WO1985004049A1 - Improved integrated circuit structure having intermediate metal silicide layer and method of making same - Google Patents

Improved integrated circuit structure having intermediate metal silicide layer and method of making same Download PDF

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Publication number
WO1985004049A1
WO1985004049A1 PCT/US1985/000307 US8500307W WO8504049A1 WO 1985004049 A1 WO1985004049 A1 WO 1985004049A1 US 8500307 W US8500307 W US 8500307W WO 8504049 A1 WO8504049 A1 WO 8504049A1
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WIPO (PCT)
Prior art keywords
layer
metal
oxide
poly silicon
poly
Prior art date
Application number
PCT/US1985/000307
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French (fr)
Inventor
Robin W. Cheung
Hugo W. K. Chan
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Advanced Micro Devices, Inc.
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Application filed by Advanced Micro Devices, Inc. filed Critical Advanced Micro Devices, Inc.
Publication of WO1985004049A1 publication Critical patent/WO1985004049A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/70—Manufacture or treatment of devices consisting of a plurality of solid state components formed in or on a common substrate or of parts thereof; Manufacture of integrated circuit devices or of parts thereof
    • H01L21/71—Manufacture of specific parts of devices defined in group H01L21/70
    • H01L21/768—Applying interconnections to be used for carrying current between separate components within a device comprising conductors and dielectrics
    • H01L21/76838—Applying interconnections to be used for carrying current between separate components within a device comprising conductors and dielectrics characterised by the formation and the after-treatment of the conductors
    • H01L21/76886—Modifying permanently or temporarily the pattern or the conductivity of conductive members, e.g. formation of alloys, reduction of contact resistances
    • H01L21/76889—Modifying permanently or temporarily the pattern or the conductivity of conductive members, e.g. formation of alloys, reduction of contact resistances by forming silicides of refractory metals
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/70—Manufacture or treatment of devices consisting of a plurality of solid state components formed in or on a common substrate or of parts thereof; Manufacture of integrated circuit devices or of parts thereof
    • H01L21/71—Manufacture of specific parts of devices defined in group H01L21/70
    • H01L21/768—Applying interconnections to be used for carrying current between separate components within a device comprising conductors and dielectrics
    • H01L21/76801—Applying interconnections to be used for carrying current between separate components within a device comprising conductors and dielectrics characterised by the formation and the after-treatment of the dielectrics, e.g. smoothing
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/70—Manufacture or treatment of devices consisting of a plurality of solid state components formed in or on a common substrate or of parts thereof; Manufacture of integrated circuit devices or of parts thereof
    • H01L21/71—Manufacture of specific parts of devices defined in group H01L21/70
    • H01L21/768—Applying interconnections to be used for carrying current between separate components within a device comprising conductors and dielectrics
    • H01L21/76801—Applying interconnections to be used for carrying current between separate components within a device comprising conductors and dielectrics characterised by the formation and the after-treatment of the dielectrics, e.g. smoothing
    • H01L21/76829—Applying interconnections to be used for carrying current between separate components within a device comprising conductors and dielectrics characterised by the formation and the after-treatment of the dielectrics, e.g. smoothing characterised by the formation of thin functional dielectric layers, e.g. dielectric etch-stop, barrier, capping or liner layers
    • H01L21/76834—Applying interconnections to be used for carrying current between separate components within a device comprising conductors and dielectrics characterised by the formation and the after-treatment of the dielectrics, e.g. smoothing characterised by the formation of thin functional dielectric layers, e.g. dielectric etch-stop, barrier, capping or liner layers formation of thin insulating films on the sidewalls or on top of conductors
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/70—Manufacture or treatment of devices consisting of a plurality of solid state components formed in or on a common substrate or of parts thereof; Manufacture of integrated circuit devices or of parts thereof
    • H01L21/71—Manufacture of specific parts of devices defined in group H01L21/70
    • H01L21/768—Applying interconnections to be used for carrying current between separate components within a device comprising conductors and dielectrics
    • H01L21/76838—Applying interconnections to be used for carrying current between separate components within a device comprising conductors and dielectrics characterised by the formation and the after-treatment of the conductors
    • H01L21/76895—Local interconnects; Local pads, as exemplified by patent document EP0896365
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L23/00—Details of semiconductor or other solid state devices
    • H01L23/52—Arrangements for conducting electric current within the device in operation from one component to another, i.e. interconnections, e.g. wires, lead frames
    • H01L23/522—Arrangements for conducting electric current within the device in operation from one component to another, i.e. interconnections, e.g. wires, lead frames including external interconnections consisting of a multilayer structure of conductive and insulating layers inseparably formed on the semiconductor body
    • H01L23/532—Arrangements for conducting electric current within the device in operation from one component to another, i.e. interconnections, e.g. wires, lead frames including external interconnections consisting of a multilayer structure of conductive and insulating layers inseparably formed on the semiconductor body characterised by the materials
    • H01L23/53204—Conductive materials
    • H01L23/53271—Conductive materials containing semiconductor material, e.g. polysilicon
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/40—Electrodes ; Multistep manufacturing processes therefor
    • H01L29/41—Electrodes ; Multistep manufacturing processes therefor characterised by their shape, relative sizes or dispositions
    • H01L29/417—Electrodes ; Multistep manufacturing processes therefor characterised by their shape, relative sizes or dispositions carrying the current to be rectified, amplified or switched
    • H01L29/41725—Source or drain electrodes for field effect devices
