SEMICONDUCTOR DEVICE WITH SHORT-LENGTH ELECTRODE AND FABRICATION PROCESS THEREFOR

Abstract: A semiconductor device (Field Effect Transistor) is fabricated by coating a wafer with oxygen impervious silicon nitride and thereafter oxidising an exposed surface. By vertically etching the oxide layer by means of reactive ion etching, an oxide abutment or bird-beak (24) is formed with a perpendicular wall (40) and sloping roof (42). By depositing an electrically conductive conformal layer (46) of thickness equal to a desired electrode length and then vertically etching this conductive layer, an electrode (16) of short length is left on the perpendicular wall (40) of the abutment or bird-beak (24). Source and drain regions (12 and 14) are formed by ion implantation adjacent the abutment or bird-beak (24) joined by a charge conduction channel (50) which is partially constricted due to the doping profile produced by the varying thickness of the oxide layer to establish a predetermined resistance and protection from avalanche breakdown.

Description: This invention relates to semiconductor devices, such as field-effect transistors (FETs) wherein one electrode, such as a gate electrode, is to be of short length. More particularly, the invention relates to a process of fabrication of such electrode to provide an asymmetric lightly-doped drain (LDD) structure suitable for large scale integration avoiding most of the optical diffraction limitations associated with photo-lithographic masking. Semiconductor devices such as FETs can operate over a wide range of frequencies. Often, it is desirable to increase the upper frequency capability of the devices by reducing the physical dimensions of key components of the device. In the case of an FET, the length of the gate is reduced to accommodate the higher frequencies. With conventional photolithographic techniques of construction, there are diffraction limitations on the minimum size of gates which can be produced. Many logic and memory circuits employ large numbers of FETs, and are constructed in the form of an integrated circuit (IC) chip, such large arrays of FETs being referred to as large scale or very large scale integration (VLSI) depending on the number of circuit elements on the chip. Since the gate in each of the many FETs on the chip is subject to the foregoing diffraction limitation, there is an upper limit to the frequency response characteristics of FET circuit construction on such chips. The same limitation applies to the construction of other electrodes in such semiconductor devices. Heretofore, no suitable manufacturing step has been employed for extremely narrow lines below say, 0.5 microns in width. According to one aspect of the invention, a method of fabricating a semiconductor device comprises covering a portion of a surface of a substrate of semiconductor material with an oxygen-impervious layer leaving an exposed area where an electrode is to be formed, oxidising the exposed area to form an oxide of the semiconductor material with the edge of the layer raised due to the formation of oxide thereunder, is characterised in the further steps of directionally etching away exposed portions of the oxide to leave an abutment of oxide having a wall substantially perpendicular to the surface of the substrate, removing the layer from said substrate and abutment, re-oxidising the substrate to form a re-oxidised layer, depositing a conformal layer of electrically conductive material over said re-oxidised layer to a thickness equal to the desired length of said electrode, and directionally etching said conductive layer to leave the electrode on the wall of the abutment. According to a second aspect of the invention, a semiconductor device comprises a semiconductor substrate, an oxide insulating layer formed on the surface thereof and having an electrode formed on said oxide layer, and is characterised in that the oxide layer has an abutment formed therein with a wall extending substantially perpendicular to the surface and a sloping roof connected to the upper edge of the wall, and in that the electrode is formed on the vertical wall of the abutment. In the preferred embodiment, the process begins by covering a portion of the wafer, up to the location where the gate is to be formed, with an oxygen impervious coating of silicon nitride, Si₃N₄. Then the wafer is exposed to oxygen which diffuses into the exposed silicon as well as penetrating a relatively small distance under the edge of the nitride layer. The silicon and oxygen interact to form silicon dioxide, SiO₂, which has a larger volume than the reacting silicon. The resulting expansion raises the height of the exposed surface of the wafer and also bends the end of the nitride layer upwardly from the original wafer surface to form a shape known as a bird beak. Heretofore, it has been the general principle to avoid the formation of a bird beak to preserve a smooth surface to the semiconductor structure. With the invention, however, the bird beak is created deliberately since it enables the formation of a supporting silicon dioxide abutment which maintains the gate structure in a position normal to the surface of the semiconductor wafer