Method for deposition on a semiconductor wafer

A nitride-coated boat is used in a method for depositing phosphosilicate glass (PSG) on semiconductor wafers. Because stainless steel is the material typically used for the boat for PSG deposition, the linear coefficient of thermal expansion of the boat is significantly greater than that of PSG. Consequently, cooling the boat after PSG deposition tends to cause flaking of PSG from the boat. The nitride-coating on the boat buffers the contraction differential between the PSG and stainless steel to significantly reduce flaking of PSG.

FIELD OF THE INVENTION 
This invention relates, in general, to semiconductor purposes, and more 
particularly to deposition on a semiconductor wafer. 
BACKGROUND OF THE INVENTION 
Deposition on semiconductor wafers is typically achieved by inserting a 
boat, which holds a number of wafers, into a chamber, commonly known as a 
tube. While inside the tube, gases are introduced which cause a deposition 
on the wafers. While the wafers are receiving the deposition, so is the 
boat which is holding the wafers. The deposition on the boat tends to 
flake off after removal of the boat from the tube. For example, in the 
case of depositing phosphosilicate glass (PSG), phosphorous doped silicon 
oxide, a stainless steel boat is typically used which has a different 
coefficient of expansion than PSG. Consequently, as a PSG-coated boat 
changes temperature, PSG particles flake off the boat. Upon removal of the 
boat from the tube, the wafers can be removed, but not before at least 
some flaking occurs. If new wafers are placed in the boat, PSG particles 
from the boat will flake onto those wafers. Subsequent deposition of PSG 
will not be effected where the particles have flaked onto the wafers. 
These wafers are then removed and cleaned. The cleaning process removes 
the particles, leaving a hole in the PSG coating. Any semiconductor die 
formed in an area which includes such a hole will be defective. 
It is desirable to wait until the temperature of a PSG-coated boat has 
stabilized before placing wafers in the boat. Even so, flaking of PSG 
particles continues, presumably because they have been loosened during the 
cool down period. Although quartz boats will reduce flaking substantially, 
quartz boats are not as durable as stainless steel, and break much more 
easily. Stainless steel boats for PSG deposition are much more desirable 
than quartz boats. 
SUMMARY OF THE INVENTION 
An object of the subject invention is to provide an improved method for 
deposition on a semiconductor wafer. 
Another object of the invention is to provide a stainless boat with reduced 
flaking for use in deposition on semiconductor wafers. 
Yet another object of the invention is to provide a method for reducing 
flaking of PSG onto semiconductor wafers. 
These and other objects of the subject invention are achieved by a 
nitride-coated, metal boat used for holding a semiconductor wafer in a 
process for depositing a material on the wafer, wherein the material has a 
significantly different linear coefficient of thermal expansion than that 
of the metal of the nitride-coated boat.

DESCRIPTION OF A PREFERRED EMBODIMENT 
Shown in FIG. 1 is an apparatus 10 for providing boats 11, 12, 13 and 14 
with improved flaking characteristics. Apparatus 10 includes a tube 16 
having a pump 17 at one end and gas entry pipes 18 and 19 at the other 
end. Containers 21 and 22 of nitrogen (N.sub.2) and ammonia (NH.sub.3), 
respectively, are coupled to tube 16 via pipe 18 under the control of flow 
controllers 23 and 24, respectively. Containers 26 and 27 of nitrogen and 
dichorosilane (S.sub.i H.sub.2 Cl.sub.2, hereinafter referred to as DCS), 
respectively, are coupled to tube 16 via pipe 19 under the control of flow 
controllers 28 and 29, respectively. Pipes 18 and 19 are shown for 
convenience as one being over the other but in practice are preferably 
side by side. Heating coils 31 surround tube 16 to heat tube 16 to a 
desired temperature. A controller 32 provides signals to flow controllers 
23, 24, 28, and 29. Tube 16 is of conventional quartz crystal which is 
commercially available. Controller 32, pump 17, and heating coils 31 are 
all commercially available and known for use in nitride deposition. 