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/40—Electrodes ; Multistep manufacturing processes therefor
    • H01L29/43—Electrodes ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H01L29/49—Metal-insulator-semiconductor electrodes, e.g. gates of MOSFET
    • H01L29/4916—Metal-insulator-semiconductor electrodes, e.g. gates of MOSFET the conductor material next to the insulator being a silicon layer, e.g. polysilicon doped with boron, phosphorus or nitrogen
    • H01L29/4925—Metal-insulator-semiconductor electrodes, e.g. gates of MOSFET the conductor material next to the insulator being a silicon layer, e.g. polysilicon doped with boron, phosphorus or nitrogen with a multiple layer structure, e.g. several silicon layers with different crystal structure or grain arrangement
    • H01L29/4933—Metal-insulator-semiconductor electrodes, e.g. gates of MOSFET the conductor material next to the insulator being a silicon layer, e.g. polysilicon doped with boron, phosphorus or nitrogen with a multiple layer structure, e.g. several silicon layers with different crystal structure or grain arrangement with a silicide layer contacting the silicon layer, e.g. Polycide gate
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
    • H01L2924/0001—Technical content checked by a classifier
    • H01L2924/0002—Not covered by any one of groups H01L24/00, H01L24/00 and H01L2224/00
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S148/00—Metal treatment
    • Y10S148/019—Contacts of silicides
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S148/00—Metal treatment
    • Y10S148/133—Reflow oxides and glasses
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S148/00—Metal treatment
    • Y10S148/147—Silicides
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S257/00—Active solid-state devices, e.g. transistors, solid-state diodes
    • Y10S257/903—FET configuration adapted for use as static memory cell
    • Y10S257/904—FET configuration adapted for use as static memory cell with passive components,, e.g. polysilicon resistors
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S438/00—Semiconductor device manufacturing: process
    • Y10S438/978—Semiconductor device manufacturing: process forming tapered edges on substrate or adjacent layers

Definitions

  • This invention relates to integrated circuit structures. More particularly, this invention relates to an integrated circuit structure having an intermediate metal suicide layer.
  • an undoped oxide layer over a polycrystalline silicon (hereinafter referred to as poly silicon or poly) layer having lightly doped portions to provide resistive paths or poly loads.
  • This second, undoped, oxide layer is applied to prevent diffusion of the dopants from getting to the poly load area of the poly silicon layer.
  • the use of an additional doped oxide layer over this oxide layer to provide a lower melting point oxide whereby the step coverage can be smoothed out means that contact openings must penetrate three layers of oxide before contacting the desired portion of the silicon substrate, such as, for example, the drain or source of an MOS transistor. This, in turn, can provide inadequate metal coverage in the contact opening resulting in thinning of portions of the metal on the wall of the contact opening.
  • the metal silicide layer is connected to the poly load structure masked by the oxide, eliminating the need for an additional interconnect layer.
  • Figure 1A is a sectional view of a portion of an integrated circuit device constructed in accordance with the invention.
  • Figure 1B is a sectional view of an integrated circuit device constructed in accordance with the prior art.
  • Figures 2A-2G are sectional views of the process steps performed to arrive at the structure of figure 1A.
  • Figure 3 is a sectional view of a portion of a CMOS device constructed in accordance with the invention.
  • Figure 4 is sectional end view of the structure of figure 1A taken along lines IV-IV.
  • Figure 1A illustrates an integrated circuit structure utilizing the invention in contrast to an integrated circuit structure having the same active and passive components formed therein in accordance with the prior art as shown in figure 1B.
  • a silicon wafer or substrate 4 has formed thereon an enhancement mode field effect transistor (FET) 6 and a depletion mode FET 8.
  • FET 6 has a gate 6a, a source 6b, and a drain 6c.
  • FET 8 has a gate 8a, a source 8b, and a drain 8c
  • Metal silicide segments 60a, 60b, and 60c of figure 1A in accordance with the invention, overlay poly silicon segments 10a, 10b, and 10c which overlaying segments together make connection to various portions of the circuit. Segments 10a/60a make connection to a poly silicon segment 12 which may, for example, be a word line in a static RAM cell.
  • Segments 10b/60b make connection to a portion 14 of the N+ layer contiguous with source 6b.
  • Portion 14 may, for example, be a voltage supply line with segments 10b/60b providing a low resistance strap.
  • Segments 10c/60c provides a low resistance quasi-buried contact (QBC) of a poly silicon segment 18 to a portion 20 of the N+ layer contiguous with drain 6c.
  • QBC quasi-buried contact
  • metal silicide layer in the system of the invention, is formed in situ by depositing, on the surface of the poly silicon, a layer of a metal, such as titanium capable of reacting with the silicon, and then subsequently, sintering the materials to form titanium silicide in situ on the surface of the poly silicon layer.
  • a metal such as titanium capable of reacting with the silicon
  • the metal silicide layer is first applied by codepositing silicon and the metal portion of the silicide to form the metal silicide layer after which the poly silicon layer is applied.
  • two distinct layers are applied to the substrate; first the metal silicide layer and then the poly silicon layer.
  • the metal silicide forms in situ on the surface of the previously laid down and patterned poly silicon segments or strips without forming an extra layer from the standpoint of additional thickness.
  • the metal silicide layer is formed by first patterning a deposited layer of poly silicon over the substrate and any previous layers thereon followed by application of a metal, such as a refractory metal, capable of reacting with the poly silicon to form the metal silicide.
  • a metal such as a refractory metal
  • metals include titanium, molybdenum, and tungsten.
  • the metal is preferably also one that is easily applied to the poly silicon as by sputtering or vapor deposition.
  • the metal used should be further capable of reacting with the poly silicon at a temperature of from about 600-800oC, i.e., high enough to permit diffusion of the silicon into the metal reactant to from silicide, yet low enough to inhibit unwanted diffusion of the silicon to other parts of the device (e.g., the gap between two patterned poly silicon lines).
