Polyimide formulation for forming a patterned film on a substrate

A composition for use in a process for the deposition of patterned thin metal films on integrated circuit substrates, the composition comprising an admixture of a thermoplastic polyimide resin and a coumarin dye dissolved in a substituted phenol solvent. Optionally a polar solvent having a boiling point greater than 160.degree. C. and a low boiling organic compound (70.degree.-150.degree. C.) may be incorporated in the composition.

BACKGROUND OF THE INVENTION 
I. Field of the Invention 
This invention relates to an underlay composition for use in the 
fabrication of integrated circuits, and in particular to a lift-off 
process involved in such fabrication. 
II. The Prior Art 
One method used for the manufacture of integrated circuits involves the 
formation of vacuum deposited thin metal films which are etched in the 
presence of etch resistant photoresist layers to provide the selected 
pattern. This, in effect, involves the traditional photoengraving or 
photolithographic etching technique. However, with the continued 
miniaturization or semiconductor integrated circuits, to achieve greater 
component density and smaller units in large scale integrated circuitry, 
additional processes have been developed wherein photoresist layers are 
patterned over a substrate, metal is vacuum deposited over the photoresist 
layer and subsequently the photoresist is removed, leaving fine metal 
linework or patterns on the substrate. 
As an example of such additional processes developed by the art to obtain 
fine minute resolution is a process wherein a bottom layer or underlay of 
a nonphotosensitive polymeric material is formed on a silicon substrate 
which may already contain previously patterned layers as well. Then a 
photoresist composition is deposited on the underlay and is exposed to a 
selected pattern of radiation such as electron beam or ultraviolet light. 
Openings are formed in the photoresist layer by development of the 
radiation exposed portions of the photoresist layer as well as aligned 
portions of the polymeric underlay. A thin metal film is then deposited on 
the undeveloped portions of the photoresist layer and on the substrate 
through the apertures formed in the photoresist and underlay. 
Pattern formation of the deposited thin film layer on the substrate is 
achieved by subsequent removal of the photoresist/underlay composite 
together with the excess thin film by immersing the substrate in a solvent 
which dissolves the polymeric underlay. This just described method for the 
formation of patterned metallic films is referred to in the art as the 
"lift-off" process, and by such process, lateral widths of thin metallic 
films are deposited on silicon wafer substrates and spaced in the order of 
0.5 mils or less. An example of a prior art teaching of such a lift-off 
process is contained in U.S. Pat. No. 4,004,044. 
Polymeric compositions which are to be used as underlay material must be 
thermally stable at the temperatures, e.g., 210.degree.-230.degree. C., at 
which deposition of thin metallic films occurs to avoid decomposition 
and/or insolubilization of the polymeric underlay. Thermoplastic polyimide 
polymers may be used as underlay materials because of their high thermal 
stability, e.g. in excess of 400.degree. C. Polyimides are generally 
defined as polymers having the repeating imide linkage 
##STR1## 
in the main chain and are derived from an aromatic dianhydride such as 
pyromellitic anhydride and an aliphatic or aromatic diamine. 
Thermoplastic polyimides are well known to the art. An example of such 
material is commercially available and sold under the trademark XU 218 by 
Ciba-Geigy Corporation of Ardsley, New York. The polyimide is formed by 
first condensing benzophenonetetracarboxylic dianhydride with 
5(6)-amino-1-(4'-aminophenyl)-1,3,3-trimethylindane (DAPI) and then 
heating at 25.degree. C. until it is fully polymerized. XU 218 is supplied 
as a solid powder, has a density of 1.2 grams per cubic centimeter and a 
glass transition temperature (Tg) of 320.degree. C. 
Polyimide formulations used as underlay materials may have incorporated 
therein a radiation absorbing dye to improve linewidth resolution and 
uniformity in the deposited photoresist imaging layer. Thus, it has been 
determined that linewidth control problems can arise due to light 
scattering and reflection from the substrate-underlay interface during 
exposure of the photoresist imaging layer to radiation such as ultraviolet 
light. The light scattering and reflection can cause the photoresist to be 
exposed to an undesirable high dose of ultraviolet light, which in turn 
leads to undesirable linewidth variation. It has been found by the art, 
e.g. U.S. Pat. No. 4,362,809 that the light-scattering and reflection 
phenomenon can be reduced by incorporating in the polyimide composition a 
radiation absorbant dye. When the imaging source is ultraviolet radiation 
the dye used is one which will absorb light at a wavelength of 350-500 
nanometers (nm), with a maximum absorbance preferably in the 400-500 nm 
range. 
