Edge masking spin tool

An apparatus and method for spin coating a substrate with a liquid material which prevents the material from contacting the edge or backside of the substrate or forming an edge bead at the periphery of the substrate is disclosed. A spin chuck is equipped with a conformable elastomer which seals the edge of the substrate and forms a continuous surface with a planar surface of the substrate for the liquid material to flow off the substrate unimpeded during the spin coating process. As the elastomer is resilient, small variations in substrate size and shape are automatically compensated. None of the substrate area is lost to masking and/or removal processes as in the prior art, allowing the entire substrate area to be available for further processing.

BACKGROUND OF THE INVENTION 
This invention generally relates to the spin application of photoresist, 
polyimide, and other liquid materials. More particularly, it relates to an 
apparatus and method for spin applying the liquid materials to a planar 
top surface of an electronic substrate so that the edge and back of the 
substrate are not coated with the material. 
Liquid coating materials such as photoresist and polyimide are most often 
applied to electronic substrates such as semiconducting wafers, 
chip-carrying substrates, and printed circuit boards by methods including 
spin, spray, and dip. These materials are used for a variety of purposes 
including photolithography, mask layers and electrical insulators. A spin 
application is preferred for these materials in semiconductor processing 
because it generally provides a thin, uniform coating. 
However, there are at least two major problems encountered during spin 
apply process. The first problem is that a thick layer of material known 
as an edge bead usually forms near the substrate edge. The edge bead is 
the result of surface tension within the film that draws the material in 
from the substrate edge. Because the thickened material does not cure as 
fully as the thinner coating on the rest of the surface, it degrades 
during further processing and is a source of particulate contamination. If 
the thicker material is photoresist, lithography processing is generally 
impossible in this area. The edge bead is therefore a concern for 
integrated circuits and high density substrates. For semiconductor wafers, 
this excess thickness of resist at the edge of the wafer is responsible 
for a higher defect level at the wafer edge, lower yield, and lower 
reliability for chips located near the edge of the wafer. 
The second problem is that the coating material often wets the wafer edge 
and creeps around to the back of the wafer during the spinning. Material 
in these locations is also a source of particulate contamination. 
Photoresist on the wafer back can prevent proper leveling and focusing of 
the lithography tool during a subsequent photoexposure step. 
Thus, in conventional processing, the edge bead and the material on a wafer 
edge and back must be removed before further processing. Typically, the 
removal is accomplished by a chemical spray directed at the wafer before 
the resist is baked. However, it is known that resist dissolving chemicals 
cause bulging of the resist adjacent to the removed resist. This degraded 
resist is also a source of particulate contamination, especially if the 
wafer is subjected to an ultraviolet hardening step. For this reason, an 
additional segment of edge resist, that exposed to the resist removing 
chemical, must also be removed. This additional segment of resist is 
removed by exposing and developing the edge region. The two removal steps 
cause 1 to 2 mm of resist to be lost from for the outer edge of the top 
surface of the wafer. 
U.S. Pat. No. 4,086,870 describes an edge masking technique using a knife 
edge cover plate mask which prevents resist from coating an outer ring of 
the wafer. As resist is usually much thinner than a knife edge, the knife 
edge appears as a wall impeding the free flow of material off the spinning 
wafer, causing non-uniformity in thickness and splashes of particles which 
land back on the wafer. Furthermore, capillary action forces resist under 
the knife edge cover plate allowing the resist to wet the wafer edge and 
back. Variations in wafer thickness and irregularities in the wafer 
surface or in the thin knife edge surface provide gaps, increasing the 
amount of leaking under the mask. Also, the top cover plate covers part of 
the active area of the wafer. The loss of semiconductor real estate due to 
the knife edge becomes more important as the industry moves to larger 
wafer sizes, thus losing a greater number of potential chip sites at the 
edge of the wafer. Finally, the complicated mechanism makes automation 
difficult. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to eliminate edge bead 
on a substrate during coating of a planar substrate with a liquid material 
in a spin apply process. 
It is another object of the invention to avoid interference with material 
flowing off the substrate during spinning. 
It is another object of the invention to seal the edge and back of a 
substrate from contact with the liquid coating material, accommodating 
variations in substrate size and shape. 