  • the device After the application of the metal coating over the silicon, the device is heated in a heat pulsed sintering environment to a temperature of about 600-800oC and held at this temperature for from 10 to 20 seconds to permit reaction to occur forming the metal silicide and inhibit the side diffusion of silicon.
  • the resultant metal silicide should have a resistivity of less than 5 ohms per square.
  • the remaining poly should have a thickness of about .25 to .33 that of the silicide.
  • the ratios of amounts of poly to metal deposited thereon must be controlled.
  • the thickness ratio of silicon to metal should be kept to a 3:1 ratio or higher to provide the desired final ratio of silicide to poly.
  • Figures 2A-2G are cross-sectional views which illustrate the processing steps used in fabricating a typical N channel device utilizing the invention such as the device shown in figure 1A. As will be shown in figure 3, other devices can also be constructed utilizing the invention;, the devices illustrated in figures 1A, 2A-2G, and 3 being only representative of the invention. Representative process and characteristic parameters such as dopant concentration, implantation depth, layer thickness, and electrical resistance are tabulated in Appendices 1-3 below.
  • Figure 2A shows wafer 4 after it has been subjected to the following steps: preoxidation, barrier oxidation, nitride depostion source/drain mask, and field doping. More particularly, a thin barrier oxide layer is deposited by heating the wafer in an oxygen atmosphere, and a silicon nitride layer is then deposited by standard techniques. The source/drain regions are masked by selectively removing portions of the nitride layer and underlying oxide layer leaving nitride/oxide layer segments 30a and 30b over respective regions 32 and 33 to define the source/drain mask. The region 35 that is not masked is then subjected to the ion implantation of a P-type impurity to define a channel stop region 36.
  • Figure 2B shows wafer 4 after it has been subjected to the following additional steps: field oxidation, implant oxidation I, enhancement mask and doping, depletion mask and doping, and gate oxidation.
  • a field oxide layer 37 is formed from region 35 by heating the wafer in an atmosphere of steam.
  • the nitride/oxide mask that defines region 35 is then removed by a chemical wet etch and Kooi ribbon oxidation.
  • a very thin oxide layer (not shown) is grown to randomize the ion implantation.
  • region 32 then has P-type impurities implanted while region 33 has N-type impurities implanted. Portions of the implanted layers within regions 32 and 33 will ultimately define the channels for FETs 6 and 8.
  • a gate oxide layer is grown over regions 32 and 33 by heating the wafer in an atmosphere of oxygen and hydrogen chloride, this gate oxide layer, being very thin, is not illustrated.
  • Figure 2C shows wafer 4 after it has been subjected to the following additional steps: poly silicon (gate) deposition and doping, poly silicon (gate) definition, implant oxidation II, and source/drain doping and annealing.
  • a poly silicon layer is deposited by a chemical vapor deposition ("CVD") process, and phosphorus added by deposition and diffusion by heating in an atmosphere of phosphorus oxychloride or by ion implantation. Selected portions are then removed to leave segments 12 and 18 and polysilicon gates 6a and 8a. The portions of regions 32 and 33 not occupied by gates 6a and 8a are then subjected to the implantation of N-type impurities, such as arsenic, to define the source and drain of each of FETs 6 and 8.
  • CVD chemical vapor deposition
  • FIG. 2D shows wafer 4 after it has been subjected to the following additional steps:
  • a layer 40 of silicon dioxide is deposited by a CVD process, densified to remove pin-holes, and has portions removed over regions 45a, 45b, and 45c to define contact regions. These regions are defined by an etching step which removes the CVD oxide (as well as the thin layer of oxide over N+ layer portions 14 and 20).
  • the opening over region 45a exposes a portion of poly silicon segment 12; the opening over region 45b exposes N+ layer portion 14 that is contiguous with source 6b; and the opening over region 45c exposes a portion of polysilicon layer segment 18 and N+ layer portion 20 in the substrate to define a QBC region.
  • Figure 2E shows wafer 4 after being subjected to the following steps: deposition of poly silicon layer, doping of poly silicon layer, and masking and selective removal of the poly silicon.
  • a 1000-3000 angstrom layer of poly silicon is deposited by a chemical vapor deposition (CVD) process.
  • the poly silicon layer is then doped, for example, by ion implantation, with phosphorous, boron, or arsenic to control the resistance of the poly load resistor which will be formed from a portion of this layer.
  • the layer is then masked to define a poly load resistor 10d and contact segments 10a, 10b, and 10c
  • FIG 2F a further oxide layer has been deposited on wafer 4 by a CVD process and then masked and selectively etched away to leave only a portion 50a over poly load resistor 10d. It will be particularly noted here that the almost complete removal of this layer of oxide is in contrast to the prior art oxide layer 50 shown in figure 1B which is only etched away in the contact areas.
  • a layer of a refractory metal such as titanium
  • the wafer is then sintered causing the titanium to react with a majority of the underlying poly silicon segments to form a metal silicide, e.g. titanium silicide segments 60a, 60b, and 60c, in situ on the surface of the remaining poly silicon segments 10a, 10b, and 10c.
  • a metal silicide e.g. titanium silicide segments 60a, 60b, and 60c
  • the formation over all of the poly .silicon, except for the portion covered by the CVD oxide, of a silicide layer or strip above this poly silicon layer results in the effective segregation or separation of one or more portions of the entire doped poly silicon strip or line to form poly load resistor 10d out of only that portion of the doped polysilicon under CVD oxide 50a. In essence, then the remainder of the high resistance poly silicon strip is shorted out by the application of the low resistance silicide (conductive) layer over it.
  • the final product includes the application of a further oxide layer 62 of PVX which is doped to lower its melting point.