A polyimide solution commercially available from Ciba-Geigy which has been 
evaluated as an underlay material for use in lift-off processes is 
composed of a mixture of about 15 percent by weight XU 218 and 5 percent 
by weight Orasol Yellow 4GN a monoazo dye dissolved in a 
gamma-butyrolactone solvent. The dye dissolved in a gamma-butyrolactone 
solvent absorbs light in the 350-500 nm range. 
One of the disadvantages to the use of the Orasol 4GN dye is that the dye 
exhibits marginal thermal stability at 230.degree. C. Deposition of the 
thin metal films is normally performed at this temperature whereby the dye 
is vulnerable to decomposition. The outgassing that can result from dye 
decomposition will cause undesirable dimensional changes or distortion of 
the deposited metal lines. 
A second disadvantage to the use of the Orasol 4GN dye is that high 
concentrations of dye, e.g. in the order of 5 percent by weight, are 
required to prevent linewidth variations due to light scattering. The 
presence of the high dye concentration has been found to materially 
increase the time necessary to effect lift-off of the polyimide underlay 
film. 
A further disadvantage in the use of the commercially available polyimide 
formulations is that the presence of high boiling point solvents such as 
gamma-butyrolactone (b.p. 203.degree. C.) causes "edge pull back" or 
volume concentration of the film after it is cast on the substrate. Edge 
pull back results in the formation of a 1 to 2 mm.diameter band of 
relatively thin (e.g. 1-1.2 microns) film material to form at the edges of 
the substrate as compared to the thickness of the remainder of the cast 
film, e.g., about 1.8-2.0 microns, normally applied as the underlay. 
During processing of the coated substrates, as by argon sputter cleaning of 
film surfaces, the heat generated during such processing causes the 
relatively thin band of polymeric film at the edge of the substrate to be 
more firmly bonded thereto whereby removal of the underlay with 
conventional solvents is rendered impossible thereby preventing lift-off 
of the photoresist layer and the deposited metal layer. 
A thermally stable underlay composition that would be thermally stable at 
temperatures in excess of 230.degree. C., which could be cast on 
semiconductor substrates without the occurrence of edge pull back and 
which could be easily and completely removed in lift-off processing would 
be highly advantageous and much prized by workers in the art. 
It is thus an object of the present invention to provide a thermally stable 
underlay composition that can be used for fabrication of semiconductors at 
temperatures in excess of 230.degree. C.; which can be readily and 
completely removed in lift-off processing. 
SUMMARY OF THE INVENTION 
The present invention fulfills the above-stated objective by providing a 
composition suitable for use as an underlay in a lift-off process which 
composition is comprised of a mixture of (a) a thermoplastic polyimide 
polymer and (b) a coumarin dye having the formula 
##STR2## 
wherein R.sub.1, is hydrogen, R.sub.2 is hydrogen or an alkyl group having 
1 to 4 carbon atoms, R.sub.3 is --NHR where R is an alkyl group containing 
1 to 4 carbon atoms or N(C.sub.2 H.sub.5).sub.2, R.sub.4 is hydrogen or an 
alkyl group having 1 to 4 carbon atoms, R.sub.5 is hydrogen or CF.sub.3 
and R.sub.6 is hydrogen, 
##STR3## 
--CN, --COCH.sub.3 or --CO.sub.2 C.sub.2 H.sub.5, the admixture being 
dissolved in a substituted phenol solvent having the formula 
##STR4## 
where R is hydrogen, --CH.sub.3 or --OCH.sub.3. 
The substituted phenol solvents used in the practice of the present 
invention generally have a boiling point in the range of 
110.degree.-210.degree. C. and preferably about 130.degree. to about 
170.degree. C. in order to provide the desired evaporation rate for 
controlling application of the underlayer film. 
As will hereinafter be further illustrated by the practice of the present 
invention, use of the polyimide formulations of the present invention 
dissolved in substituted phenol solvents substantially eliminates edge 
pull back. The incorporation of a dye of the coumarin type in the 
polyimide formulation provides an underlay composition having improved 
thermal stability due to higher thermal stability of this class of dye. 