It is a further object of the invention to allow automatic placement and 
removal of substrates on an apparatus designed according to the present 
invention. 
It is another object of the invention to coat the entire top surface of the 
substrate without reducing the coated area with the means used to 
accomplish the other objects of the invention. 
These and other objects of the invention are accomplished by using a 
rotating chuck having a conformal elastomer, the elastomer positioned to 
surround the periphery of the substrate when placed on the chuck for the 
spin coating process. In the preferred embodiment, the elastomer is an 
inflatable elastomer in the shape of an O-ring or D-ring in the 
approximate shape of the substrate to be coated. When inflated, the 
elastomer makes a sealing contact and masks the substrate edge. 
The inflatable elastomer provides a pressurized seal to the substrate, 
eliminating the concern about capillary action leaks to the edges or 
backside of the substrate. Since pressure is equally applied to all points 
on the substrate edge, defects or small variations in substrate size and 
shape are automatically accommodated. The elastomer masks only the edge of 
the substrate, so the liquid material covers the full substrate surface to 
the edge of the substrate. Thus, the full surface can be used for 
electronic devices and more devices can be built on each substrate. 
Once the substrate is in position for spin coating, the top surface of the 
elastomer forms a substantially continuous surface with the top surface of 
the substrate and is also substantially planar to the top surface of the 
substrate. As the seal does not extend above the substrate surface, the 
flow of resist off the substrate is not impeded by the elastomer. Thus, 
resist uniformity is not degraded. Edge beading is eliminated since the 
resist film smoothly crosses the substrate-elastomer boundary. The surface 
tension force pulling in the resist is transferred from the substrate edge 
to the outside edge of the elastomer or the outside edge of a frame. As 
the elastomer is substantially planar to the substrate, there is also no 
wall against which resist thickness would build up. The structure is low 
cost and simple compared to the prior art. No springs, clamps, top cover 
plates, or knife edges are needed. 
While the continuous surface provided at the substrate periphery transfers 
much of the edge bead effect to the outer edge of the spin chuck, a small 
edge bead can form with remaining material on the substrate. To prevent 
this, the substrate is given a few seconds of extended spin to remove 
sufficient solvent to prevent an edge bead from forming after the 
elastomer is deflated and removed. 
In another embodiment of the invention, the substrate is forced into a firm 
elastomeric ring which conforms to and seals the substrate edge. The force 
is provided by vacuum pressure. In yet another embodiment of the 
invention, the natural position of a hollow O-ring or D-ring of elastomer 
is conformal to the periphery of the substrate. A vacuum is applied to 
deflate the elastomer ring to allow the substrate to be placed in or 
removed from the coating apparatus.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring to FIGS. 1 and 2, a substrate 11 is mounted on a vacuum chuck 13 
to which is also fixed a frame 15 of a height substantially planar with 
the top surface of the substrate. In FIGS. 1A and 1B, the chuck 13 is 
mated to a sealed shaft 12 which is made to spin upon command from a 
spinner tool (not shown). The chuck 13 and frame 15 support an inflatable 
elastomer 17 which is inflated to make a pressurized sealing contact with 
the edge of the substrate 11. The elastomer is an elastic, rubber-like 
substance and can be as natural or synthetic rubber or other similar 
elastic materials. Rubbers, elastic plastics, nitriles and fluorocarbons 
of sufficient durability and solvent resistance may be used with the 
present invention. The substrate 11 is held in place by a vacuum supplied 
by vacuum line 14. The elastomer is inflated by forcing in air or a fluid 
through a connecting valve 19, much like a bicycle tire with a removable 
supply line (not shown). More complicated schemes for supplying the 
inflating pressure to the elastomer 17 would occur to one skilled in the 
art such as supply lines provided in the chuck 13. The height of the 
inflated elastomer is substantially planar to the top surface of the 
substrate to form a substantially continuous surface to avoid the 
possibility of impeding the flow of resist, polyimide, or other liquid 
coating material off the substrate as the substrate 11 spins. Thus, if the 
substrate is a semiconducting wafer, with a thickness of about 20 mils, 
the inflated elastomer has a cross sectional diameter of approximately 20 
mils. If the substrate is a chip carrier, with a thickness of 75 to 250 
mils, the inflated elastomer will have a correspondingly larger cross 
sectional diameter. While one skilled in the art would understand that a 
variety of liquid coating materials can be used in the present invention, 
the detailed description will refer to the coating material as resist or 
photoresist. 
