# Patent

## Title
Gate stack having nitride layer

## Abstract

A semiconductor structure includes a semiconductor substrate, a gate layer containing silicon on the semiconductor substrate, a metallic layer on the gate layer, and a nitride layer on the metallic layer. The gate layer contains a P+ region and an N+ region.

## Background
BACKGROUND
Modern integrated circuits are constructed with up to several million active devices, such as transistors and capacitors, formed in and on a semiconductor substrate. Interconnections between the active devices are created by providing a plurality of conductive interconnection layers, such as polycrystalline silicon and metal, which are etched to form conductors for carrying signals. The conductive layers and interlayer dielectrics are deposited on the silicon substrate wafer in succession, with each layer being, for example, on the order of 1 micron in thickness.
A gate structure is an element of a transistor. FIG. 1 illustrates an example of a gate stack 8. A semiconductor substrate 10 supports a gate insulating layer 16, which overlaps doped regions (source/drain regions) in the substrate (12 and 14), and the gate insulating layer supports a gate 18, which is typically polycrystalline silicon. On the gate is a metallic layer 30. The metallic layer may be separated from the gate by one or more other layers, such as nitrides, oxides, or silicides, illustrated collectively as barrier layer 20. The metallic layer may in turn support one or more other layers (collectively 40), such as nitrides, oxides, or silicides. Oxide 22 may be formed on the sides of the gate to protect the gate oxide at the foot of the gate stack; and insulating spacers 24 may be formed on either side of the gate stack. Furthermore, contacts to the source/drain regions in the substrate, and to the gate structure, may be formed.
Self-aligned contacts (SAC) allow the design of a semiconductor device to have a distance between the gate and the via contact to the substrate, to be at most one-half the minimum gate width; the contact may even be designed to overlay the gate. Typically, SAC uses a nitride layer on the gate stack, together with spacers that include nitride, to prevent a misaligned contact from electrically contacting the gate itself. If the nitride were not present, then the etch used to form the hole which will become the contact would pass through the dielectric layer all the way to the gate. When present, the nitride layer and spacers act as an etch stop, preventing misalignment from forming a hole all the way to the gate, and therefore allowing design of the device to have a much smaller average distance between the contact and the gate.
The nitride layer on the gate stack has at least a thickness of 800 angstroms when used for forming SAC. If used only for other purposes, such as an etch-stop layer or a hard mask, a thickness of less than 800 angstroms is used. Also, the thickness of at least 800 angstroms is the thickness after the dielectric layer has been formed; the nitride layer is usually thicker when originally formed, allowing for a loss of about 500 angstroms during the gate etch (i.e. thickness for the hard mask function), and a loss of about 200 angstroms during nitride spacer formation.
There is an ongoing need to reduce the size of the elements within integrated circuits and semiconductor structures. As the size of an element is reduced, it becomes more likely that irreversible changes will occur in the compositions of the various layers and regions within an element. These changes are typically related to diffusion, which is dependent on the elevated temperatures used for processing and the length of time the temperatures are maintained. It is thus desirable to perform processing at cooler temperatures and/or to reduce the time necessary to carry out the processing.
BRIEF SUMMARY
In a first aspect, the present invention is a method of making a semiconductor structure, comprising depositing a nitride layer on a metallic layer. The metallic layer is on a gate comprising silicon, the gate is on a semiconductor substrate, and the gate comprises a P+ region and an N+ region.
In a second aspect, the present invention is a semiconductor structure comprising a semiconductor substrate, a gate comprising silicon on the semiconductor substrate, a metallic layer on the gate, and a nitride layer on the metallic layer. The gate comprises a P+ region and an N+ region.
In a third aspect, the present invention is a semiconductor structure comprising a semiconductor substrate, a gate comprising silicon on the semiconductor substrate, a barrier layer on the gate, a metallic layer on the barrier layer, and a nitride layer on the metallic layer. The gate comprises a P+ region and an N+ region, the barrier layer comprises tungsten, and the metallic layer comprises tungsten.
The term “distance” means the length between the closest edges of the two objects.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a gate stack structure.
FIGS. 2-7 illustrate a method of forming the structure of FIG. 8.
FIG. 8 shows a gate stack of the present invention.
FIG. 9 shows the gate stack of FIG. 8 after further processing.
FIG. 10 shows a gate layer having a P+ region (64) and an N+ region (63) separated by a region (6) which is on an isolation region (4) of the substrate, having a width of at most 0.4 microns.



