FIELD EFFECT TRANSISTOR DEVICE AND METHOD OF MAKING SAME

Abstract: A field effect transistor with a lightly doped drain and a heavily doped source is formed utilizing a single reactive ion etched silicon dioxide spacer. A gate electrode of polycrystal­ line silicon on a thin layer of silicon dioxide grown on a silicon substrate of one conductivity type is utilized in the formation of a pair of shallow, lightly doped regions of opposite conductivity type in the substrate on each side of the gate electrode. A pair of silicon dioxide spacers is provided, each on a respective side of the gate electrode and overlying a respective shallow, lightly doped region, by anisotropically etching a layer of silicon dioxide deposited over the structure. One of the spacers is masked and the other spacer is removed by reactive ion etching. Thereafter, a pair of deep, heavily doped regions of opposite conductivity type are formed in the substrate not masked by the gating electrode and the insulating space to complete the formation of the drain and source regions of the device.

Description: FIELD EFFECT TRANSISTOR DEVICE AND METHOD OF MAKING SAME The present invention relates in general to field effect transistor devices and in particular to field effect transistor devices which are of small dimensions yet which have high performance characteristics and are relatively easy to fabricate. In the prior art (Fabrication of High-Performance LDDFET'S with Oxide Sidewall-Spacer Technology, IEEE Transactions on Electron Devices, Vol. ED-29, No. 4, April 1982), a field effect transistor is described comprising a substrate of silicon semiconductor material of one conductivity type having a planar surface on which a thin layer of insulating material is formed. A gate electrode is provided on the thin layer of insulating material and is utilized for the formation of a pair of shallow lightly doped regions of opposite conductivity type in the substrate on each side of the gate electrode. Next, an insulating spacer is provided on each side of the gate electrode overlying each of the shallow, lightly doped regions. Thereafter, a pair of deep, heavily doped regions of opposite conductivity type are formed in the substrate not masked by the gate electrode and the insulating spacers to form the source and drain regions of the device. The device so formed is referred to as a lightly doped drain field effect transistor as the region of the drain adjacent the gate electrode is constituted of a shallow lightly doped region. In the prior art device described, the structure of the source is the same as the structure of the drain with a shallow lightly doped region extending between a more heavily doped region and the portion of the substrate underlying the gate. While it is desirable to provide a lightly doped drain region in order to improve the performance of the device, such as to increase the'voltage at which drain-to-source punch through occurs, inclusion of such lightly doped regions in the source increases the source resistance and impairs the performance of the device. An object of the present invention is to provide a field effect transistor of small dimensions which has a lightly doped drain and a heavily doped source. Another object of the present invention is to provide a simple method of making a field effect transistor of small dimensions which has a lightly doped drain and a heavily doped source. In carrying out the invention in one illustrative embodiment thereof, there is provided a substrate of silicon semicorductor material of one conductivity type having a planar surface. A thin layer of insulating material is formed on the planar surface. A gate electrode having a pair of opposite sides is formed on the thin layer of insulating material. A first lightly doped shallow region of opposite conductivity type and a second lightly doped shallow region of opposite conductivity type are formed in the substrate adjacent the planar surface using the gating electrode as a mask. The first region forms in the substrate a first PN junction having an edge in the planar surface and adjacent one side of the gating electrode. The second shallow region forms in the substrate a second PN junction having an edge in the planar surface and adjacent the other side of the gating electrode. The first and second regions have a concentration of activators of a first value. A first insulating spacer and a second insulating spacer each including first and second opposed sides and a base are simultaneously formed. The first side of the first spacer is contiguous to the aforementioned one side of the gating electrode. The base of the first spacer lies on the exposed surface of the thin layer of insulating material. The first side of the second spacer is contiguous to the other side of the gating electrode. The base of the second spacer lies on the exposed surface of the thin layer of insulating material. Next, a reactive ion etching mask is provided over a portion of the gate electrode including the aforementioned one side thereof and over the first insulating spacer. Thereafter, the exposed surface of the substrate is reactive ion etched using a gas which reactive ion etches silicon dioxide and which does not reactive ion etch silicon to remove selectively the second insulating spacer. A first deep region of opposite conductivity type is formed in the substrate adjacent the planar surface. The first deep region forms in the substrate a third PN junction having an edge in the planar surface and adjacent the second side of the first insulating spacer. The first deep region has a concentration of activators of a second value substantially greater than the aforementioned first value. A second deep region of opposite conductivity type is formed in the substrate adjacent the planar surface. The second deep region forms a fourth PN junction in the substrate having an edge in the planar surface and adjacent the other side of the gating electrode. The second