DETAILED DESCRIPTION OF THE INVENTION 
Thermoplastic polyimide polymer compositions of the XU 218 type are 
preferred in the practice of the present invention. Other polyimide 
compositions which may be used in the practice of the present invention 
include the condensation product of pyromellitic dianhydride and DAPI, the 
condensation product of pyromellitic anhydride and a mixture of DAPI and 
dimethylene and the condensation product of benzophenone tetracarboxyl 
dianhydride and a mixture of DAPI and methylene dianiline. 
Coumarin dyes which are useful in the practice of the present invention 
have an absorptive maximum between 250 and 550 nm and include: Coumarin 6, 
Coumarin 30, Coumarin 152, Coumarin 153, Coumarin 314, Coumarin 334, 
Coumarin 337, Coumarin 355 and bis-3,3'(7-diethylamino) coumarin. A 
coumarin dye particularly useful in the practice of the present invention 
is Oracet Yellow 8GF, otherwise known in the art as Coumarin 7. 
Suitable substituted phenol solvents useful in the practice of the present 
invention include anisole and dimethyoxy benzene. 
In practice, it has been observed that during spin coating application of 
the polyimide formulations of the present invention to a semiconductor 
substrate, thin strands or whiskers of coating material may form on the 
edge of the substrate. The formation of whiskers is undesirable because 
the whiskers may break off during spin coating and fall back on the wafer, 
causing contamination and thereby reduce product yields. The formation of 
whiskers on the edge of the substrate during spin coating may be avoided 
by incorporating a low boiling (i.e. about 70.degree. to about 150.degree. 
C.) liquid organic compound into the solvent portion of the present 
invention. The low boiling organic liquid comprises about 1 to about 15 
percent by weight and preferably about 5 to about 10 percent by weight of 
the total solvent used to prepare the compositions of the present 
invention. The low boiling organic liquid is selected from substituted 
aromatic hydrocarbons such as xylene, toluene and ethyl benzene ethers 
such as tetrahydrofuran and dixoane and aliphatic ketones. The preferred 
boiling point range for the low boiling point organic compound is from 
about 100.degree. C. to about 130.degree. C. Further, if desired, polar 
solvents having a boiling point greater than 160.degree. C., generally 
160.degree. to 200.degree. C., such as amides, lactones and ketones may be 
used in combination with the substituted phenol solvent. Examples of these 
polar solvents include dimethyl formamide, gamma-butyrolactone and 
N-methyl-pyrrolidone. The advantage of using ancillary solvents such as 
gamma-butyrolactone is that they have a higher boiling point (160.degree. 
C.-200.degree.C.) than the substituted phenol solvent which prevents the 
deposited film from drying too fast. The ancillary solvent may be used in 
combination with the substituted phenol solvent at a weight ratio of about 
50:50 to 90:10 and preferably about 70:30. 
The compositions of the present invention are prepared by mixing the 
thermoplastic polyimide polymer with the coumarin dye in the substituted 
phenol solvent or mixture of the substituted phenol solvent with a polar 
solvent and low boiling organic liquid compound as described above. 
Generally the film solutions of the present invention contain from about 
10 to about 20 percent by weight of the polyimide resin. Generally, the 
coumarin dye will be present in the formulation at concentrations of about 
0.1 to about 1% by weight based on the weight of the solution or about 1 
to about 3 percent by weight of the total solids. The quantity of 
substituted phenol solvent or mixture of solvents and the composition of 
the mixture are selected to obtain the specific thickness and coating 
quality desired in accordance with known coating techniques. 
The compositions of the present invention may be employed as lift-off films 
or masking films which are coated on a suitable support by any customary 
procedure, as by spin coating, dipping, brushing, rolling spraying and the 
like. The particular technique employed depends on the consistency, 
viscosity and solids content of the solution. Spin coating at 2000 to 
10,000 rpm for 1 to 90 seconds has been found to be acceptable and results 
in a uniform lift-off layer. When used as a lift-off film composition of 
the present invention the thickness of the on the support may range from 
about 0.5 to about 8 microns and for most lift-off processes a thickness 
of about 1.5 to about 2.5 microns is preferred. When used as a masking 
film, the thickness of the deposited film depends on the etch procedure 
employed as is well known to the art. 