  • This layer serves as a dielectric between the metal sili cide and a further metal layer 65 which may be aluminum.
  • the device Before masking, the device may be heated to a temperature of from about 900-950°C to soften the PVX oxide to smooth out the step coverage.
  • the PVX oxide layer is then masked and selectively etched to provide contacts 65b and 65c between poly silicon segment 12 and metal silicide/poly silicon segments 60b/10b as well as provide contact 65a to the N+ region adjacent source 8b of depletion mode FET 8.
  • a metal layer 65 of aluminum or aluminum silicon is then applied and, after masking, etched to form contacts 65a, 65b and 65c
  • a protective oxide layer 80 is then applied in a final fabrication step prior to testing, mounting and attachment of leads.
  • Figure 3 illustrates the invention as embodied in a CMOS device having an N well 24 in which a P channel device 8' is formed having a source 8b', a drain 8c', and a gate 8a'. Parameters for such a device are also listed in the Appendices.
  • the lower melting point of the PVX oxide can also be used to permit the rounding or sloping of the edge 62a of the etched contact openings for contacts 65a, 65b and 65c to provide better metal fill in and avoid thinning of the sidewalls of the metal contact. This also provides better step coverage and planarization of the surface.
  • the dopant in the PVX layer must be the same as that used for the source and drain since heating the PVX after opening the contacts can result .in some of the dopant from the PVX entering the openings and depositing on the doped substrate.
  • contacts 65a, 65b and 65c are further enhanced by the elimination of the intermediate silicon dioxide layer 50 of prior art figure 1B except for CVD portion 50a over the poly load resistor 10d. This results in contacts 65a, 65b and 65c needing to only penetrate two thicknesses of oxide rather than three.
  • the invention provides an improved process and resulting device wherein the use of an intermediate silicide layer formed in situ in the surface of a polysilicon layer eliminates stepping between a metal layer and a silicide layer and permits subsequent higher temperature processing to provide better step coverage. Furthermore, by providing the silicide layer above the poly silicon layer or strips, and therefore the silicide portion of the contacts for the poly load resistor above the poly silicon, the CVD layer, used in the prior art to protect the poly load resistor from external doping, can be, in essence, eliminated by leaving the CVD oxide, after the anisotropic etching, only above the poly load resistor (except for the rounded portions of CVD oxide adjacent the poly silicon strips).
  • channel stop 36 1 micron 10 16 -10 17 B enhancement 32 1 micron 10 16 B depletion 33 0.3-0.6 micron 10 16 -10 17 As source/drain 8b, 8c 0.2-0.4 micron 10 20 As (N dopant)
  • source/drain 40 poly silicon gates 30 metal silicide 60a, 60b, 60c 5 poly silicon 10a, 10b, 10c 10 9

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Abstract

An improved integrated circuit structure characterized by enhanced step coverage and a method of making it. The structure comprises a base layer of silicon, a first oxide layer on the silicon layer, strips of poly silicon (10) having selected portions thereof reacted with a metal capable of forming a metal silicide (60) in situ on the surface of the poly silicon strips, a further oxide layer (62) over the metal silicide, and a metal layer (65) providing electrical contact to selected portions of the structure. The construction makes it possible to remove all of an intermediate oxide layer during manufacture except for an oxide layer (50a) above the poly load resistor (10d). This elimination of one oxide layer, together with the integration of the conductive metal silicide and underlying poly silicon into one layer and the rounding of the metal silicide edge with oxide spacers (58) via anisotropic etching of the intermediate oxide layer, permits better step coverage for the resulting structure .

Description

IMPROVED INTΕEGRATED CIRCUIT STRUCTURE HAVING INTERMEDIATE METAL SILICIDE LAYER AND METHOD OF MAKING SAME BACKG ROUND OF THE INVENTION
1. Field of the Invention
This invention relates to integrated circuit structures. More particularly, this invention relates to an integrated circuit structure having an intermediate metal suicide layer.
2. Background Art In the production of integrated circuit structures, such as, for example, static RAM chips, metal step coverage of contacts in the application of the final layers or rear end of the process is less than satisfactory when an intermediate metal layer is used. The presence of such an intermediate metal layer complicates the step coverage problem because higher temperature processing cannot be used to smooth out the step coverage since, if sufficient heat is used to soften the outer oxide layer to smooth out the step, the heat may adversely affect the intermediate metal layer.
Furthermore, it is customary to provide an undoped oxide layer over a polycrystalline silicon (hereinafter referred to as poly silicon or poly) layer having lightly doped portions to provide resistive paths or poly loads. This second, undoped, oxide layer is applied to prevent diffusion of the dopants from getting to the poly load area of the poly silicon layer. The use of an additional doped oxide layer over this oxide layer to provide a lower melting point oxide whereby the step coverage can be smoothed out means that contact openings must penetrate three layers of oxide before contacting the desired portion of the silicon substrate, such as, for example, the drain or source of an MOS transistor. This, in turn, can provide inadequate metal coverage in the contact opening resulting in thinning of portions of the metal on the wall of the contact opening.
While the use of an intermediate conductive layer of metal suicide is not unknown, prior uses of such a metal suicide have been made by concurrently sputtering silicon and a refractory metal to form a metal suicide layer which, after selective etching, is covered by a poly silicon layer. Subsequent doping of the poly layer results in the formation of resistive paths between the metal suicide portions. However, this use of two conductive layers provides a further step which must be covered during the subsequent application of further layers.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an improved integrated circuit structure and a process for making the structure wherein a metal suicide layer replaces a metal layer to permit higher temperature processing of subsequently applied layers.
