate electrodes of the MOS transistors are formed of a first-level film of polycrystalline silicon, the power supply line is formed of a second-level film of polycrystalline silicon (relatively thick and heavily doped), and the load resistors are formed of a third-level film of polycrystalline silicon (relatively thin and undoped). By virtue of this arrangement, when the thick first polycrystalline silicon film is processed, it is possible to suppress the reduction in the film thickness of the thick first polycrystalline silicon film under the second polycrystalline silicon film, and even when the film thickness of the first polycrystalline silicon film is slightly reduced, there is no effect on the resistance, so that changes in the manufacturing process is allowed, and this facilitates process control. In addition, it is possible to obtain a semiconductor device which enables miniaturization and large scale integration.

These and other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings.

FIG. 1 is a circuit diagram showing a conventional static random-access memory;

FIG. 2 is a sectional view of a conventional high-resistance element;

FIG. 3 is a sectional view showing a first embodiment of the present invention;

FIG. 4 is a sectional view showing a second embodiment of the present invention;

FIGS. 5 and 6A-6F are sectional views, and FIG. 7 a plan view, which show in combination a third embodiment of the present invention; and

FIG. 8 is a sectional view showing a fourth embodiment of the present invention.

The present invention will be described hereinunder in detail by way of embodiments and with reference to the accompanying drawings.

Referring first to FIG. 3, which shows the cross section of a first embodiment of the high-resistance element according to the present invention, polycrystalline silicon films 3 and 31, which are formed from a first-level conductor film (resistance film) having a thickness of 100 to 1000 nm, is formed on a silicon dioxide (SiO.sub.2) layer which is formed on a silicon substrate 1 and which has a thickness of 50 to 500 nm. The first-level polycrystalline silicon films 3 and 31 have an impurity such as arsenic added thereto at a concentration of 10.sup.18 to 10.sup.21 cm.sup.-3 to provide a low-resistance layer. A thin polycrystalline silicon film 4 which is formed from a second-level conductor film (resistance film) and which has a thickness of 10 to 200 nm is provided in such a manner that two end portions of the silicon film 4 are in direct contact with the first-level polycrystalline silicon films 3 and 31 without any SiO.sub.2 interposed therebetween. The second-level polycrystalline silicon film 4 has no impurity added thereto. An inter-level insulator 5 such as SiO.sub.2 having a thickness of 100 to 1000 nm is formed on portions of the first-level polycrystalline silicon films 3 and 31 which are not in contact with the second-level polycrystalline silicon film 4.

In the above-described structure, the second-level polycrystalline silicon film 4 having no impurity added thereto defines a high-resistance area, and the first-level polycrystalline silicon films 3 and 31 define interconnection areas which are respectively led to metal electrodes made of aluminum or the like. When the area of contact between the first-level polycrystalline silicon films 3, 31 and the second-level polycrystalline silicon film 4 is subjected to annealing in a manufacturing process carried out thereafter (for example, an annealing carried out at temperatures of 900.degree. C. to 950.degree. C., for 10-60 minutes), an impurity such as arsenic is redistributed into the contact area (including in the contact area of the second-level polycrystalline silicon film) from the heavily-doped first-level polycrystalline silicon films 3 and 31 to provide an electrical contact. Accordingly, the length of the high-resistance area is equal to a value obtained by subtracting an impurity diffusion length from the distance between the first-level polycrystalline silicon films 3 and 31, and thus determined by using a self-alignment technique, which means that the structure in accordance with this embodiment is suitable for achieving larger scale integration. Since the film thickness of the second-level polycrystalline silicon film 4 is sufficiently thinner than that of the first-level polycrystalline silicon films 3 and 31 (for example, when the first-level film has a thickness of about 2000.ANG., the second-level film has a thickness of 500-700.ANG.), when the second-level polycrystalline silicon film 4 is processed by, for example, dry etching, even if over-etching is carried out due to nonuniformity in film thickness and etching rate the reduction in film thickness of the first-level polycrystalline silicon films 3 and 31 is very small as compared with the initial film thickness thereof. Accordingly, there is substantially no change in resistance as a result of the reduction in the film thickness. Further, since the first-level polycrystalline silicon films 3 and 31 have a low resistance, i.e., 10 to 100 .OMEGA./.quadrature., they can be employed as interconnections. In addition, when contact holes are provided in the insulator 5 in order to connect the first-level polycrystalline silicon film portions 3 and 31 to respective metal electrodes of aluminum or the like, since the silicon films 3 and 31 have a relatively large film thickness there is no fear of the first-level polycrystalline silicon films 3 and 31 disappearing as a result of the over-etching which is necessary due to nonuniformity in film thickness of insulator 5 and nonuniformity in etching rate in the insulator 5, which disappearance would otherwise result in no electrical connection between the second-level polycrystalline silicon film 4 and the metal electrodes 6 of aluminum or the like, the net result of the present invention also including an improvement in the production yield in a manufacturing process.

This embodiment relates to a static MOS memory cell which employs the high-resistance element according to the present invention.