In practice, when the composition of the present invention is used as a 
lift-off film, the solution of lift-off film material is applied, as an 
underlay film, by spin coating, on a semiconductor substrate. The 
thickness of the layer is controlled by the viscosity of the material 
deposited on the substrate and the rate at which it is spun during the 
deposition. Typically, the thickness is in the range of 1.5 to 2.5 
microns, and preferably from about 1.8 to 2.0 microns when used in 
integrated circuit applications. 
In the fabrication of integrated circuits, the substrate may be a 
semiconductor material or a semiconductor substrate having a surface layer 
of an electrically insulative material, such as silicon dioxide or silicon 
nitride (Si.sub.3 N.sub.4). In a typical multilayer process, the substrate 
is first coated with an adhesion promoter such as an aminosilane which is 
applied using standard spin coating or dip coating techniques. 
Subsequently, the polyimide underlay formulation is applied to the 
substrate, and the coated substrate or wafer is allowed to dry. When 
anisole is used as the solvent in the polyimide lift-off film layer 
formulations of the present invention, the solvent is sufficiently 
volatile (b.p. 154.degree. C.) that an intermediate drying step before 
application of a photoresist layer is unnecessary and successive coatings 
may be applied to the polyimide underlay without drying. However, a 
200.degree. C. bake for 10 minutes is typically used to drive off any 
residual solvent. A barrier layer or resin glass (typically a siloxane) is 
applied over the polyimide underlayer using spin coating techniques. The 
resin-glass layer is baked typically at 200.degree. C. for a period 
ranging from 10-30 minutes. Thereafter a photosensitive material is 
applied as the photoresist over the surface of the resin-glass barrier 
layer. The overlying photoresist layer is then exposed to a suitable 
source of radiation, such as ultraviolet, in a pattern of light 
corresponding to the desired preselected configuration on the substrate. 
The exposed areas of the photoresist layer are developed using an aqueous 
base developer to provide a pattern. The pattern is transferred through 
the barrier layer of resin-glass using a CF.sub.4 plasma etch. 
Subsequently, the pattern is transferred through the polyimide underlayer 
using oxygen ion reactive etching. Both the CF.sub.4 plasma and oxygen 
reaction ion etching are performed using commercially available equipment 
in a manner known to the art. 
In alternative multilayer processes for the fabrication of integrated 
circuits using the polyimide underlay films of the present invention, a 
barrier layer is not employed, and the photoresist is applied directly 
over the polyimide layer. In these alternative multilayer processes either 
the photoresist composition is formulated to be more etch resistant than 
the polyimide underlayer so that after exposure and development of the 
pattern, reactive ion etching is used to transfer the pattern through the 
polyimide underlayer or the photoresist film is silyated to impart etch 
resistance to the photoresist prior to reactive ion etching development of 
the polyimide underlayer. 
After the formation of the aperture pattern, a thin metallic film is 
deposited, as by vacuum deposition, at a temperature of 100.degree. to 
300.degree. C. on the photoresist layers and the substrate through the 
apertures etched in the photoresist and polyimide underlay. The metal may 
be any metal conventionally used for integrated circuit metallization, 
that is, aluminum-copper alloys, copper, chromium, silver, tantalum, gold 
and combinations thereof. The thin metallic films generally have a 
thickness in the order of 0.5 to 1.0 micron. 
After deposition of the thin metallic film, the polyimide underlay of the 
present invention can be readily and rapidly removed by conventional 
lift-off techniques as by imersion in a solvent which dissolves or swells 
the polyimide layer without affecting the thin metallic film. Suitable 
solvents include, acetone, butyl acetate, trichloroethylene and cellosolve 
acetate. A preferred lift-off technique is to immerse the substrate in a 
solvent such as N-methyl-2-pyrrolidinone for about 5 to 10 minutes. 
Ultrasonic agitation conventionally used in lift-off processing is not 
required in the present process. Removal of the deposited layers leaves a 
thin metal film in the desired preselected configuration on the substrate. 
The present invention is illustrated by the following Examples:

EXAMPLE I 
A polyimide formulation to prepare an underlay film for use in a lift-off 
process was prepared by dissolving a mixture of 15% by weight XU 218 and 
5% by weight Oracet Yellow 8GF in a solvent mixture comprised of anisole 
and gamma-butyrolactone at an anisole:butyrolactone weight ratio of 70:30. 
Oracet Yellow 8GF has the formula 
##STR5## 
A series of quartz discs were spin coated with the polyimide formulation 
and then baked for 10 minutes at 100.degree. C. followed by 30 minutes at 
240.degree. C. to prepare a 1.9 micron thick polyimide film on the quartz 
disc. The ultraviolet absorbance of the baked films was measured with a 
uv-visible spectrophotometer and is recorded in Table I below. 