It is ah another object of the invention to provide an improved integrated circuit structure and a process for making the structure wherein a metal suicide layer is used beneath a doped oxide layer instead of a metal layer whereby a subsequently applied doped oxide layer may be softened at high temperature to provide better step coverage without interfering with the underlying conductive layer of metal suicide.
It is yet another object of the invention to provide an improved integrated circuit structure and a pro cess for making the structure wherein the use of an intermediate metal suicide layer eliminates an intermediate oxide layer from the contact area thereby reducing the thickness of oxide layers which must be removed by a subsequent contact etch whereby better step coverage is provided.
It is a further object of the invention to provide an improved integrated circuit poiy load structure and a process for making the structure wherein a metal silicide interconnection layer is formed in situ by selective reaction with a poly silicon layer in areas unmasked by oxide. The metal silicide layer is connected to the poly load structure masked by the oxide, eliminating the need for an additional interconnect layer.
It is yet a further object of the invention to provide an improved integrated circuit structure and a process for making the structure wherein an intermediate metal silicide layer is provided having rounded oxide corners formed by anisotropic etching to provide better step coverage when subsequent layers are applied.
It is a still further object of the invention to provide an improved integrated circuit structure wherein conductive paths are formed in poly silicon, previously doped to provide a preselected resistivity and patterned to form strips, by the in situ formation of metal silicide with a portion of said poly silicon strips whereby poly load resistors are formed which are connected at their ends with portions of the conductive metal silicide.
These and other objects of the invention will be apparent from the drawings and description of the preferred embodiments.
DESCRIPTION OF THE DRAWINGS
Figure 1A is a sectional view of a portion of an integrated circuit device constructed in accordance with the invention.
Figure 1B is a sectional view of an integrated circuit device constructed in accordance with the prior art.
Figures 2A-2G are sectional views of the process steps performed to arrive at the structure of figure 1A.
Figure 3 is a sectional view of a portion of a CMOS device constructed in accordance with the invention.
Figure 4 is sectional end view of the structure of figure 1A taken along lines IV-IV.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Figure 1A illustrates an integrated circuit structure utilizing the invention in contrast to an integrated circuit structure having the same active and passive components formed therein in accordance with the prior art as shown in figure 1B.
In both figures 1A and 1B, a silicon wafer or substrate 4 has formed thereon an enhancement mode field effect transistor (FET) 6 and a depletion mode FET 8. FET 6 has a gate 6a, a source 6b, and a drain 6c. FET 8 has a gate 8a, a source 8b, and a drain 8c Metal silicide segments 60a, 60b, and 60c of figure 1A, in accordance with the invention, overlay poly silicon segments 10a, 10b, and 10c which overlaying segments together make connection to various portions of the circuit. Segments 10a/60a make connection to a poly silicon segment 12 which may, for example, be a word line in a static RAM cell. Segments 10b/60b make connection to a portion 14 of the N+ layer contiguous with source 6b. Portion 14 may, for example, be a voltage supply line with segments 10b/60b providing a low resistance strap. Segments 10c/60c provides a low resistance quasi-buried contact (QBC) of a poly silicon segment 18 to a portion 20 of the N+ layer contiguous with drain 6c. It will be immediately noticed that oxide layer 50 in prior art figure 1B has been eliminated in figure 1A except in the poly load area where this layer is designated as 50a.
It will also be noted that poly silicon segments
10a', 10b', and 10c' respectively overlay metal silicide segments 60a', 60b', and 60c' in prior art figure IB while the metal silicide segments 60a, 60b, and 60c overlay poly silicon segments 10a, 10b, and 10c in figure 1A. This is because the metal silicide layer, in the system of the invention, is formed in situ by depositing, on the surface of the poly silicon, a layer of a metal, such as titanium capable of reacting with the silicon, and then subsequently, sintering the materials to form titanium silicide in situ on the surface of the poly silicon layer. In the prior art practice exemplified by figure 1B, the metal silicide layer is first applied by codepositing silicon and the metal portion of the silicide to form the metal silicide layer after which the poly silicon layer is applied. Thus, in the prior art, two distinct layers are applied to the substrate; first the metal silicide layer and then the poly silicon layer. In the practice of the invention, the metal silicide forms in situ on the surface of the previously laid down and patterned poly silicon segments or strips without forming an extra layer from the standpoint of additional thickness.
In accordance with the invention, the metal silicide layer is formed by first patterning a deposited layer of poly silicon over the substrate and any previous layers thereon followed by application of a metal, such as a refractory metal, capable of reacting with the poly silicon to form the metal silicide. Examples of such metals include titanium, molybdenum, and tungsten.
The metal is preferably also one that is easily applied to the poly silicon as by sputtering or vapor deposition.
The metal used should be further capable of reacting with the poly silicon at a temperature of from about 600-800ºC, i.e., high enough to permit diffusion of the silicon into the metal reactant to from silicide, yet low enough to inhibit unwanted diffusion of the silicon to other parts of the device (e.g., the gap between two patterned poly silicon lines).
After the application of the metal coating over the silicon, the device is heated in a heat pulsed sintering environment to a temperature of about 600-800ºC and held at this temperature for from 10 to 20 seconds to permit reaction to occur forming the metal silicide and inhibit the side diffusion of silicon. The resultant metal silicide should have a resistivity of less than 5 ohms per square.
It should be noted here that it is preferable that not all of the poly silicon on which the metal deposits, react with the metal to form the desired metal silicide layer. It has been found that a small remaining layer of poly beneath the metal silicide serves to strengthen the metal silicide layer. Preferably, after reaction, the remaining poly should have a thickness of about .25 to .33 that of the silicide. To achieve this, the ratios of amounts of poly to metal deposited thereon must be controlled. The thickness ratio of silicon to metal should be kept to a 3:1 ratio or higher to provide the desired final ratio of silicide to poly.