Referring to FIG. 4, which shows a cross section of a static MOS memory cell in accordance with this embodiment, a p-type well 7 is formed in an n-type silicon substrate 1, and an n-channel MOS transistor is formed in the p-type well 7, the MOS transistor consisting of n-type diffusion layers 8 (forming the source and drain of the MOS transistor, e.g., an MOS field effect transistor); a gate oxide 9; and a gate electrode 11 made of polycrystalline silicon, a metal silicide or a metal.

Such gate electrode 11 is formed from a first-level conductive film of the static MOS memory cell. An MOS transistor which is adjacent to the described MOS transistor is electrically isolated from the latter by a relatively thick field oxide layer having a thickness of 200 to 2000 nm. A second-level polycrystalline silicon film 3 which has an impurity such as arsenic added thereto at a concentration of 10.sup.17 to 10.sup.21 cm.sup.-3 and which has a film thickness of 100 to 1000 nm is formed on the MOS transistor through an insulator layer 10 having a film thickness of 100 to 1000 nm. A third-level polycrystalline silicon film 4 which has no impurity added thereto and which has a film thickness of 20 to 200 nm is provided in such a manner that one end of the third-level polycrystalline silicon film 4 is in direct contact with the second-level polycrystalline silicon film 3 without any insulator or the like interposed therebetween. The other end of the third-level polycrystalline silicon film 4 is connected to the n-type diffused region 8 through a contact hole provided in the insulator layer 10 above the n-type diffused region 8. A small current required for a static operation is supplied to the diffused region 8 defining the drain of the MOS transistor from the second-level polycrystalline silicon film 3 connected to a power supply (e.g., 5 V) through the third-level high-resistance polycrystalline silicon film 4. In addition, it is possible to set the resistance to any desired value by varying the film thickness of the third-level polycrystalline silicon film 4 as desired. Therefore, it is possible to provide a static MOS memory cell including a small-sized high-resistance element and having a structure suitable for achieving larger scale integration by reducing the film thickness of the third-level polycrystalline silicon film 4.

In this embodiment, the second-level polycrystalline silicon film in Embodiment 2 is also employed as an electrode which is connected to an n-type diffusion layer. Referring to FIG. 5, which shows the cross section of a static MOS memory cell in accordance with this embodiment, a power supply voltage is applied to the second-level polycrystalline silicon film 3 through a metal electrode 6 made of aluminum or the like. For example, the metal electrode can be directly connected to the second-level polycrystalline silicon film 3. The third-level polycrystalline silicon film 4 is not directly connected to the diffusion layer 8 of the MOS transistor but is connected thereto through a second-level polycrystalline silicon film 31. Similarly, an aluminum electrode 61 is connected to the diffusion layer 8 through a second-level polycrystalline silicon film 311. It should be noted that the second-level polycrystalline silicon films 3, 31 and 311 have an impurity such as arsenic added thereto at a concentration of 10.sup.18 to 10.sup.21 cm.sup.-3 so as to be lowered in terms of resistance, whereas the third-level polycrystalline silicon film 4 has no impurity added thereto so as to define a high-resistance area.

According to this embodiment, the length of the high-resistance area is determined by the distance between the second-level polycrystalline silicon films 3 and 31, and it is possible to produce a static MOS memory including a load resistance element having a high resistance by reducing the film thickness of the third-level polycrystalline silicon film 4 in a manner similar to that of the above-described Embodiments 1 and 2. Further, since the metal electrode 6 made of aluminum or the like is not directly connected to the diffused region 8 of the MOS transistor but connected thereto through the second-level polycrystalline silicon film 311, the silicon film 311 can be formed in such a manner as to be stacked on the upper side of the gate electrode. It is therefore unnecessary to provide any margin for alignment between the gate electrode and the contact hole, which means that the structure in accordance with this embodiment is suitable for achieving larger scale integration. In addition, since all the connections with the aluminum electrodes 6 and 61 are obtained on the second-level polycrystalline silicon films 3, 31 and 311, the film thickness of the insulator 5 provided with contact holes can be made uniform, so that it is possible to improve the production yield.

The above-described semiconductor device is produced as follows.

