High rate silicon nitride deposition method at low pressures

Details High rate silicon nitride deposition method at low pressures High rate silicon nitride deposition method at low pressures

1999-09-15

2003-01-14

Mills, Gregory (Department: 1763)

Semiconductor device manufacturing: process

Coating of substrate containing semiconductor region or of...

Insulative material deposited upon semiconductive substrate

C156S089150

Reexamination Certificate

active

06506691

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to methods for chemical vapor deposition (CVD) of silicon nitride, and more particularly to a method for CVD of silicon nitride employing a novel combination of flow rate, temperature and pressure to achieve improved film properties at a high rate of deposition at low pressure.
2. Brief Description of the Prior Art
Silicon nitride is commonly used in the manufacturing of semiconductor devices wherein it is deposited onto substrates (i.e. wafers) by chemical vapor deposition. Deposition is carried out in a variety of commercially available hot wall and cold wall reactors by placing a substrate in a vacuum chamber, heating the substrate and introducing dichlorosilane and ammonia. Deposition rates of approximately 10-70 angstroms per minute are achieved for low pressure processes (less than 1 Torr) as described in
Chemical Vapor Deposition for Microelectronics
(A. Sherman, Noyes Publications, Park Ridge, N.J. (1987), p. 77), and 50-100 angstroms per minute for high pressure processes (10-200 Torr) as described in detail in U.S. Pat. Nos. 5,482,739 and 5,629,043. Higher deposition rates of 2,000 angstroms per minute have been reported for plasma enhanced depositions as reported in U.S. Pat. No. 5,399,387, however silicon nitride films deposited by plasma enhancement do not have the same properties as silicon nitride deposited without plasma enhancement.
A typical prior art vertical furnace low pressure chemical vapor deposition (LPCVD) system is depicted in FIG.
1
and includes a chamber having a quartz tube
10
and chamber seal plate
12
into which is inserted a boat
14
for carrying a plurality of substrates
16
. Dichlorosilane and ammonia gases enter the gas injection tube (or tubes)
18
from the gas inlet tube (or tubes)
20
through the chamber seal plate
12
. The gases exit the process chamber through the seal plate
12
and out the exhaust port
24
. A plurality of heater elements
26
are separately controlled and adjustable to compensate for the well-known depletion of the feed gas concentration as the gas flows from the gas injection tube
18
to the chamber exhaust port
24
. This type of deposition system typically operates in the 200 mTorr to 500 mTorr range (200×10
−3
Torr to 500×10
−3
Torr). Typical gas flows are 30 sccm of dichlorosilane, 200 sccm of ammonia and 1-2 slm of a carrier gas such as nitrogen or hydrogen. Operating at these low concentrations of dichlorosilane and ammonia results in low deposition rates of typically 10-60 angstroms per minute. Operation at higher concentrations of reactant gases results in non-uniform deposition across the substrates and great differences in the deposition rate from substrate to substrate. Increased flow rates could improve the deposition uniformity at higher pressures, however increased gas flow increases the reactive gas pressure at the injection tube holes causing gas phase nucleation resulting in particles being deposited on the substrates. Other problems associated with this reactor include film deposition on the interior quartz tube
10
and gas injection tube
18
. This unwanted deposition decreases the partial pressure of the reactive feed gas concentration near the surface of the substrate
16
resulting in a reduced deposition rate and potential contamination caused when film deposited on the wall of tube
10
and injector tube
18
flakes off and deposits on the substrates
16
. In addition, a major problem is the accumulation of reactant gas by-products, mainly composed of ammonium chlorides at the exhaust tube resulting in contamination of the substrates. Attempts have been made to deposit silicon nitride using silane in place of dichlorosilane to eliminate the ammonium chloride by products, however such attempts have resulted in non-acceptable film uniformity.
Another prior art reactor is illustrated in FIG.
2
and described in detail in U.S. Pat. No. 5,108,792. A substrate
28
is placed on a rotating substrate carrier
30
, enclosed in a vacuum tight chamber having an upper quartz dome
32
and a lower quartz dome
34
and associated chamber wall
36
. The substrate
28
is heated by upper lamps
38
and lower lamps
40
. Reactant gases are injected through gas input tube
42
and exhausted through exhaust tube
44
. This reactor overcomes some of the limitations of the vertical furnace reactor of FIG.
1
. The reactor can be operated at higher pressures than vertical LPCVD furnaces and does not have an injector tube and its associated problems.
U.S. Pat. No. 5,482,739 entitled “Silicon Nitride Deposition” describes the process conditions and reactor modifications required to deposit silicon nitride at a rate of 70 angstroms per minute in the reactor described in U.S. Pat. No. 5,108,792. The reactor modifications reduce the accumulation of ammonium chloride by-products. The typical process pressure range is 10-200 Torr for depositing silicon nitride on a silicon substrate. This process pressure is approximately 2 orders of magnitude higher than the nominal deposition pressure of a similar LPCVD process.
Increased deposition rates are very desirable, resulting in higher machine productivity and more importantly reducing the time the substrates are exposed to high temperatures, i.e. >600° C. Reduced time at high temperatures is important during the fabrication of semiconductor devices as the device sizes become smaller, because elevated temperatures, i.e. >600° C., for any extended time cause unwanted changes in semiconductor device structure. A disadvantage of the prior art low pressure methods is their low deposition rate. A disadvantage of the prior art high pressure methods is that operating at high pressure can cause a gas phase reaction which can produce particulate contamination on the wafer.
U.S. Pat. No. 5,551,985 by Brors et al. describes a CVD reactor that provides improved uniformity in heating a wafer, and a highly uniform gas flow across the surface of a wafer. U.S. patent application Ser. No. 08/909,461 filed on Aug. 11, 1997, and Ser. Nos. 09/228,835 and 09/228,840 filed on Jan. 12, 1999, the disclosures of which are incorporated herein by reference, describe wafer chambers in which related processes may also be used.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for rapid deposition of a silicon nitride film having a highly uniform, smooth surface.
It is a further object of the present invention to provide a method of operating a CVD reactor that optimizes the rate and uniformity of deposition of silicon nitride.
It is a still further object of the present invention to provide a method of operating a CVD reactor that results in a high degree of uniformity in deposition of silicon nitride from one run to another.
It is another object of the present invention to provide a method of operating a CVD reactor that optimizes the rate and uniformity of deposition of silicon nitride, and substantially reduces the accumulation of ammonium chloride by-products.
Briefly, a preferred embodiment of the present invention includes a method of operating a CVD reactor providing a novel combination of wafer temperature, gas flow and chamber pressure resulting in both rapid deposition and a uniform, smooth film surface. According to the method, a wafer is placed in a vacuum chamber wherein a reactant gas flow of silane and ammonia is directed in parallel with the wafer surface via a plurality of temperature controlled gas injectors, the gas being confined to a narrow region above the wafer. The gas is injected at a high velocity, causing the deposition rate to be limited only by the rate of delivery of unreacted gas to the wafer surface and the rate of removal of reaction byproducts. The high velocity gas stream passing across the wafer has the effect of thinning the layer adjacent the wafer surface containing reaction by-products, known as the “boundary layer,” resulting in faster delivery of the desired reactant gas to the wafer surfa

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