DETAILED DESCRIPTION
The present invention includes forming a nitride layer on a gate stack at lower temperatures than are typically used, by plasma enhanced chemical vapor deposition (PECVD). The present invention is particularly suited to forming a gate stack including a gate having both P+ and N+ doping regions, where the lower temperature of the PECVD inhibits excessive diffusion of dopants.
Referring to FIG. 2, a gate insulating layer 102 is on a semiconductor substrate 100. The semiconductor substrate may be a conventionally known semiconductor material. Examples of semiconductor materials include silicon, gallium arsenide, germanium, gallium nitride, aluminum phosphide, and alloys such as Si1-xGex and AlxGa1-xAs, where 0≦x≦1. Preferably, the semiconductor substrate is silicon, which may be doped or undoped. The gate insulating layer 102 may be a conventionally known insulating material. For example, the gate insulating layer may contain silicon oxide or silicon oxynitride.
Referring to FIG. 3, a gate layer 105 may be formed on the gate insulating layer. The gate layer may contain a variety of semiconducting materials. Typically, a gate layer contains polycrystalline silicon (poly) or amorphous silicon. The gate layer may be doped with one type of dopant (P+ or N+), or it may contain both types of dopants in discrete regions. A split gate is a gate layer containing both P+ and N+ doping regions.
In the case of a split gate, those regions of the gate that are P+ doped (such as with B or BF2+) are over N− doped channel regions of the substrate, forming a PMOS; those regions of the gate that are N+ doped (such as with As+ or phosphorus+) are over P− doped channel regions of the substrate, forming an NMOS. The P+ and N+ doping regions of the gate are separated by a region which is on an isolation region of the substrate; this isolation region has a width of at most 0.4 microns, more preferably at most 0.36 microns. The doping of the regions of the gate is preferably carried out after forming the gate, by masking and doping each region separately, or by an overall doping of the gate with one dopant type, and then masking and doping only one region with the other dopant type (counter doping).
Referring to FIG. 4, a barrier layer 115 may optionally be formed on the gate layer. The optional barrier layer may contain a variety of materials, including nitrides, silicides, and oxides, and is preferably a conductive material. For example, the barrier layer may contain refractory silicides and nitrides. Preferably, the barrier layer contains silicon nitride, or a nitride or silicide of a metal such as tantalum, titanium, niobium or tungsten, for example tungsten nitride.
Referring still to FIG. 4, a metallic layer 125 may be formed on the gate layer, or the barrier layer 115, if it is present. The metallic layer 125 may contain a variety of metal-containing materials. For example, a metallic layer may contain aluminum, copper, tantalum, titanium, tungsten, or alloys or compounds thereof. The metallic layer may be formed, for example, by physical vapor deposition (PVD) of the metal, or by low pressure chemical vapor deposition (LPCVD) of a mixture of a metal halide and hydrogen.
Referring to FIG. 5, a barrier layer 135 may optionally be formed on the metallic layer. The formation of the second optional barrier layer may be performed as described for the first optional barrier layer 115.
Referring still to FIG. 5, nitride layer 145 may be formed on the metallic layer by a variety of methods, including chemical vapor deposition (CVD). Preferably, the nitride layer is silicon nitride formed by PECVD. The nitride layer may vary in composition, so that the top of the nitride layer is anti-reflective, for example so that the top of the nitride layer is silicon rich silicon nitride, or silicon oxynitride; this layer may also act as a hard mask to protect the nitride layer during subsequent etches. Alternatively, a separate anti-reflective layer (ARC) may be formed.
Preferably, the nitride layer is formed rapidly at a relatively low temperature. For example, if the gate layer contains both P+ and N+ doping regions, diffusion of the dopants may occur if the wafer is maintained at sufficiently high temperatures for a prolonged period of time. Thus, it is desirable that any high temperature processing is performed only for relatively short periods of time. Likewise, it is desirable that any lengthy processing is carried out at relatively low temperatures. Preferably, the nitride layer is formed at a temperature of at most 750° C., if the atmosphere is substantially devoid of oxygen, or in a reducing environment. Under typical conditions, a temperature of at most 600° C. is preferred, at most 450° C. is more preferred. A temperature of at least 350° C. is preferred. The depositing of the nitride layer is preferably carried out at a temperature and for a time that does not result in substantial diffusion between the P+ region and the N+ region in a split gate.