FIGS. 6A to 6D respectively show steps of the process for producing the static MOS memory according to the present invention. First, a p-type well 7 having an impurity concentration of 10.sup.15 to 10.sup.17 cm.sup.-3 is formed in, for example, an n-type silicon substrate 1 by ion implantation and thermal diffusion, as conventionally done, and a thick field oxide 2 having a thickness of 200 to 100 nm is partially formed on the p-type well 7 by conventional LOCOS technique (see FIG. 6A). Next, a gate oxide film 9 having a thickness of 10 to 100 nm is formed, and impurity ions such as boron ions are implanted in order to control the threshold voltage of an MOS transistor. Then, a gate electrode 11 is formed using, for example, polycrystalline silicon (that is, a first-level polycrystalline silicon film) and impurity ions such as arsenic ions are implanted with said gate material employed as a mask, and activation is effected to form n-type diffusion layer 8 having an impurity concentration of 10.sup.17 to 10.sup.21 cm.sup.-3 (see FIG. 6B). Then, an insulator film 10 such as SiO.sub.2 having a thickness of 100 to 1000 nm is deposited by CVD (chemical vapor deposition), and contact holes are formed by photolithography and dry etching at positions where the n-diffusion layer 8 is to be connected to a second-level polycrystalline silicon film (see FIG. 6C). Then, a second-level polycrystalline silicon film 3 having a thickness of 100 to 1000 nm is deposited by CVD, and after an impurity such as arsenic has been added to the second-level polycrystalline silicon film 3 at a concentration of 10.sup.18 to 10.sup.21 cm.sup.-3, patterning is effected by photolithography and dry etching (see FIG. 6D). Next, a third-level polycrystalline silicon film 4 having a thickness of 10 to 200 nm is deposited by CVD, and then similarly subjected to patterning (see FIG. 6E). Finally, an SiO.sub.2 film 5 having a thickness of 100 to 1000 nm is deposited by CVD and provided with contact holes which reach the second-level polycrystalline silicon film 3. Aluminum having a thickness of 500 to 2000 nm is deposited and subjected to patterning to form electrodes 6 (see FIG. 6F). It should be noted that it is preferable for stabilizing the resistance of the high-resistance element to employ an SiO.sub.2 film 5 which contains no phosphorus or a double layer film which consists of an SiO.sub.2 film containing no phosphorus and an SiO.sub.2 film containing phosphorus (a phosphosilicate glass film).

The planar layout of the static MOS memory cell in accordance with this embodiment will be explained with reference to FIG. 7. As illustrated, a second-level polycrystalline silicon film 3 is disposed within the memory as a feeder line for supplying a power supply voltage (e.g., 5 V) and connected to an aluminum electrode in the periphery of the memory. Other second-level polycrystalline silicon films 31 and 311 are formed by patterning so as to be disposed above contact holes 15 which are provided in such a manner as to reach the diffusion layers of MOS transistors. Further, contact holes 16 for providing connection between the second-level polycrystalline silicon films 311 and aluminum electrodes (data lines) 6 are formed above the silicon films 311. High-resistance elements are formed from third-level polycrystalline silicon films 4 and therefore lead to no increase in the memory cell area.

It should be noted that, although this embodiment has been described by way of n-channel MOS transistors, p-channel MOS transistors may also be employed to obtain similar advantages.

This embodiment relates to a method of connecting together first- and second-level polycrystalline silicon films in the high-resistance element in accordance with Embodiment 1.

Referring to FIG. 8, which shows the cross section of the high-resistance element in accordance with this embodiment, side wall spacers 14 defined by an insulator such as SiO.sub.2 are formed at the side walls of the first-level polycrystalline silicon film 3, by conventional techniques for forming side wall spacers (such as forming an SiO.sub.2 layer all over the substrate (e.g., by CVD) and etching the SiO.sub.2 layer, by reactive ion etching, so that SiO.sub.2 remains only at the side walls of the polycrystalline silicon film 3). As for the process for forming the side wall spacers 14, see Tsang, et al., "Fabrication of High Performance LDDFET's with Oxide Sidewall-Spacer Technology", in IEEE Transactions on Electron Devices, Vol. ED-29, No. 4 (April 1982), pages 590-596, the contents of which are incorporated herein by reference. Such side wall spacers can have an aspect ratio (of side wall height (thickness) to width) of from 1 through 4, for example. Of course, the aspect ratio depends on the thickness of insulator 14; that is, the thicker the insulator layer provided, the wider the side wall (and the smaller the aspect ratio). The side wall spacers 14 are provided so that the second-level polycrystalline silicon film 4 is in contact with only the upper surface of the first-level polycrystalline silicon film 3.

This embodiment enables a reduction in the distance by which an impurity in the first-level polycrystalline silicon film 3 is redistributed into the second-level polycrystalline silicon film 4 during an annealing step. In addition, it is also possible to gentle the slope of the side walls of the first-level polycrystalline silicon film 3.

It should be noted that, although in Embodiments 1 to 4 a polycrystalline silicon has been exemplarily shown as the material for the resistance film, it is not necessarily limitative, and any insulator material (or other material) which is conductive to only a required extent, such as Ta.sub.2 O.sub.5 or ion-implanted SiO.sub.2, may be employed.

As will be understood from the above description, it is possible, according to the present invention, to obtain a high-resistance element having a reduced occupied area. It is also possible to obtain a high-resistance element having excellent resistance controllability. Further, it is possible to obtain a high-resistance element while facilitating the manufacturing process control. In addition, it is possible to produce a static MOS memory cell which includes a high-resistance element with a reduced necessary area and which is suitable for achieving larger scale integration, with ease and with an excellent production yield.

It is further understood by those skilled in the art that the foregoing description describes preferred embodiments of the disclosed device and that various changes and modifications may be made in the invention without departing from the spirit and scope thereof.