from which the IC is constructed. In accordance with a feature of the invention, advantage is taken of the electrical insulating properties of the silicon dioxide beneath the nitride layer by forming the silicon dioxide into an insulating abutment. This is accomplished by a reactive ionic etch of the exposed silicon dioxide followed by a stripping of the nitride layer which leaves an abutment with a straight vertical wall at the site where the gate is to be formed. There follows a conformal deposition of a doped polysilicon layer upon the wafer and the abutment to a depth equal to the desired length of the gate. A vertically oriented reactive ion etch to the depth of the polysilicon layer removes all of the layer except for the gate alongside the vertical wall of the abutment. The source and drain are then developed on opposite sides of the gate. A further feature of the invention is found in the construction of the source and the drain by ion implantation. The depths of the source and the drain regions depend on the stopping distance of the dopant atoms, the depth following the profile of the abutment in the active region of the drain. There results an improved impurity profile in the active region of the drain which offers protection from avalanche with reduced resistance to current flowing between the source and the drain, i.e. an asymmetric lightly-doped drain structure. The invention will now be particularly described with reference to the accompanying drawings, in which Figure 1 shows steps A to H in the formation of an FET in accordance with the present invention. With reference to the drawing, there are depicted eight steps labelled A to H illustrating a process incorporating an insulating abutment for the formation of a gate electrode in a field-effect transistor. Because of the avoidance of diffraction effects, the gate has a length less than that which can be obtained with conventional lithographic technology to provide a higher frequency response than is generally attainable. In addition, the distribution of dopant impurities in the drain electrode of the FET is more precisely controlled than with conventional lithographic technology to permit a reduced total value of drain resistance while providing increased protection from avalanche and consequential burn-out of the FET. While this description is concerned with the construction of a gate electrode of an FET, it will be understood that the same procedure and abutment can be employed for forming other components of a semiconductor device. As shown in Figure 1H, a transistor 10 comprising a source 12, a drain 14 and a gate 16 is substantially complete except for the attachment of leads. The leads are shown diagrammatically as a source lead 18, a drain lead 20 and a gate lead 22, and are attached in subsequent conventional manufacturing steps (not shown). The transistor includes an abutment 24 of insulating material such as silicon dioxide. The abutment 24 is upstanding from substrate 26 of the transistor 10 at the location of the gate 16 and serves as a guide in the formation of gate 16. The fabrication process begins with step A wherein a wafer 28 of semiconductor material is provided to serve as the substrate 26, the wafer 28 being coated with a layer 30 of silicon nitride. By way of example in the explanation of the invention, it is presumed that the wafer 30 is composed of p-conductivity type silicon, it being understood that other materials may be utilised. The lay er 30 may be deposited by any suitable process such as chemical vapour deposition (CVD) or sputter deposition. The layer 30 is impervious to oxygen and protects the upper surface of the substrate 26 where covered by the layer 30. The layer 30 may have a thickness in the range of 50 to 100nm (500 to 1000 angstroms). An edge 32 of the layer 30 defines the location wherein the abutment 24 and the gate 16 are to be constructed. In step B, the wafer is exposed to oxygen at a temperature in the range of 900 to 1100 C. Oxygen reacts with the silicon of the wafer 28 to build up a layer 34 of silicon dioxide on the exposed surface of the wafer 28. The area of the wafer 28 which is protected by the layer 30 is not oxidised except for a region near the edge 32 wherein there is a penetration of the oxygen to build up oxide which lifts the edge 32 of the layer 30 to form a structure known as a bird beak 36. It will be noted that approximately 45 percent of the silicon dioxide is above the original surface 38 and 55 percent of the silicon dioxide is beneath the surface 38. The thickness of the layer 34 is in the range typically of 300 to 500 nm (3000 to 5000 angstroms). The length of the bird beak 36 is also approximately 300 to 500 nm (3000 to 5000 angstroms). At step C, the oxide layer 34 is removed by reactive ion etching done in a vertical direction, the portion of the oxide beneath the bird beak 36 being shielded from the etchant by the silicon-nitride layer 30. Thus, only the exposed portion of the silicon dioxide is etched to provide a substantially perpendicular wall 40 of the abutment 24. The etching of the oxide is accomplished with a well known reactive ion etch (RIE) which is selective to the silicon dioxide