Figures 2A-2G are cross-sectional views which illustrate the processing steps used in fabricating a typical N channel device utilizing the invention such as the device shown in figure 1A. As will be shown in figure 3, other devices can also be constructed utilizing the invention;, the devices illustrated in figures 1A, 2A-2G, and 3 being only representative of the invention. Representative process and characteristic parameters such as dopant concentration, implantation depth, layer thickness, and electrical resistance are tabulated in Appendices 1-3 below.
Figure 2A shows wafer 4 after it has been subjected to the following steps: preoxidation, barrier oxidation, nitride depostion source/drain mask, and field doping. More particularly, a thin barrier oxide layer is deposited by heating the wafer in an oxygen atmosphere, and a silicon nitride layer is then deposited by standard techniques. The source/drain regions are masked by selectively removing portions of the nitride layer and underlying oxide layer leaving nitride/oxide layer segments 30a and 30b over respective regions 32 and 33 to define the source/drain mask. The region 35 that is not masked is then subjected to the ion implantation of a P-type impurity to define a channel stop region 36.
Figure 2B shows wafer 4 after it has been subjected to the following additional steps: field oxidation, implant oxidation I, enhancement mask and doping, depletion mask and doping, and gate oxidation.
More particularly, a field oxide layer 37 is formed from region 35 by heating the wafer in an atmosphere of steam. The nitride/oxide mask that defines region 35 is then removed by a chemical wet etch and Kooi ribbon oxidation. Prior to doping regions 32 and 33, a very thin oxide layer (not shown) is grown to randomize the ion implantation. In a series of mask layer deposition and definition steps, region 32 then has P-type impurities implanted while region 33 has N-type impurities implanted. Portions of the implanted layers within regions 32 and 33 will ultimately define the channels for FETs 6 and 8. After these enhancement and depletion region definition and implantation steps, a gate oxide layer is grown over regions 32 and 33 by heating the wafer in an atmosphere of oxygen and hydrogen chloride, this gate oxide layer, being very thin, is not illustrated.
Figure 2C shows wafer 4 after it has been subjected to the following additional steps: poly silicon (gate) deposition and doping, poly silicon (gate) definition, implant oxidation II, and source/drain doping and annealing.
More particularly, a poly silicon layer is deposited by a chemical vapor deposition ("CVD") process, and phosphorus added by deposition and diffusion by heating in an atmosphere of phosphorus oxychloride or by ion implantation. Selected portions are then removed to leave segments 12 and 18 and polysilicon gates 6a and 8a. The portions of regions 32 and 33 not occupied by gates 6a and 8a are then subjected to the implantation of N-type impurities, such as arsenic, to define the source and drain of each of FETs 6 and 8.
Figure 2D shows wafer 4 after it has been subjected to the following additional steps:
CVD oxide deposition and densification 1, and QBC mask.
More particularly, a layer 40 of silicon dioxide is deposited by a CVD process, densified to remove pin-holes, and has portions removed over regions 45a, 45b, and 45c to define contact regions. These regions are defined by an etching step which removes the CVD oxide (as well as the thin layer of oxide over N+ layer portions 14 and 20). The opening over region 45a exposes a portion of poly silicon segment 12; the opening over region 45b exposes N+ layer portion 14 that is contiguous with source 6b; and the opening over region 45c exposes a portion of polysilicon layer segment 18 and N+ layer portion 20 in the substrate to define a QBC region.
Figure 2E shows wafer 4 after being subjected to the following steps: deposition of poly silicon layer, doping of poly silicon layer, and masking and selective removal of the poly silicon.
A 1000-3000 angstrom layer of poly silicon is deposited by a chemical vapor deposition (CVD) process. The poly silicon layer is then doped, for example, by ion implantation, with phosphorous, boron, or arsenic to control the resistance of the poly load resistor which will be formed from a portion of this layer.
The layer is then masked to define a poly load resistor 10d and contact segments 10a, 10b, and 10c
In figure 2F a further oxide layer has been deposited on wafer 4 by a CVD process and then masked and selectively etched away to leave only a portion 50a over poly load resistor 10d. It will be particularly noted here that the almost complete removal of this layer of oxide is in contrast to the prior art oxide layer 50 shown in figure 1B which is only etched away in the contact areas.
It should be further noted here that a high selectivity etch will be used on this oxide film which will stop at the densified oxide layer 40 beneath. Using an anisotropic plasma etch, the rounded oxide portions 58 as best seen in figure 4 will be formed adjacent the edges of the poly silicon segments or strips, thus further smoothing out the geometry of the layers resulting in better step coverage.
In figure 2G a layer of a refractory metal, such as titanium, has been applied, for example, by sputtering. In accordance with the invention, the wafer is then sintered causing the titanium to react with a majority of the underlying poly silicon segments to form a metal silicide, e.g. titanium silicide segments 60a, 60b, and 60c, in situ on the surface of the remaining poly silicon segments 10a, 10b, and 10c. In those portions where there is no underlying poly silicon, e.g., portion 50a of CVD oxide over poly load resistor 10d, no metal silicide will form, and the metal will remain in metallic form. This metal is then selectively removed from the substrate .with an acid and/or base etch, such as a solution of ammonium hydroxide, hydrogen peroxide, and water.
Thus, the formation over all of the poly .silicon, except for the portion covered by the CVD oxide, of a silicide layer or strip above this poly silicon layer (which has been previously lightly doped to provide a preselected resistivity and patterned to provide poly silicon strips) results in the effective segregation or separation of one or more portions of the entire doped poly silicon strip or line to form poly load resistor 10d out of only that portion of the doped polysilicon under CVD oxide 50a. In essence, then the remainder of the high resistance poly silicon strip is shorted out by the application of the low resistance silicide (conductive) layer over it.