In FIG. 2, resist 21 has been spin coated on the substrate 11, and the 
resist has spun out onto the elastomer 17 and frame 15. Typical 
thicknesses for resist and other materials range from 0.5 to 10 um. The 
curves between substrate and frame are reasonably smooth, so the resist 21 
forms a continuous film connecting substrate 11, elastomer 17, and frame 
15. Surface tension forces allow an edge bead 23 of thick photoresist to 
form only on the outer edge of the frame 15 where there is a discontinuity 
in the resist, and that is far from the substrate edge 25. The edge bead 
is generally 1 to 5 um thicker than the rest of the resist. Thus, the 
surface tension force that normally causes thicker photoresist on the 
substrate edge is transferred from the edge of the substrate to the outer 
edge of the frame, where it is inconsequential for substrate yield or 
reliability. 
The resist 21 on the substrate 11 is found to be uniform across its entire 
surface 26 and past the edge of the surface where it curves down to form 
the top of the edge of the substrate 25. Because of the sealing by the 
pressurized elastomer 17, an extended region along the vertical edge 29 
and back side 31 of the substrate 11 is free of resist. Experimental 
results have shown that the resist 21 maintains a continuous film and is 
able to bridge a small gap between a wafer 11 and a surrounding ring 17. 
Pressure sealing is not needed to maintain the continuous film from 
substrate 11 to ring 17, but only to prevent resist 21 from wetting the 
substrate edge 29 and back 31. 
The elastomer 17 may have several cross sectional shapes including O-ring 
and D-ring, with one side attached to the frame 15 as shown in FIG. 2. 
While much of the edge bead effect is transferred to the elastomer 17 or 
frame 15, a smaller edge bead can form on the substrate 11 if the spin is 
prematurely stopped. The resist 21 must be relatively immobile before the 
elastomer 17 is deflated so surface tension will not then be able to pull 
resist 21 away from the edge of the substrate 11, causing an edge bead. 
However, the resist 21 must not be cured so hard that it flakes into 
particles when the elastomer 17 is deflated. One way to remove solvents is 
by extending the spinning of the substrate 11 until the resist 21 is 
immobile, just past tacky. Another method to prevent edge bead would be to 
heat the wafer slightly after the chuck stops. Using heating, the extended 
spin is not necessary to set the resist before the O-ring is separated 
from the substrate. 
In processing substrates, first the substrate 11 is inserted in the fixture 
13, held by vacuum provided by vacuum line 14, then the nearby elastomer 
17 is inflated so that it seals against the substrate edge 29. The 
inflated elastomer 17 will conform to small variations in substrate size 
and shape, provide sealed masking, and provide an easily removable 
structure as compared with a rigid overhang. Resist 21 is applied using 
standard application procedures. The elastomer 17 masks the substrate edge 
29 without extending above the surface of the substrate, so it does not 
restrict the flow of photoresist off the substrate during the apply and 
spin steps. The elastomer 17 is then removed by deflating it slightly. The 
substrate 11 is removed. The elastomer 17 is spray cleaned and spun dry. 
The elastomer 17 must be able to stand up to solvent spray. A perfluoro 
elastomer such as Kalrez.TM., coating an elastomer tubing will withstand 
most solvents, including N-butyl acetate and cellosolve acetate. A 
preferred elastomer ring 17 is a Kalrez.TM. coated-Viton.TM. inflatable 
O-ring. Viton is a fluoroelastomer, and other elastomers may be used as 
the body of the O-ring. As Teflon.TM. a fluoropolymer withstands most 
solvents, it is a possible coating for the O-ring. However, as Teflon is 
not an elastomer, Kalrez is preferred. Kalrez, Vitron and Teflon are 
trademarks of the DuPont Corporation. An alternate cleaning method is to 
expose resist on the elastomer to light and then spray a developer. In 
this case, a broader range of elastomers can be used, but in addition to 
the light exposure and developer spray, it is also necessary to rinse the 
elastomer with deionized water to remove developer before the next wafer 
is inserted. 