and does not harm either the silicon of the wafer or the silicon-nitride of the layer 30. At step D the silicon nitride layer 30 is removed with a standard well-known wet chemical etchant to provide a structure composed of the wafer 28 modified by the development of the abutment 24. The abutment 24 rises from a location beneath the original surface 38 to a location above the original surface 38 of the wafer 28. Removal of the layer 30 leaves a slanting roof 42 to the abutment 24, the roof 42 joining the upper edge of the perpendicular wall 40. In step E, the semiconductor structure of step D is again subjected to an oxygen atmosphere at an elevated temperature of 900 to 1100°C for the development of a further oxide layer 44. The silicon dioxide of the layer 44 develops on both sides of the abutment 24 upon the reaction of the oxygen with the silicon. During the growth of the layer 44, there is relatively little development of additional oxide at the site of the abutment 24 because the silicon dioxide therein acts as a barrier to further reaction between the oxygen and the silicon. The process continues with step F wherein the semiconductor is exposed to silicon which is deposited by chemical vapour deposition (CVD), this deposition being accomplished conformally with the result that a polycrystalline layer 46 with n-conductivity type doping develops uniformly upon the layer 44 as well as on the wall 40 of the abutment 24. The polycrystalline form of the silicon in the layer 46 is referred to as polysilicon and is grown to a thickness equal to the desired length of the gate 16, typically in the range of 200 to 500 nm (2000 to 5000 angstroms). The polysilicon is deposited by use of silane (SiH₄) which, at a temperature in the range of 600 to 700°C produces the desired layer of polysilicon heavily doped with a suitable impurity to influence its conductivity. In step G, the polysilicon of the layer 46 is etched away by reactive ion etching through a depth equal to the depth of the layer 46. During the etching, the appropriate plasma ions are directed by an electric field perpendicular to the original surface of the wafer 28 and, therefore, such e tching does not eat into the surface of the layer 44 parallel to the wall 40. As a result of the etching, all of the polysilicon is removed except for the region contiguous to the wall 40 and to the layer 44, this remaining region of polysilicon serving as the gate 16. Finally, in step H, source and drain are developed by ion implantation of an n-conductivity type dopant such as arsenic or phosphorus directed at the upper surface of the semiconductor device. The depth of implantation is dependent on the stopping distance of the dopant atoms at the implanting energy and the material at the upper surface of the semiconductor device. Therefore, there develops an implanted layer, the bottom of which is indicated by a line 48, that has a profile which substantially follows the profile of the top surface of the semiconductor device, including the region of the abutment 24 and the gate 16. This results in a bump in the profile 48 at the site of the abutment 24 and the gate 16. The region of the ionically doped layer to the left of and beneath the abutment 24 forms a drain region 14, while the portion of the ionically implanted layer to the right of the gate 16 forms a source region 12. Upon the application of suitable positive voltages to the gate 16 and to the drain 14 relative to the source 12, a charge conduction channel 50 develops between the drain region 14 and the source region 12. The channel 50 passes beneath the electrically-insulating silicon-dioxide layer 44 and the bottom of the silicon dioxide of the abutment 24. While the line 48 passes above the channel 50, it is noted that the implantation of the dopant occurs to a depth governed by the laws of projectile kinetics and statistics and, accordingly, there is some dopant present at the region of the channel 50. A feature of the invention is found in the reduced amount of dopant in the region of the channel 50. By selection of a suitable voltage in the ion implantation, and by selection of a suitable height for the abutment 24, the amount of dopant in the drain side of the channel 50 can be carefully controlled so as to provide adequate protection from avalanche breakdown while avoiding excessive amounts of electrical resistance in the drain 14. Such resistance is dependent on the amount of doping, and increases with increased doping. Thereby, the semiconductor device, as shown at step H, can be used effectively as a field-effect transistor with minimal power loss due to the presence of resistance in the path of the channel 50 between the regions of the drain 14 and the gate 16. As noted above, this is accomplished with adequate protection from avalanche and the subsequent damage which might result from excessive heating of the transistor. The invention is applicable to materials other than silicon, and for the development of electrode structures other than that of the gate of an FET. The foregoing process also applies to a substrate of n-conductivity type silicon in which case the source and drain regions would be doped with a p-conductivity