The final product, as illustrated in Figure 1A, includes the application of a further oxide layer 62 of PVX which is doped to lower its melting point. This layer serves as a dielectric between the metal sili cide and a further metal layer 65 which may be aluminum. Before masking, the device may be heated to a temperature of from about 900-950°C to soften the PVX oxide to smooth out the step coverage. The PVX oxide layer is then masked and selectively etched to provide contacts 65b and 65c between poly silicon segment 12 and metal silicide/poly silicon segments 60b/10b as well as provide contact 65a to the N+ region adjacent source 8b of depletion mode FET 8. A metal layer 65 of aluminum or aluminum silicon is then applied and, after masking, etched to form contacts 65a, 65b and 65c A protective oxide layer 80 is then applied in a final fabrication step prior to testing, mounting and attachment of leads.
Figure 3 illustrates the invention as embodied in a CMOS device having an N well 24 in which a P channel device 8' is formed having a source 8b', a drain 8c', and a gate 8a'. Parameters for such a device are also listed in the Appendices.
If an N channel process is used rather than CMOS technology, the lower melting point of the PVX oxide can also be used to permit the rounding or sloping of the edge 62a of the etched contact openings for contacts 65a, 65b and 65c to provide better metal fill in and avoid thinning of the sidewalls of the metal contact. This also provides better step coverage and planarization of the surface. However, the dopant in the PVX layer must be the same as that used for the source and drain since heating the PVX after opening the contacts can result .in some of the dopant from the PVX entering the openings and depositing on the doped substrate.
It should also be noted in this regard that the over all topology with respect to contacts 65a, 65b and 65c is further enhanced by the elimination of the intermediate silicon dioxide layer 50 of prior art figure 1B except for CVD portion 50a over the poly load resistor 10d. This results in contacts 65a, 65b and 65c needing to only penetrate two thicknesses of oxide rather than three.
Thus, the invention provides an improved process and resulting device wherein the use of an intermediate silicide layer formed in situ in the surface of a polysilicon layer eliminates stepping between a metal layer and a silicide layer and permits subsequent higher temperature processing to provide better step coverage. Furthermore, by providing the silicide layer above the poly silicon layer or strips, and therefore the silicide portion of the contacts for the poly load resistor above the poly silicon, the CVD layer, used in the prior art to protect the poly load resistor from external doping, can be, in essence, eliminated by leaving the CVD oxide, after the anisotropic etching, only above the poly load resistor (except for the rounded portions of CVD oxide adjacent the poly silicon strips). This was not feasible when the silicide, including the silicide interface between the metal contacts and the poly load resistor, was beneath the poly silicon layer. This elimination of an intermediate layer of oxide, except over the poly load resistor, further enhances the topology adjacent the contact areas by reducing the number of oxide layers, and hence the total thickness, which the metal contacts must penetrate.
While the invention has been described with respect to its preferred embodiments, minor modifications may be made which do not depart from the scope of the invention which is defined by the appended claims .
Having thus described the invention, what is claimed is:
Appendix 1 - Implantation Parameters
Region Depth Dopant (molecules/cm3)
channel stop 36 1 micron 1016-1017 B enhancement 32 1 micron 1016 B depletion 33 0.3-0.6 micron 1016-1017 As source/drain 8b, 8c 0.2-0.4 micron 1020 As (N dopant)
*source/drain 8b',8c' 0.3-0.5 micron 1020 B (P dopant)
*N well 24 4.0 micron 1016 P
*for CMOS device
Appendix 2 - Layer Parameters
Layer Thickness Dopant (molecules/cm3) barrier oxide 200-700 A nitride 30a,30b 500-1000 A field oxide 37 0.7-1 micron implant oxide 100 A gate oxide 300 A poly silicon gate 7a, 8a 0.2-0.5 micron 1020-1021 P *gate 8a' 0.2-0.5 micron 1020-1021 P CVD oxide 40 0.2-0.5 micron metal silicide
60a, 60b, 60c 0.1-0.3 micron poly silicon 0.1-0.3 micron 10 19 As
10a, 10b, 10c PVX oxide 62 0.5-1.0 micron 4-8% P
*for CMOS device Appendix 3 - Electrical Characteristics
Layer/Region Resistivity (ohms/square)
source/drain 40 poly silicon gates 30 metal silicide 60a, 60b, 60c 5 poly silicon 10a, 10b, 10c 109

Claims

We claim:
1. An improved integrated circuit structure characterized by enhanced step coverage comprising a base layer of silicon, a first oxide layer on said substrate, a layer of poly silicon having selected portions thereof reacted with a metal capable of forming a metal silicide in situ in said poly silicon layer, a further oxide layer over said metal silicide, and a metal layer providing electrical contact to selected portions of said structure, said enhanced step coverage being provided by the elimination of an intermediate metal layer by the formation of said metal silicide in said poly silicon layer.
2. The structure of claim 1 wherein said metal reacted with said layer of poly silicon is a metal which may be applied at a temperature of less than 800º to said structure prior to reaction.
3. The structure of claim 2 wherein said metal reactant is applied to said poly silicon layer at a temperature sufficiently low to inhibit any excess side diffusion of poly siicon to the intended gaps between two adjacent poly silicon lines.
4. The structure of claim 1 wherein said metal reacted with said poly silicon is a refractory metal.
5. The structure of claim 4 wherein said metal is selected from the group consisting of titanium, molybdenum, and tungsten.
6. The structure of claim 1 wherein said metal is reacted with said poly silicon at a temperature of from 600-800ºC to form said metal silicide.
