In situ resist control during spray and spin in vapor

Spin coating of resist on a semiconductor wafer is done in a controlled chamber, starting with introducing a resist solvent vapor into the chamber from a nozzle, applying the resist by spraying a very thin layer of the resist material, monitoring and adjusting the resist thickness during spinning in vapor, and then removing solvent from the chamber. The result is a saving in resist material and enhanced coating uniformity.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to manufacturing apparatus for use in 
photolithography, and more particularly to apparatus for to optimizing the 
quantity of photoresist applied to a substrate. 
2. Description of Related Art 
In photolithography, photoresist (also usually referred to in the art as 
resist and referred to both ways herein interchangeably) is deposited upon 
a work piece to be patterned by the photolithographic process. The expense 
of the quantity of photoresist used is the most or nearly the most 
expensive part of the cost of the process currently, depending upon prices 
of the materials used. Unfortunately, conventional resist spinners waste 
expensive resist material. In an application to manufacturing of 
semiconductor wafers, for example, a thick film of photoresist in the form 
of a puddle millimeters in height is delivered to cover most of a 
semiconductor wafer. Then the surplus photoresist material is spun off the 
semiconductor wafer surface, leaving an ultimate thickness of photoresist 
which is only on the order of several micrometers on the surface of the 
wafer. A very economical process currently requires 5 ml of photoresist 
material for each six inch diameter wafer. 
U.S. Pat. No. 4,800,836 of Yamamoto et al for "Resist Coating Apparatus" 
neither introduces nor removes vapor from the chamber in which the resist 
is dumped on the substrate. No spraying is employed and there is no delay 
after application. 
U.S. Pat. No. 4,416,213 of Sakiya for "Rotary Coating Apparatus" does not 
control the environment of the deposited resist. 
U.S. Pat. No. 4,290,384 of Ausschnitt et al for "Coating Apparatus" 
suggests use of a mist but no spray and no spinning. 
Japanese J6001248-A 85.01.19 (8509) Fujitsu, describes a resist coating 
method in which the atmosphere of resist solvent vapor is formed adjacent 
a semiconductor substrate and the resist is coated onto the semiconductor 
substrate. The solvent is discharged from one side of a porous disc. The 
resist is dropped onto the substrate. There is no spraying and no savings 
in resist material. 
Japanese J02113518A 90.04.25 (9023) Mitsubishi Denki KK describes a resist 
coating device with a circular passage to introduce resist mist at a place 
surrounding the wafer on a spin chuck. 
Japanese J02100314A 90.04.12 (9021) NEC Corp. describes an inner wall cup 
to coat a film of solvent or gas of photoresist. 
Copending, commonly assigned U.S. patent application Ser. No. 07/784,290 
filed Oct. 29, 1991 of Cardinali and Lin for "A Material Saving Resist 
Spinner and Process" describes resist coating with the 
spray-and-spin-in-vapor (SASIV) technique. With SASIV, the resist spin off 
is done in a vapor-rich environment so that spray resist application can 
be used to save resist material with a gain in thickness uniformity. 
In the past resist thickness has varied even when employing the 
improvements in SASIV. That is a problem which needs to be overcome. 
Accordingly, there is a need to eliminate variations which occur with the 
SASIV process and the like. 
In accordance with this invention, resist thickness is monitored and fine 
tuned in situ to achieve an accurate final thickness independent of 
variations of resist viscosity, vapor concentration, air flow, spin speed, 
and many other parameters.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The technology exists to deliver a coating several micrometers (.mu.m) 
thick upon a semiconductor wafer. For example, even so coarse a technology 
as spray painting could be employed for that purpose broadly. However, the 
uniformity of thickness and the control of the absolute thickness are far 
below those required for semiconductor processing. 
The process of this invention involves spraying resist upon the work piece 
to cover it with only a few micrometers of photoresist. At the same time, 
the photoresist material is prevented from drying rapidly by provision of 
a controlled environment. A third control feature employed in accordance 
with this invention is the use of high speed spinning to control the 
thickness and uniformity of the resist. Because the final step of the 
process is similar to that used in conventional resist processing, the 
thickness and uniformity control are performed to at least as good a 
standard. Their performance are enhanced because the resist can planarize 
more fully using this process which retains vapor pressure longer. 