type dopant such as boron. The depth of the drain region 14, as well as the depth of the source region 12 (between the layer 44 and the line 48) is in the range of approximately 100 to 200 nm (1000 to 2000 angstroms). During the ionic implantation of the source and drain regions, a dose of 10¹⁴-10¹⁵ atoms per square centimetre is generally used. It is noted that the foregoing process employs a standard form of polysilicon gate structure, and is compatible with present VLSI technology. The length of the gate 16 has been determined by a non-lithography dependent technique, that is, the length of the gate is determined by the thickness of the chemical vapour deposition of the polysilicon. The thickness of the polysilicon is readily selected and controlled and is not subjected to optical diffraction limitations associated with lithography. Since the gate length is equal to t he thickness of the polysilicon layer, an extremely short gate length can be established without limitations as to diffraction effect. It is noted that all portions of the gate structure and the intervening regions with the source and the drain can be precisely controlled as to their geometry. In addition, a light level of doping can be accurately established for the drain side of the channel 50 to provide the above-noted advantages of reduced power loss and protection from avalanche breakdown. The polysilicon typically is heavily doped with an impurity such as arsenic to provide good conduction in the gate 16. The slanting roof 42 of the abutment 24 provides for the gradual elongation of the lightly doped drain portion of the channel 50 so as to spread out the electric field distribution and avoid any strong radiance which might otherwise result in highly localised concentration of electric field which in turn causes breakdown.

Claims: 1. A process for fabricating a semiconductor device comprising covering a portion of a surface of a substrate (26) of semiconductor material with an oxygen-impervious layer (30) leaving an exposed area where an electrode (16) is to be formed, oxidising the exposed area to form an oxide of the semiconductor material with the edge of the layer (30) to raise due to the formation of oxide thereunder, characterised in the further steps of directionally etching away exposed portions of the oxide to leave an abutment (24) of oxide having a wall (40) substantially perpendicular to the surface of the substrate, removing the layer (30) from said substrate (26) and abutment (24), re-oxidising the substrate to form a re-oxidised layer (44), deporting a conformal layer (46) of electrically conductive material over said re-oxidised layer (44) to a thickness equal to the desired length of said electrode (16), and directionally etching said conductive layer (46) to leave the electrode (16) on the wall (40) of the abutment (24). 2. A process as claimed in claim 1, further including doping a surface region of said substrate (26) to a predetermined depth profile (48) to provide a charge conduction band (50) which is partially constricted at one side of said abutment (24). 3. A process as claimed in claim 2, in which said doping step is accomplished by ion implantation. 4. A process as claimed in any preceding claim, in which at least one of said directional etching steps is accomplished by reactive ion etching. 5. A process as claimed in any preceding claim in which the substrate is of silicon, in which the oxygen impervious layer in the silicon nitride and in which the conductive layer is of doped polycrystalline silicon. 6. A semiconductor device comprising a semiconductor substrate (26), an oxide insulating layer (44) formed on the surface thereof and having an electrode formed on said oxide layer (44), and is characterised in that the oxide layer (44) has an abutment (24) formed therein with a wall (40) extending substantially perpendicular to the surface and a sloping roof (42) connected to the upper edge of the wall, and in that the electrode (16) is formed on the vertical wall (40) of the abutment (24). 7. A semiconductor device as claimed in claim 6, in which a dopant is disposed within said substrate (26) beneath said oxide layer (44) with a doping profile rising into said abutment partially to constrict a charge conduction channel (50) formed beneath the oxide layer (44) thereby to provide a predetermined resistance and protection from avalanche. 8. A device as claimed in claim 7 in the form of a field-effect transistor comprising a drain region (14) formed at the side of said abutment (24) adjacent said sloping roof (42) and a source region (12) formed at the side of said abutment (24) adjacent said perpendicular wall. 9. A device as claimed in claim 8, i n which the substrate is of one conductivity type (N or P) and the source and drain regions are of opposite conductivity type (P or N). 10. A device as claimed in any one of claims 6 to 9, in which the substrate (26) is of silicon, the abutment (24) and oxide layer (44) are of silicon dioxide, and the electrode is of doped polycrystalline silicon.

Inventors: FANG, FRANK FU, GROSSMAN, BERTRAND M.
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: H01L21
IPC codes: H01L21, H01L29