deep region has a concentration of activators substantially equal to said second value. The second deep region of opposite conductivity type contains the second shallow region of opposite conductivity type. The novel features which are believed to be characteristic of the present invention are set forth with particularity in the appended claims. The invention itself, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings wherein: Figure 1 is a plan view of a field effect transistor in accordance with the present invention. Figure 2 is a sectional view of the device of Figure 1 taken along section lines 2-2 showing the internal structure thereof. Figure 3 is another sectional view of a portion of the device of Figure 1 taken along section lines 3-3 thereof also showing internal structure thereof. Figures 4A-4G are schematic illustrations of vertical cross sections of a piece of semiconductor wafer in the process of fabrication of a field effect transistor in accordance with the present invention. Referring now to Figures 1, 2 and 3, there is shown a portion of an integrated circuit including a field effect transistor 10 made in accordance with the present invention. The device includes a substrate of monocrystalline silicon semiconductor material of P-type conductivity and relatively high resistivity, for example, 2 ohm-cm. The substrate may be conveniently 10 mils thick. A trench 13 about a micron wide and about a micron deep is formed in the planar surface 12 of the substrate 11 and filled with an insulating material 14 such as silicon dioxide surrounding an active region 15 of the substrate isolating this region from other such regions formed in the integrated circuit substrate. The sides of the active region 15 form a square in the plane of the major surface 12 of the substate. A thin layer 17 of silicon dioxide is provided on the planar surface 12. A gate electrode 18 of poly crystalline silicon having sides 18a and 18b, and base 18c is provided on the insulating layer 17. An insulating spacer 19 having a first side 19a, a second side 19b and a base 19e is provided. First side 19a of the spacer 19 has a height equal to the height to the side 18a of the gating electrode 18 and is contiguous therewith. The base 19e is contiguous with the top surface of the thin insulating layer 17. A first lightly doped shallow region 21 of N-type conductivity is located adjacent the planar surface and underlying the base of the insulating spacer 19 forming a PN junction 23 with the substrate. The insulating spacer 19 may be formed by covering the major surface of the substrate 11 with silicon dioxide, for example, by chemical vapor deposition and thereafter anisotropically etching the layer of silicon dioxide until all portions of the layer are completely removed except for a spacer 19 on side 18a of the electrode 18 and another spacer element 20 (not shown) on the other side 18b of the gate electrode 18. The spacer 19 is masked and spacer 20 on the other side of the gate electrode 18 is reactive ion etched using a gas such as trifluoromethane (CHF3) which does not react or reacts minimally with the polycrystalline silicon material of the gate electrode 18 and which does react with silicon dioxide thereby selectively removing the insulating spacer 20. The thin layer 17 of silicon dioxide is formed by thermally growing silicon dioxide in the exposed portions of the planar surface. Gate electrode 18 is formed by depositing a layer of polycrystalline silicon overlying thin insulating layer 23 and thereafter patterning the layer by photolithographic masking and etching techniques well known in the art. A heavily doped, deep region 25 of N-type conductivity is provided in the substrate adjacent the planar surface 12 and forms a PN junction 26 with the substrate. The PN junction 26 has an edge 27 in a planar surface 12 adjacent the bottom base 19e and the side 19b of the insulating spacer 19. An impurity or donor activator concentration in the region 25 is substantially greater than the donor activator concentration in the region 21. Another region 28 of N-type conductivity is provided in the substrate adjacent the major surface 12 and side 18b of the gating electrode 18 forming a PN junction 29 therewith. The PN junction 29 has an edge in the planar surface 12 and adjacent the side 18b of the gating electrode 18. The net impurity or donor concentration in the region 28 of N type conductivity is substantially greater than the donor concentration of the shallow, lightly doped region 21 and is substantially equal to the doping concentration in the region 25. The regions 25 and 28 of N type conductivity are formed by initially depositing by ion implantation a predetermined quantity of donor activators, such as arsenic of a dosage of about 1015 ions per cm2, upon the major surface 12 of the substrate over these regions and thereafter activating the arsenic ions by heating at a suitable temperature and for a short time. A thick layer 30 of silicon dioxide is chemically vapor deposited on the major surface of the substrate 11. Thereafter the thick layer 30 of silicon dioxide is patterned to form openings 31, 32 and 33 therein exposing the source region 28, the drain region 25 and the gate electrode 18. Thereafter a layer of aluminum is sputtered over the surface of the resultant structure into openings 31, 32 and 33. The layer of aluminum is then patterned to remove portions thereof leaving the conductive connections (not shown) making contact with the source region 28, the drain region 25 and the gate electrode 18, respectively. The device is then heated to a temperature of about 550 0C for a time to bond the conductive connections of aluminum to the source region 28, the drain region 25 and the gate electrode 18. Subsequently, a conductive connection 35 of aluminum is formed on the opposite surface of the substrate. Reference is now made to Figures 4A-4G which illustrate the manner of fabrication of the field effect