FIG. 1 shows an embodiment of a system for practicing the present 
invention. As described in copending application Ser. No. 07/784,290 a 
reasonably airtight spinner chamber 10 (i.e. an enclosed, controlled 
chamber) houses a rotating chuck 12 driven by shaft 11. The chuck 12 
carries a work piece, i.e. substrate, in the form of a semiconductor wafer 
14. A vaporizing nozzle 16 delivers liquid solvent droplets 17 into 
chamber 10 for the purpose of preparing the atmospheric concentration of 
solvent for addition of droplets 19 of photoresist to the chamber 10. This 
is initiated prior to the step of spray coating liquid photoresist 
droplets 19 onto wafer 14. 
Next, the liquid resist 19 is applied to the wafer 14 from the spray nozzle 
18 to form a thin film 20 of resist on the top surface of wafer 14. After 
the desired uniformity and wet thickness are achieved by spinning, the 
vapor can be removed from chamber 10 to facilitate proper drying, by a 
conventional vent (not shown for convenience of illustration, but see FIG. 
2 of my copending application Ser. No. 07/784,290). Alternatively, the 
wafer 14 is relocated to a low vapor environment. Subsequently, the wafer 
14 is baked in a conventional manner. 
The resist thickness resulting from a spin coating process is dependent on 
the viscosity of the resist material and the spin speed. There is a well 
known relationship, 
##EQU1## 
where T is the resist thickness, 
.eta. viscosity, and 
s the spin speed. 
In actual practice, other parameters such as the air flow and vapor 
pressure near the wafer also affect the final resist thickness. 
This invention takes advantage of the vapor environment during resist 
coating with the spray-and-spin-in-vapor (SASIV) technique disclosed in 
copending U.S. patent application Ser. No. 7/784,290 filed Oct. 29, 1991 
of Cardinali and Lin for "A Material Saving Resist Spinner and Process." 
With SASIV, the resist spin off is done in a vapor-rich environment so 
that spray resist application can be used to save resist material with a 
gain in thickness uniformity. Here, the resist thickness reproducibility 
is improved by monitoring the resist thickness in the initial phase of the 
spin cycle uses a spin speed that is slightly lower than the target speed 
for the final thickness. This lower spin speed makes the resist thickness 
slightly larger than target. Because the resist is still completely wet in 
the vapor-rich environment, the spin speed can be fine tuned according to 
in situ measurements taken during the initial phase of the spin cycle. 
Any thin-film thickness measurement technique can be used. Referring again 
to FIG. 1, in accordance with this invention, the wafer chamber 10 has a 
source 40 of light, connected by fiber optic line 41 to a light sender 42 
adapted to resist thickness measurement and beam 43 hits the resist film 
20 and is reflected as beam 44 directed at light receiver 45 comprising an 
optical element adapted for resist thickness measurement. The sender 42 
and the receiver 45 employ polychromatic interferometry to monitor the 
resist thickness. This thickness information is used to fine tune the spin 
speed for the target resist thickness. The output of receiver 45 is an 
electronic signal which passes on electrical cable 46 to thickness 
measurement module 47. Module 47 performs signal analysis of standard thin 
film thickness measurement techniques to calculate the resist thickness, 
as done by the prometrix thin film thickness measurement tool. 
The thickness measurement module 47 supplies signals on line 48 to spin 
speed control module 49, which controls a variable drive motor (indicated 
by line 50) attached to shaft 11. 
A typical process sequence is shown in the flow chart in FIG. 2. After the 
vapor spray 60 and resist spray 62 steps and the start of spinning 66 as 
in SASIV, the resist thickness is measured 68. Then the spin speed 78 can 
be increased according to the measured thickness 68 depending upon the 
decision on target thickness 70. With the spin speed increased but still 
slightly below the predicted speed, this cycle can be repeated until the 
thickness reaches a highly accurate target thickness for the wet resist. 
The relationship of the wet and dry thicknesses can be predetermined. 
The concentration of the solvent vapor should be kept high to prevent 
drying of the resist, yet not saturated such that the solvent may condense 
on a wafer 14, causing a non-uniform coating. The amount and duration of 
the solvent spray is a function of the duration and the quantity of the 
solvent in the resist which is sprayed upon wafer 14. A longer resist 
spray time or a higher resist dilution can use a shorter solvent spray 
duration. 