“Substantial diffusion between the P+ region and the N+ region” of the gate means that the threshold voltage (VT) one or both of the PMOS or NMOS changes by more than 20 mV, more preferably 10 mV, even more preferably 5 mV. In order to determine if a nitride deposition results in substantial diffusion between the P+ region and the N+ region of a particular split gate, a single PMOS or NMOS is formed, with the other part of the gate forming a comparatively very large reservoir of the opposite doping type, separated by an isolation region of the same size as the actual device. The PMOS or NMOS is formed using the nitride deposition, and using BTBAS (tetra(bis-t-butylamino)silane, also known more simply as bis-t-butyl amino silane) and ammonia at about 550° C. for spacer deposition, and low temperature selective oxidation, as used in the example.
Referring to FIGS. 6-8, each layer may be patterned to form the gate stack. The patterning may be accomplished, for example, by conventional photolithographic and etching techniques. Referring to FIG. 6, the nitride layer may be etched to form a patterned nitride layer 150, for example by forming a patterned photoresist on nitride layer 145 (FIG. 5) and then etching the exposed portions of the layer. The nitride etching may be carried out by conventional nitride etch techniques, for example by exposure to a plasma formed from a mixture of CF4 and CHF3.
Referring to FIG. 7, the patterned nitride layer may be used as a hard mask for the etching of the metallic layer 125 (FIG. 6) to form a patterned metallic layer 130. Referring to FIG. 8, the patterned nitride layer and the patterned metallic layer may be used as a hard mask for the etching of the gate layer 105 (FIG. 7) to form patterned gate layer 110. The gate etching may be carried out by conventional gate etch techniques, for example by exposure to a plasma formed from chlorine, hydrobromic acid and/or oxygen.
FIG. 8 thus illustrates a gate stack 200 which may be formed on a semiconductor wafer. Semiconductor substrate 100 supports a gate insulating layer 102, which in turn supports a gate layer 110. The gate layer supports a metallic layer 130, which may optionally be separated from the gate layer by barrier layer 120. The metallic layer may optionally support a barrier layer 140. The nitride layer 150 is on the metallic layer 130, or optionally on the layer 140 above the metallic layer.
Further processing of the gate structure may include forming sidewall oxide regions 160 on gate layer 110 and forming spacers 170 (preferably containing nitride) on the sides of the stack. Furthermore, a dielectric layer 180 maybe formed on the nitride layer, and contacts 190 formed through the dielectric to the substrate, as illustrated in FIG. 9, as well as contacts to the gate itself. After dielectric layer is formed, the nitride layer has a thickness of at least 800 angstroms, preferably at least 1100 angstroms, so that it may be used to allow formation of SAC. Other processing may be used to complete formation of semiconductor devices from the semiconductor structure. For example, source/drain regions 12, 14 may be formed in the substrate, additional dielectric layers may be formed on the substrate, and contacts and metallization layers may be formed on these structures. These additional elements may be formed before, during, or after formation of the gate stack.
The related processing steps, including the etching of the gate stack layers and other steps such as polishing, cleaning, and deposition steps, for use in the present invention are well known to those of ordinary skill in the art, and are also described in Encyclopedia of Chemical Technology, Kirk-Othmer, Volume 14, pp. 677-709 (1995); Semiconductor Device Fundamentals, Robert F. Pierret, Addison-Wesley, 1996; Wolf, Silicon Processing for the VLSI Era, Lattice Press, 1986, 1990, 1995 (vols 1-3, respectively), and Microchip Fabrication 4rd. edition, Peter Van Zant, McGraw-Hill, 2000.
The semiconductor structures of the present invention may be incorporated into a semiconductor device such as an integrated circuit, for example a memory cell such as an SRAM, a DRAM, an EPROM, an EEPROM etc.; a programmable logic device; a data communications device; a clock generation device; etc. Furthermore, any of these semiconductor devices may be incorporated in an electronic device, for example a computer, an airplane or an automobile.
EXAMPLE
Example 1
Formation of a Gate Structure
The following detailed steps were used to form the gate stack having a split gate:















Nitrogen-containing gate oxide formation



Poly deposition—single amorphous gate deposit



Mask for P-doping



P+ poly implantation



N-well implantation



P-channel implantation



P+ poly implantation strip resist



Mask for N-doping



P-well implantation



N+ poly implantation



N-channel implantation



N+ poly implantation strip resist



Tungsten PVD, sputtering (nitrogen + argon, then argon only)



Nitride—PECVD



Deposit ARC and Resist



Etch mask for nitride



Nitride etch—ARC and silicon nitride



Remove resist



Tungsten and Poly etch



Selective oxidation



N doping of source/drain shallow extension region mask



As implant



Boron halo implant



Stripping &  cleaning



P doping of source/drain shallow extension region mask



P-type (boron) implant



As halo implant



Stripping &  cleaning



Nitride spacer deposition (BTBAS chemistry)



Spacer etch



source/drain implant



Dielectric deposition/planarization/mask for contacts



self-aligned contact (SAC) etch



SAC etch clean using N2/H2 or NF3H2/Ar, and then solvent










The silicon nitride layer had a thickness of 1300 angstroms (although the actual amount deposited was greater since silicon nitride is lost during the poly etch), the tungsten layer had a thickness of 400 angstroms, the tungsten nitride layer had a thickness of 75 angstroms, and the poly layer had a thickness of 800 angstroms. The contacts having a width of 0.13 microns at the top, and a width of 0.05 microns at the bottom.
The resist material was removed by ashing, and the stack was cleaned by treating the wafer with EKC265TM (EKC, Hayward, Calif.; a mixture of 2-(2 aminoethoxy) ethanol, hydroxylamine and catechol) by spinning with spraying (using a spray tool) at 65-85° C. for 10 minutes, then 2 minutes at 20° C. followed by rinsing with deionized water, to prevent undesirable oxidation of the tungsten.
The exposed sides of the poly were covered with a layer of oxide about 50-70 angstroms thick by the selective oxidation. This was carried out by exposing the stack to a mixture of hydrogen and oxygen (5-15% steam) at a temperature of 750° C. to selectively oxidize the poly relative to the tungsten and tungsten nitride.
Etching of the nitride (nitride on poly contact mask etch) was carried out with a plasma at a pressure of 35 mT, at a power of 280 W, a temperature of 15° C. The gas composition for the main etch was CHF3 at 30 sccm, Ar at 60 sccm, and O2 at 10 sccm. The clean was carried out with plasma ashing in two steps, followed by a solvent clean:
Step 1:
pressure of 2 T, temp. of 185° C., microwave power of 800 W, gas: O2 at 3750 sccm, N2 at 375 sccm;
Step 2:
same values, except temp. of 200° C. and microwave power of 1400 W.
Solvent Clean:
EKC 265TM, with a spray tool: temp. of 70° C. for 10 minutes, and an extra 2 minutes at 20° C., followed by rinsing with deionized water and then spin drying in N2. Alternatively this may be done with H2SO4 at 150° C. twice for 10 minutes each and then spin drying in N2.
Etching to form contacts (SAC etch) was carried out with a plasma at a pressure of 55 mT, a power of 500 W, a temperature of 35° C., with the magnet at 20 Gauss, a gas of CF4 at 5 sccm, CHF3 at 10 sccm, C2H2F4 at 10 sccm, and Ar at 90 sccm, as the ARC etch; and as the main etch a pressure of 55 mT, a power of 500 W, a temperature of 35° C., with the magnet at 25 Gauss, a gas of CHF3 at 80 sccm, C2H2F4 at 8 sccm, and Ar at 90 sccm. The clean was carried out with plasma ashing in two steps, followed by a solvent clean:
Step 1:
pressure of 400 mT, temp. of 20+/−5° C., RF power of 420 W, gas: O2 at 400 sccm.
Step 2:
pressure of 750 mT, temp. of 20+/−5° C., RF power of 420 W, gas: N2 at 400 sccm, H2 at 400 sccm, and NF3 at 5 sccm; or alternatively:
pressure of 750 mT, temp. of 40+/−5° C., RF power of 350 W, gas: CF4 at 20 sccm, N2/5% H2 at 200 sccm, and O2 at 500 sccm.
Solvent Clean:
EKC 265TM, with a spray tool: temp. of 70° C. for 10 minutes, and an extra 2 minutes at 20° C., followed by rinsing with deionized water and then spin drying in N2. Alternatively this may be done with H2SO4 at 150° C. twice for 10 minutes each and then spin drying in N2.

## Claims

1. A method of making a semiconductor structure, comprising:
depositing a first barrier layer on a continuous metallic layer, the continuous metallic layer disposed on a second barrier layer, the second barrier layer disposed on a gate layer comprising silicon, and the gate layer disposed on a semiconductor substrate, wherein the gate layer comprises a P+ region and an N+ region, and wherein the P+ and N+ regions are separated by a region which is on an isolation region of the substrate having a width of at most 0.4 microns; and
depositing a nitride layer on the first barrier layer, the depositing comprising a PECVD process carried out at a temperature of approximately 350° C.

2. The method of claim 1, wherein the continuous metallic layer comprises tungsten.

3. A method of making a semiconductor device, comprising:
forming a semiconductor structure by the method of claim 1, and
forming a semiconductor device from the semiconductor structure.

4. A method of making an electronic device, comprising:
forming a semiconductor device by the method of claim 3, and
forming an electronic device comprising the semiconductor device.

5. The method of claim 1, further comprising forming a dielectric layer on the nitride layer.

6. The method of claim 5, further comprising forming a contact through the dielectric layer to the substrate.

7. The method of claim 6, wherein a distance between the gate layer and the contact to the substrate is at most one-half a minimum gate width.

8. The method of claim 1, wherein the second barrier layer comprises titanium silicide.

9. The method of claim 1, wherein the first barrier layer comprises titanium.

10. The method of claim 1, wherein the second barrier layer comprises titanium.

11. The method of claim 1, wherein the nitride layer has a thickness of at least 1100 angstroms.