transistor of Figures 1-3 in accordance with the present invention. Elements in Figures 4A-4G identical to elements of Figures 1-3 are identically designated. Initially, a wafer 11 of monocrystalline semiconductor material of relatively high resistivity, for example 2 ohms cm, having a convenient thickness, for example,#l0 mils, and a P-type conductivity is provided. The wafer may be of any convenient diameter on which a very large number of individual field effect transistors, such as shown in Figures 1-3 may, be formed. Diagrams of Figures 4A-4G show the processing of the wafer as it affects the individual transistor on the wafer. After suitable cleaning of the surface of the wafer, a trench 13 about 1 micron wide and about 1 micron deep is etched into the silicon substrate utilizing photolithographic masking and etching techniques well known in the art to form an active region 15 of square outline of 3.5 microns on a side. Thereafter, the wafer is exposed to a mixture of silane and oxygen at a temperature of about 9000C for a sufficient period of time to fill the trenches 13 with silicon dioxide and also cover the exposed surfaces 12 of the wafer of silicon. The wafer is next planarized. To this end the wafer is covered with a suitable photoresist such as Hunt type 704 photoresist. Thereafter, the resist and the silicon dioxide are reactive ion etched in an atmosphere of trifluoro-methane (CHF3) and oxygen to produce the structure shown in Figure 4A in which an active region 15 is isolated from other active regions by the isolation regions 14 of silicon dioxide formed in the substrate 11. In the next step of the process, shown in Figure 4B, a layer 17 of silicon dioxide, referred to as gate oxide, is thermally grown over the surface 12 of the substrate. A layer of polycrystalline silicon 0.5 micron thick is then deposited over the layer of gate oxide 17. The polycrystalline layer is suitably patterned using photolithographic masking and etching techniques well known in the art to form the gate electrode 18 one micron in length from side 18a to side 18b and 0.5 micron thick. Using the gate 18 as a mask arsenic ions of a dosage of 2 x 1013 ions per cm2 at 100 kilovolts are implanted and then flash annealed in the substrate 11 to form lightly doped shallow regions 21 and 22 of N-type conductivity having an activator concentration of about 1Ql8 atoms per cm3. In the next step of the process, shown in Figure 4C, substrate 11 is covered with a layer 37 of chemically vapor deposited silicon dioxide 0.5 micron thick using silane and oxygen. Thereafter, the layer 35 of silicon dioxide is anisotropically etched in trifluoromethane (CHF3), which etches silicon dioxide at a much more rapid rate than it etches silicon, to remove the silicon dioxide from the wafer until only spacer portions 19 and 20 remain as shown in Figure 4D. The height of the spacers 19 and 20 is about 0.5 micron. The width of the bottom portions of the spacers 19 and 20 is also about 0.5 micron. In the next step of the process shown in Figure 4E, a layer of photoresist is deposited over the exposed surface of the wafer covering the gate electrode 18, the insulating spacers 19 and 20, and the exposed portions of the planar surface 12. The photoresist is a Shipley type AZ1470 positive photoresist 1 micron thick. The photoresist is patterned by photolithographic masking and etching techniques well known in the art to provide a mask 39 which covers a portion of the gate electrode 18, the insulating spacer 19, and a portion of the planar surface 12 as shown. The alignment of the mask 39 fs not critical. The edge 39a of the mask 39 would be aligned to the center line of the gate electrode 18. A deviation of the alignment of edge 39a up to less than about one-half of the distance between edges 18a and 18b of the gate electrode 18 could be tolerated. In the next step of the process, the exposed surface of the substrate is reactive ion etched in trifluoromethane (CHF3) and argon to remove the spacer 20 and the portion of silicon dioxide layer 17 beneath spacer 20, to form the structure shown in Figure 4F. Thereafter, the photoresist pattern is removed. Regions adjacent the planar surface not masked by the gate electrode 18 or the insulating spacer 19 are implanted with arsenic ions of a dosage of 1015 atoms per cm2 at 200 kilovolts. Thereafter, the resultant structure is flash annealed 0 at 1000 C for about 10 seconds to activate the ion implants to form the deep, heavily doped regions 25 and 28 in the substrate, as shown in Figure 4F. Heavily doped region 28 completely contains the lightly doped region 22 formed adjacent the planar surface 12 and also heavily doped region 25 contains a portion of the lightly doped region 21 formed adjacent the planar surface 12, as shown in Figure 4F. The next step of the process is illustrated in Figure 4G. The surface of the wafer is covered with a layer 30 of silicon dioxide deposited thereon by chemical vapor deposition using silane and oxygen to a thickness of about 0.5 micron. Thereafter, the layer 30 of silicon dioxide is patterned using photolithographic masking and etching techniques well known to those skilled in the art to provide openings 31 and 32 to the source region 28 and the drain region 25 and to provide opening 33 (Pig. 1) to the gate electrode 18. At this point in the process, metal contacts (not shown) are provided to the source region 28, the drain region 25 and the gate electrode 18 of the transistor formed in the active region 15. While the embodiments of the invention described above, N channel transistors were formed on P-type silicon substrates, P-channel transistors could as well have been formed on N-type conductivity substrates in a similar manner. While the invention has been described in specific embodiments, it will be understood that modifications may be made by those skilled in the art and it is intended by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the invention.