Another embodiment of the invention is illustrated in FIGS. 3A and 3B in 
which the edge seal is obtained by pulling the substrate 11 down with a 
vacuum force into a resilient elastomer ring 41 located along the 
substrate edge. FIG. 3A illustrates the substrate and apparatus before 
vacuum is applied, and FIG. 2B illustrates after vacuum is applied. In 
FIG. 2A, the substrate 11 is resting on a ring of elastomer 41 above the 
vacuum chuck 13. In FIG. 2B, vacuum has been applied, and the substrate 11 
has been pulled down to the surface of the chuck 13, embedding the 
substrate edge 11 in the elastomer 41. Because of the compression of the 
elastomer 41, there is a sealing force directed at the substrate edge 11. 
The elastomer 41 is selected to be sufficiently comfortable to allow for 
small variations in size and shape of the substrate 11, as well as 
resistant to the processing solvents. 
A third embodiment of the invention resembles that shown in FIGS. 1A, 1B 
and 2 physically, however, the elastomer in its relaxed state conforms to 
a substrate placed on the spinning chuck. To place the substrate on the 
chuck at the beginning of the process, a slight vacuum is applied to 
deflate the elastomer slightly. After the substrate is in position, the 
vacuum is released and the elastomer conforms to the substrate periphery. 
To remove the substrate, the vacuum is again applied to deflate the 
elastomer. The first and second embodiments of the invention are slightly 
superior in that greater pressure can be provided at the substrate 
periphery to prevent resist coating of the substrate edge and backside. 
The process flow of the invention is illustrated in FIG. 4. In box 51, the 
substrate 11 is placed on the spin chuck fixture 13 which extends in a 
lateral direction past the edge of the substrate 11. The substrate 11 is 
held in place by vacuum. The elastomer 17, 41 is then conformed to the 
periphery of the substrate 11 so that any small variations in size and 
shape are compensated in box 53. In the case of the first embodiment, the 
elastomer 17 is inflated to seal the edge of the substrate 29. Where the 
solid elastomer 41 is used in the second embodiment, it is accomplished by 
pulling the substrate 11 down into the elastomer 41 by vacuum. In both 
cases, the top surface of the elastomer is substantially planar to the top 
surface of the substrate 27. In box 55, resist 21 is applied in a 
conventional manner. Dispense tools are well known in the art; they can be 
as simple as a piece of tubing connected to a resist reservoir equipped 
with a pump which forces a measured quantity of liquid through the tubing 
to the substrate. In box 57, the substrate 11 is spin coated with the 
resist by starting the rapid rotation of the spin chuck 13. The resist is 
set by extending the spin or bake for a few seconds to remove sufficient 
solvents so that resist 21 will not flow or bead up when the elastomer is 
removed. The resist 21 will not be so fully hardened that the resist 21 
flakes when the elastomer 17 is deflated. In box 59, the elastomer is 
removed from the periphery of the substrate. For the inflatable elastomer 
17, the elastomer 17 is deflated to allow substrate 11 to be removed. For 
the resilient ring of elastomer 41, the vacuum force is released, allowing 
the substrate 11 to be freed. The substrate 11 is removed to a hot plate 
to complete the bake process. The elastomer 17 is spray rinsed with resist 
solvent while spinning to remove excess resist 21. Spray clean tools are 
also well known to the art. Pressurized solvents are directed by means of 
a spray nozzle to the elastomer 17 in the fixture 13. Then, the spin chuck 
13 is spun at high speed to dry the elastomer 17. The substrate 11 is now 
ready for exposure and the fixture 13 is ready for the next substrate. 
While several embodiments of the invention, together with modifications 
thereof, have been described in detail herein and illustrated in the 
accompanying drawings, it will be evident that various further 
modifications are possible without departing from the scope of the 
invention. Those skilled in the art would find many obvious equivalents to 
those listed above. The embodiments discussed above describe and 
illustrate the apparatus necessary for a circular substrate such as a 
semiconductor wafer. For substrates of other shapes, the elastomer ring 
and other portions of the apparatus must be modified to correspond to 
these shapes. For example, a square or rectangular substrate such as a 
ceramic chip carrier will require a square or rectangle elastomer ring 
which is generally similar in shape and size to the periphery of the chip 
carrier. Nothing in the above specification is intended to limit the 
invention more narrowly than the appended claims.