For purposes of comparison, the procedure of Example I was repeated with 
the exception that a polyimide formulation consisting of an admixture of 
15% by weight XU 218 and 5% by weight Orasol 4GN dissolved in 
gamma-butyrolactone was used as the underlay formulation. The ultraviolet 
absorbance of these comparative baked films is also recorded in Table I. 
TABLE I 
______________________________________ 
ULTRAVIOLET ABSORBANCE 
Bake Conditions 
Dye Component 
100.degree. C./10 min. 
240.degree. C./30 min. 
______________________________________ 
Oracet Yellow 8GF 
wavelength 
440 nm 1.37 -- 
438 nm -- 1.28 
Orasol Yellow 4GN 
wavelength 
434 nm 0.573 -- 
422 nm -- 0.554 
______________________________________ 
The data recorded in Table I indicate that the optical density of the baked 
films prepared from Oracet Yellow 8GF dyed polyimides is more than twice 
that of the Orasol Yellow 4GN dyed polyimide. 
EXAMPLE II 
The thermal stability of the Oracet Yellow 8GF and Orasol Yellow 4GN dyes, 
in powdered form, were analyzed using thermo-gravimetric analysis (TGA) 
isothermally at 240.degree. C. The TGA results are recorded in Table II 
below, for purposes of comparison 
TABLE II 
______________________________________ 
% of Original Weight Remaining After 
Thermal Exposure (Minutes) 
Dye Component 
30 min. 80 min. 100 min. 
140 min. 
______________________________________ 
Oracet Yellow 8GF 
99.5 98.5 98.0 96.0 
Orasol Yellow 4GN 
96.5 70.0 62.0 60.0 
______________________________________ 
The data recorded in Table II demonstrate the substantially greater thermal 
stability of the Oracet 8GF dye as compared to the Orasol Yellow 4GN dye. 
EXAMPLE III 
A series of nitride coated wafers were spin coated with a polyimide 
formulation prepared in Example I containing 4% by weight of the Oracet 
dye and having incorporated therein 10% by weight xylene. After spin 
coating the wafers were baked at 200.degree. C. for 10 minutes and 
230.degree. C. for 30 minutes to prepare a 2.0 micron thick polyamide film 
on the wafer substrate. No edge pull back or formation of whiskers was 
evident. 
Following the application of the polyimide underlayer, an etch resistant 
barrier layer of resin-glass siloxane was spin coated over the polyimide 
underlayer following the procedure of U.S. Pat. No. 4,004,044 and the 
wafer baked at 200.degree. C. for 10 minutes. Thereafter a conventional 
photoresist composition comprised of an admixture of a 
novolak-type-phenol-formaldehyde resin and a photosensitive cross-linking 
agent was spin coated on the baked polyimide layer and baked at 80.degree. 
C. The wafers were then exposed in an ultraviolet light exposure apparatus 
at a wavelength of 436 nm through a metal mask having a pattern of 3 
micron wide bars separated by 2 micron spaces. The exposed photoresist was 
developed using an aqueous base developer and standard wet development 
technique. The developed photoresist pattern was transferred through the 
resin-glass siloxane barrier using a CF.sub.4 plasma etchant using 
commercially available etching equipment and techniques. The pattern was 
subsequently transferred through the polyimide underlayer using standard 
oxygen reactive ion etching techniques. The wafers with multilayer masking 
were then vacuum metallized with an aluminum-copper alloy. Lift off was 
accomplished by immersing the metallized wafers in 
N-methyl-2-pyrrolidinone at about 135.degree. C. The metal coating was 
lifted off from the wafer substrate within 70 minutes. 
The substrates were examined under a microscope (100.times. magnification) 
and found to be free of unlifted metal. 
Photomicrographs taken at 5000.times. magnification indicated that the 
metal lines had good resolution and fidelity to mask dimensions and were 
substantially free of linewidth variation. While specific components of 
the present system are defined above, many other variables may be 
introduced which may in any way affect, enhance, or otherwise improve the 
system of the present invention. These are intended to be included herein. 
Although variations are shown in the present application, many 
modifications and ramifications will occur to those skilled in the art 
upon a reading of the present disclosure. These, too, are intended to be 
included herein.