7. The structure of claim 6 wherein said metal is reacted with said poly silicon for from 10 to 20 seconds at said temperature in a heat pulsed sintering environment.
8. The structure of claim 1 wherein said layer of poly silicon and said metal reactant in contact therewith react to form a layer of metal silicide while leaving a small layer of silicon beneath to strengthen the metal silicide layer.
9. The structure of claim 1 wherein said metal silicide layer is the reaction product of a predetermined amount of said metal capable of reacting with said poly silicon at a temperature of 600-800°C whereby not all of said silicon in contact with said metal is reacted leaving an unreacted silicon layer beneath said metal in contact with the lower surface of said metal silicide of about .25 to .33 of the thickness of said silicide layer to reinforce said silicide layer.
10. The structure of claim 1 wherein said further oxide layer is doped to lower the melting point whereby subsequent application of a metal contact layer will provide rounded edges on said doped oxide layer to provide better step coverage.
11. The structure of claim 1 wherein said poly silicon layer comprises a poly silicon lightly doped to provide a predetermined resistivity and patterned to provide one or more strips of doped poly silicon.
12. The structure of claim 11 wherein said one or more doped poly silicon strips have rounded portions of oxide adjacent the edges of said strips formed by the deposition of a second, CVD, oxide layer over said strips followed by anisotropic etching of said second oxide layer, said rounded portions further enhancing the step coverage of said structure.
13. The structure of claim 12 wherein said anisotropic etching of said second oxide layer removes all of said second oxide layer except for said rounded edges, and one or more preselected portions above said poly silicon layer which permit formation of a poly load resistor beneath each of said preselected oxide portions, whereby said structure is further characterized by the substantial absence of one of the oxide layers above said silicon layer thereby providing further enhancement of the step coverage.
14. The structure of claim 13 wherein said metal silicide is formed in portions of said poly silicon strips not covered by said preselected portions of said second oxide to form one or more conductive strips whereby poly silicon portions beneath said preselected oxide portions do not react with said metal capable of forming a metal silicide thus forming poly load resistors out of said unreacted portions of said poly silicon beneath said preselected oxide portions, said poly load resistors being electrically connected at each end thereof to one of said conductive metal silicide strips.
15. An integrated circuit device, characterized by enhanced step coverage comprising: a substrate; at least one oxide layer on at least a portion of said substrate; one or more strips of poly silicon thereon doped to provide a preselected resistivity; rounded corners of oxide adjacent the edges of said strips formed by depositing a second oxide layer of CVD silicon dioxide over said strips followed by anisotropic etching of said second oxide layer to remove all of said second oxide layer except said rounded portions and one or more preselected portions above said poly silicon strips; a layer of metal silicide formed in portions of said poly silicon layer not covered by said second oxide portions, whereby poly load resistors are formed beneath said preselected portions of said second oxide layer, said metal silicide comprising a conductive material providing electrical coupling between selected portions of said device, including the ends of said poly load resistors; a further oxide layer overlying said metal silicide; and a metal layer over said further oxide layer; said device being characterized by improved step coverage made possible by said rounded oxide edges, substantial removal of said second oxide layer, and replacement of an intermediate metal layer by said metal silicide layer whereby said device may be heated prior to application of said metal contact layer to soften at least a portion of said further oxide layer to smooth the coverage over steps formed in said device.
16. An improved method for making integrated circuit devices having improved step coverage, said improvement comprising the steps of: forming an intermediate metal silicide layer by depositing, on a layer of poly silicon, a metal capable of reacting with said silicon to form a metal silicide; heating the device to a temperature sufficient to form said silicide; forming an oxide layer over said silicide layer; and then heating said device to soften a portion of said oxide layer to smooth out the step coverage by said oxide of underlying layers; said metal silicide layer having a melting point sufficiently high to permit said softening of said oxide layer without melting said metal silicide layer.
17. The method of claim 16 wherein said metal comprises a refractory metal.
18. The metal of claim 17 wherein said metal is selected from the group consisting of titanium, molybdenum, and tungsten.
19. The method of claim 18 wherein said device is heated to a temperature of from 600-800°C to form said metal silicide by reaction between said metal and said poly silicon.
20. The method of claim 19 wherein said device is heated to said temperature range for from 10 to 20 seconds to form said metal silicide.
21. The method of claim 16 including the step of depositing a sufficient amount of said metal to permit reaction of all of said metal and a majority of said poly silicon in contact with said metal, whereby said metal silicide layer formed by said reaction will have a poly silicon layer in contact with the lower surface of said metal silicide layer of from 0.25 to 0.33 the thickness of said metal silicide layer to thereby strengthen said silicide layer.
22. The method of claim 16 including the further steps of doping said layer of poly silicon to provide a preselected resistivity, patterning said poly silicon layer to form one or more poly silicon strips, depositing a a second oxide layer of silicon dioxide over said one or more underlying strips, and selectively removing portions of said second oxide layer by anisotropic etching to form round corners adjacent said poly silicon strips whereby subsequent step coverage is enhanced.
23. The method of claim 22 including the further step of providing one or more preselected portions of said oxide layer over said poly silicon strips by said anisotropic etching whereby said subsequent formation of said silicide layer forms a poly load resistor beneath each of said preselected portions of said oxide thus providing further enhancement of said step coverage by the effective elimination of said second layer of oxide from the resulting device.
24. The method of claim 23 including forming one or more conductive paths by said metal silicide on said resistive poly silicon strips except where said preselected portions of oxide remain whereby said poly load resistors are formed in said poly silicon strips under said preselected oxide portions and are connected adjacent the ends thereof to said conductive silicide paths.
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