Boats 11-14 are stainless steel with perforations and are shown inside tube 
16 with boat 14 nearest the pump end and boat 11 nearest the other end. To 
deposit nitride on boats 11-14, a typical procedure for depositing nitride 
on silicon wafers is used. With tube 16 maintained at 650.degree. C. by 
heating coils 31, boats 11-14 are inserted into tube 16. Tube 16 is pumped 
down by pump 17 to a vacuum of in the order of 1 milliTorr for 10 minutes. 
Nitrogen from container 21 is flowed through tube 16 to purge tube 16 at a 
pressure of about 200 milliTorr for 10 minutes. Tube 16 is then again 
pumped down. Ammonia from container 22 is flowed through tube 16 at a rate 
of 135 SCCM for 3 minutes before introducing DCS from container 27. After 
the 3 minutes DSC is flowed through tube 16 at a rate of 30 SCCM while 
ammonia is still flowing at 135 SCCM. Pressure in tube 16 is about 500 
milliTorr while both ammonia and DCS are flowing. With both DCS and 
ammonia flowing, nitride is being deposited on the wafers. Consequently, 
the time duration chosen for flowing ammonia and DSC together is directly 
related to the desired nitride thickness on boats 11-14, i.e., the thicker 
the desired nitride, the longer the required time duration. Typically, the 
time duration is determined experimentally. An ammonia to DCS ratio of 4.5 
to 1 is maintained even though the nitride reaction implies a 10 to 3 
ratio in order to ensure that the DCS fully reacts. 
After the desired time duration has lapsed, the DCS flow is stopped while 
the ammonia flow continues for an additional three minutes to ensure there 
is no DCS residue. Tube 16 is pumped down again to complete the process of 
clearing tube 16 of ammonia and DCS. Nitrogen from container 21 is flowed 
into tube 16 for 5 minutes with pump 17 turned off to begin backfilling 
tube 16. Nitrogen from container 26 is flowed until atmospheric pressure 
is reached to complete the backfilling process. Nitrogen from container 21 
is provided at a relatively low pressure compared to that provided by 
container 26. The backfilling process begins by introducing nitrogen from 
container 21 so that the pressure inside tube 16 does not change too 
rapidly. After tube 16 has been backfilled, boats 11-14 are removed. Boats 
11-14 are consequently coated with nitride. As such, boats 11-14 are 
useful for holding semiconductor wafers during an oxide deposition process 
step to reduce flaking of oxide from boats 11-14. A thickness range of 
3000-4000 A of nitride on boats 11-14 has been shown to provide 
significantly reduced flaking. Greater thicknesses of nitride would 
presumably also show significantly reduced flaking. Boats 11-14 are 
particularly useful in an oxide deposition step which includes phosphorous 
for a passivation layer. Such a layer is commonly known as PSG for 
phosphosilicate glass. A PSG layer is normally a relatively thick layer 
for which special considerations are necessary for ensuring uniformity. 
One such consideration is requiring that boats 11-14 have many 
perforations. Such perforations in a solid silicon boat cause such boat to 
be extremely fragile, essentially impractical in a production environment. 
Shown in FIG. 2 is an apparatus 36 which is very similar to apparatus 10 of 
FIG. 1 but which is set up for depositing PSG. Apparatus 36 includes a 
tube 37, having a pump 38 at one end and gas entry pipes 39 and 40 shown 
for convenience as being at the other end. In practice it may be desirable 
to have gas entry pipes 39 and 40 at the same end as pump 38. Gas entry 
pipes 39 and 40 have extensions 41 and 42, respectively, into tube 16. 
Extension 41 is comprised of four separate small pipes (not individually 
shown) which terminate at points 43, 44, 45 and 46, to correspond to boat 
locations in tube 37. Extension 42 extends the length of tube 37 
underneath boats 11, 12, 13, and 14. Extension 42 is perforated all along 
its length. In addition there is a gas entry point 47 from gas entry pipe 
40 at the end opposite to pump 38. Gas entry pipes can also be located on 
the same side of tube 16 as pump 38 but points 43, 44, 45, 46 and 47 would 
be the same. Containers 51, 52, and 53 of nitrogen, phosphine (PH.sub.3), 
and silane (S.sub.i H.sub.4), respectively, are coupled to tube 37 via 
pipe 39 under the control of flow controllers 54, 55, and 56, 
respectively. Containers 58 and 59 of silicon and oxygen (O.sub.2), 
respectively, are coupled to tube 37 via pipe 40 under the control of flow 
controllers 61 and 62, respectively. Heating coils 63 surround tube 37 to 
heat tube 37 to a desired temperature. A controller 64 provides signals to 
flow controllers 54, 55, 56, 61 and 62 and pump 38. Tube 37 is a 
conventional metal tube which is commercially available. Controller 64, 
pump 38, heating coils 63, and gas entry tubes 39 and 40 with extensions 
41 and 42, respectively, are all known to be commercially available for a 
PSG deposition. 