EXAMPLE 
An amount of 0.8 ml of approximate mixture of as follows: 
______________________________________ 
2-ethoxy ethyl acetate 5 parts 
4-butyrol acetone 1 part 
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was sprayed into an approximately 3 liter closed chamber with an air brush 
at a pressure of 20 psia for a duration of 2 seconds prior to application 
of the resist which has a 32% solid content. The liquid resist was sprayed 
upon the 3 inch wafers for approximately 2 seconds. The wet resist had 
thickness of approximately 10 .mu.m. When the resist coverage is increased 
five times by spraying liquid resist, proportionately longer, no solvent 
pre-spray is required. The resist used to spray the three inch wafers is 
about 0.05 ml per wafer. Using the conventional puddle coating process, 1 
ml of resist is required, which is about 20 times more or greater than an 
order of magnitude. To facilitate uniform spraying of resist from nozzles 
18, the wafer 14 was spun at 60 rpm and the air brush system was aimed 
slightly off center to deliver more material from nozzles 18 towards the 
periphery of the wafer 14. Spread out time ran about 20 to 30 seconds to 
yield satisfactory uniformity. 
An optimized vapor spray coating tool can further reduce the quantity of 
material consumed in the coating process. It is desirable to maintain the 
wafer 14 spinning at a high velocity rpm during the vapor introduction, so 
that if any solvent drops upon the wafer 14, it will be spread out thinly 
and uniformly. In a manufacturing system, multiple nozzles 18 can be used 
to spray more material towards the outside of the wafer 14. 
Spread Out Time 
A very important step In this invention is to allow the resist to spread 
out uniformly by delaying the thickness-controlling spinning after the 
spray of resist from nozzles 18. The duration of the spread out time 
varies as a function of as follows: 
1) the viscosity of the resist, 
2) the force of impact of the sprayed resist droplets, 
3) the vapor pressure of the solvent. 
Without the spread out delay in the example cited above, the coating was 
found to be full of radially oriented non-uniformity. 
When planarization over topography is desired, a second spread out process 
can be used after the resist spun off has stopped, so that resist piled up 
at the edges of wafer topographical features due to centrifugal force can 
now settle down and spread out flat over the surface of the substrate. 
The resultant baked resist has an average thickness of about 1.05 .mu.m and 
a standard deviation of 0.3%, whereas that of the control wafers has an 
average thickness of 1.3 .mu.m and a standard deviation of 1.5% at a spin 
speed of 3480 rpm. The larger thickness from the control wafers confirms 
that the resist has started drying as soon as it forms a puddle on the 
wafer. 
In summary, one or more nozzles is used to spray resist upon the wafer. 
Preferably, a spinning step, after deposition of the resist is provided 
for thickness and uniformity control. 
During high speed spinning, in situ thickness control takes place. 
The resist solvent vapor pressure above the vapor is maintained high before 
resist is spun off the wafer. After the desired uniformity of thickness 
and wet thickness are achieved, the solvent vapor can be removed during 
the spinning. Alternately, the solvent vapor is removed after the spinning 
and a second spread out. Solvent vapor is removed to promote rapid drying. 
The film is applied at more than 1 micrometer and spun down to 1 
micrometer. 
A delay time is deliberately introduced between the spraying of the coating 
material and the spinning steps. Ultrasonic or vibratory agitation is 
applied to the system during the delay time to shorten the duration of the 
delay time among other things. Thus the ultrasonic or vibratory agitation 
is applied to the system, during the delay time, before spinning. 
Preferably, the substrate is spun--at a high rpm during the introduction of 
vapor--at a substantially lower rpm during spraying and--at the 
thickness-regulating rpm after spraying and spread out. 
A preferred embodiment of this invention is to provide a high rpm spinning 
step during the introduction of the solvent vapor. 
A further preferred embodiment of this invention is to provide a low rpm 
spinning step during spraying of the resist material onto the substrate. 
During the final spinning step, solvent vapor pressure is maintained or 
evacuated depending upon the degree of planarization desired, with early 
evacuation providing higher throughput but less planarization. 
A second spread out delay between the final spinning step and resist drying 
can be used to further planarize the resist coating but with corresponding 
reduction in throughput. 
While this invention has been described in terms of the above 
embodiment(s), those skilled in the art will recognize that the invention 
can be practiced with modifications within the spirit and scope of the 
appended claims.