Claims: CLAIMS 1. A field effect transistor comprising: a substrate of silicon semiconductor material of one conductivity type having a planar surface, a thin layer of insulating material lying on said planar surface, a gate electrode lying on said thin layer of insulating material and having a pair of opposed sides, an insulating spacer including first and second opposed sides and a base, the first side of said spacer being contiguous to one side of said gate electrode and contiguous therewith, the base of said spacer abutting the exposed surface of said thin layer of insulating material, a first shallow region of opposite conductivity type having a first concentration of activators, located adjacent said planar surface and underlying said base of said spacer, said first shallow region forming a first PN junction with said subtrate, a first deep region of opposite conductivity type adjacent said planar surface, said first deep region forming a second PN junction with said said substrate, said second PN junction having an edge in said planar surface and adjacent said second side of said insulating spacer, said first deep region having a second concentration of activators, said second concentration of activators being substantially greater than said first concentration of activators, a second deep region of opposite conductivity type adjacent said planar surface, said second deep region forming a third PN junction having an edge in said planar surface and adjacent said other side of said gate electrode, said second deep region having a third concentration of activators substantially equal to said second concentration of activators. 2. The field effect transistor of claim 1 in which said thin layer of insulating material and said insulating spacer are constituted of silicon dioxide. 3. The field effect transistor of claim 2 in which said gate electrode is constituted of heavily doped polycrystalline silicon. 4. A method of making a field effect transistor comprising: providing a substrate of silicon semiconductor material of one conductivity type having a planar surface, forming a thin layer of insulating material on said planar surface, forming a gate electrode having a pair of opposite sides on said thin layer of insulating material, forming a first lightly doped shallow region of opposite conductivity type and a second lightly doped shallow region of opposite conductivity type in said substrate adjacent said planar surface, said first shallow region forming a first PN junction with said substrate, said first PN junction having an edge in said planar surface and adjacent one of said sides of said gating electrode, said first shallow region having a first concentration of activators, said second shallow region forming a second PN junction with said substrate, said second PN junction having an edge in said planar surface and adjacent the other of said sides of said gating electrodes, said second shallow region having a second concentration of activators equal to said first concentration of activators, forming a first insulating spacer and a second insulating spacer, each including first and second opposed sides and a base, the first side of said first spacer contiguous to one side of said gate electrode, the bottom base of said first spacer abutting the exposed surface of said thin layer of insulating material, the first side of said second spacer contiguous to the other side of said gate electrode, the bottom base of said second spacer abutting the exposed surface of said thin layer of insulating material, removing said second spacer, forming a first deep region of opposite conductivity type in said electrode adjacent said planar surface, said first deep region forming a third PN junction with said substrate, said third PN junction having an edge in said planar surface and adjacent said second side of said first insulating spacer, said first deep region having a third concentration of activators, said third concentration of activators being substantially greater than said first concentration of activators, forming a second deep region of opposite conductivity type in said substrate adjacent said planar surface, said second deep region forming a fourth PN junction having an edge in said planar surface underlying said other side of said gate electrode, said second deep region having a fourth concentration of activators substantially equal to said third concentration of activators, said fourth deep region containing said second shallow region. 5. The method of claim 4 in which said first insulating spacer and said second insulating spacer are formed by depositing a layer of silicon dioxide of a predetermined thickness over said substate and thereafter etching said layer of silicon dioxide anisotropically to the point of exposing the major surface of said substrate thereby forming said first insulating spacer and said second insulating spacer. 6. The method of claim 5 in which a reactive ion etching mask is provided over a portion of said gate electrode including said one side thereof and over said first insulating spacer, and in which the exposed surface of said substrate is reactive ion etched usinq a gas which reactive ion etches silicon dioxide and which does not reactive ion etch the material of said gate electrode thereby selectively removing said second insulating spacer. 7. The method of claim 6 in which said gate electrode is constituted of heavily doped polycrystalline silicon. 8. The method of claim 7 in which said gas is trifluoromethane.

Inventors: VOSBURGH, KIRBY GANNETT
Applicants: GENERAL ELECTRIC COMPANY
Application year: 1985
Main classification: H01L29
IPC codes: H01L29, H01L21