For PSG deposition, tube 37 is maintained at about 450.degree. C. by 
heating coils 63. Boats 11-14, after receiving a coat of nitride as 
described for FIG. 1, are inserted into tube 37 carrying semiconductor 
wafers. A typical wafer 66 is shown as being in boat 13. The semiconductor 
material of the wafers is preferably silicon. For convenience, FIG. 2 
shows 11 wafers in each boat 11-14, whereas a typical boat in practice 
preferably holds 30 wafers. Tube 37 is pumped down to a vacuum of in the 
order of 1 milliTorr. Nitrogen from container 51 is flowed through tube 37 
to purge tube 37 at a pressure of about 200 milliTorr for 3 minutes. Tube 
37 is again pumped down. Oxygen from container 59 is flowed through tube 
37 at a rate of 120 SCCM for about 6 seconds. Silane and phosphine from 
containers 53 and 52, respectively, are simultaneously flowed through tube 
37 at rates of 48 SCCM and 6.5 SCCM, respectively. Pressure in tube 37 is 
about 250 milliTorr while oxygen, silane, and phosphine are flowing. The 
oxygen, silane, and phosphine react in conventional fashion to form PSG 
and hydrogen (H.sub.2). The PSG is deposited on the wafers and boats 11-14 
while the hydrogen is removed via pump 38. The mixture of silane and 
phosphine enter tube 37 at points 43-46 while the oxygen enters at points 
47 and through tiny holes all along extension 42 to obtain uniform 
deposition on the wafers. 
After the desired deposition has been obtained, the flow of silane and 
phosphine into tube 37 is stopped while the flow of oxygen continues for 1 
minute. Tube 37 is pumped down again to complete the process of clearing 
tube 37. Nitrogen from container 51 is flowed through tube 37 for 1 minute 
then pump 38 is turned off so that tube 37 begins backfilling for 5 
minutes. Nitrogen from container 58 is flowed until atmospheric pressure 
is reached to complete the backfilling process. Boats 11-14 are then 
removed. 
As boats 11-14 cool down subsequent to removal, the stainless steel, 
nitride and PSG of boats 11-14 contract. The linear coefficients of 
thermal expansion of stainless steel, nitride, and PSG are 
17.2.times.10.sup.-6 /.degree.C., 2.5.times.10.sup.-6 /.degree.C., and 
0.5.times.10.sup.-6 /.degree.C., respectively. Consequently, boats 11-14 
contract more than the PSG coating, tending to cause PSG to flake off. The 
nitride layer between the PSG and stainless steel acts as a buffer to 
reduce flaking of PSG from boats 11-14. Boats 11-14 can be reused many 
times even though the PSG accumulates on boats 11-14. Boats 11-14 with a 
nitride coating continue to provide a significant reduction in flaking 
until at least a thickness of 20 microns of PSG accumulates on boats 
11-14. After boats 11-14 have cooled sufficiently, the wafers can be 
removed from boats 11-14 and new wafers for receiving PSG can be placed in 
the boats. 
While the invention has been described in a preferred embodiment, it will 
be apparent to those skilled in the art that the disclosed invention may 
be modified in numerous ways and may assume many embodiments other than 
that specifically set out and described above. For example, a 
nitride-coated boat may be useful in other cases than for PSG deposition. 
The nitride coating could be a buffer between any deposited material which 
has a significantly different linear coefficient of thermal expansion than 
the metal of the boat. Accordingly, it is intended by the appended claims 
to cover all modifications of the invention which fall within the true 
spirit and scope of the invention.