Process for imaging multi-layer resist structure

An image is provided by depositing a first layer of a photoresist containing a phenolic-formaldehyde novolak type polymer and an imidazole, benzimidazole, triazole, or indazoles to increase the solubility of the layer in aqueous alkaline developer after exposure to imaging radiation; depositing on the first layer a second layer of a photoresist containing a phenolic-formaldehyde novolak type polymer; the second layer having a lower degree of solubility in aqueous alkaline developer after exposure to imaging radiation; exposing the layers to imaging radiation; and developing the layers.

TECHNICAL FIELD 
The present invention is concerned with providing an image, and is 
particularly concerned with providing an image or mask of a multi-layer 
resist article. The present invention is especially concerned with 
providing what is referred to as photosensitive conformable masks whereby 
only one exposure step and one development step are required. According to 
the present invention, the bottom layer is modified so that a desired 
profile of the patterned resist is readily obtained. 
BACKGROUND ART 
In the manufacture of patterned devices such as semiconductor chips and 
carriers, the steps of defining different layers which constitute the 
desired product are among the most critical and crucial steps involved. 
Polymer films are often used in integrated circuit fabrication as a 
pattern transfer mask for the semiconductor substrates. For example, 
polymers used as a photoresist can act as a mask for etching, ion 
implantation, or lift-off to induce designated removal, doping, or 
addition to the underlying substrate, respectively. 
As the lines and spaces to be etched, however, become smaller, such as at 
about 1 micron, the photolithographic procedures for producing the 
photoresist pattern that is the etch mask are affected by such parameters 
as reflections from the surface grain structure of the metals or 
polysilicon substrate to be etched, standing wave effects, variations in 
the photosensitive material thickness, reflections from steps and 
diffraction effects. 
One technique for overcoming the problems of surface topology, reflections 
and diffractions, is to employ a multi-layer resist system known as a 
portable conformable mask (PCM) system. Such is described by Burn Jeng 
Lin, "Portable Conformable Mask --A Hybrid Near U.V. and Deep U.V. 
Patterning Technique", Proceeding of SPIE, Vol. 174, page 114, 1979, 
disclosure of which is incorporated herein by reference. The simplest 
multi-layer resist system employs a two-layer resist system which avoids 
the cost and complexity of most other multi-layer systems. The bottom 
layer is insensitive to the radiation used to image the top resist layer, 
and is preferably a resist from a polymer of methylmethacrylate such as 
polymethylmethacrylate (PMMA), that is applied over the wafer topology to 
provide a planar surface. The top layer is generally a relatively thin 
(e.g., about 1 micron or less) layer of a material that is simultaneously 
sensitive to the imaging radiation --electron beam, X-ray, or optical 
radiation --and opaque to the radiation used to expose the bottom layer. 
Typically this can be a positive photosensitive material that responds to 
the imaging radiation such as ultraviolet light used in step-and-repeat 
photolithography and is opaque to deep U.V. wavelengths used to expose 
PMMA. After the top layer is imaged and developed, the bottom layer is 
imaged by flood exposure through the top layer resist mask and developed 
using, for instance deep U.V. (about 190 nm to about 280 nm). 
A number of the suggested processes used to overcome image distortion or 
linewidth variation due to different radiations from the irregular 
surfaces require processing that is rather complex. This causes a decrease 
in the throughput and an increase in process control problems. 
It has also been suggested to use two layers of the same parent resist of 
varying molecular weights to achieve different sensitivities or 
solubilities. However, such an approach requires unique synthetic effort 
for control of molecular weight as well as tedious polymer 
characterizations. It would, therefore, be desirable to provide a method 
that is capable of overcoming image distortion or linewidth variations due 
to different radiations from irregular surfaces that does not require the 
complex processing now employed. 
SUMMARY OF THE INVENTION 
The present invention is concerned with a process for resist patterning to 
obtain the desired profile and overcome the distortion problems due to 
different reflective radiations from irregular surfaces without requiring 
special synthesis or polymer characterizations. Resist patterning pursuant 
to the present invention requires only one exposure and one developing 
step to obtain the desired imaged profile. 
In particular, the process of the present invention includes depositing on 
a substrate a first layer of a positive photoresist that includes a 
phenolicformaldehyde novolak type polymer with a diazo sensitizer and a 
modifier in an amount to increase the solubility of the layer in aqueous 
alkaline developer after exposure to image radiation. The modifier is an 
imidazole, benzimidazole, triazole, or indazole. Mixtures can be employed 
if desired. 
Deposited on the first layer is a second and different layer of a positive 
photoresist comprising a phenolicformaldehyde novolak type polymer with a 
diazo sensitizer. 
The second layer has a lower degree of solubility than the first layer in 
aqueous alkaline developer after exposure to imaging radiations. 
The first and second layers are exposed to imaging radiation in a 
predetermined pattern and developed. 
Best and Various Modes for Carrying out the Invention 
The first layer of the first resist material employed in accordance with 
the present invention is a positive resist material. 
The positive resist is a phenolic-formaldehyde novolak 
type polymer sensitized with diazo compounds. Examples of such diazo 
sensitizers are discussed on pages 48-55 of DeForest, Photoresist 
Materials and Processes, McGraw-Hill Book Company, 1975, disclosure of 
which is incorporated herein by reference. Some diazo compounds are 
derivatives of benzoquinone 1, 2-diazide-4-sulphochloride; 
2-diazo-1-napthol-5-sulphonic acid ester; napthoquinone-1, 
2-diazide-5-sulphochloride; naptho-quinone-1, 2-diazide-4 sulphochloride; 
napthoquinone 2-1-diazide-4-sulphochloride and napthoquinone 2, 
1-diazide-5-sulphochloride. The preferred diazo sensitizers are the 
diazo-naphthoquinone sensitizers. 
The phenolic component of the phenolic novolak polymer includes phenol and 
substituted phenols such as cresol. A particular example of such is 
Shipley AZ-1350 which is an cresol-formaldehyde novolak polymer 
composition. Such a positive resist composition includes therein a 
diazoketone such as 2-diazo-1-napthol-sulphonic ester. 
The composition usually contains on the order of about 15% by weight or so 
of the diazoketone compound. Examples of some other commercially available 
photosensitive materials suitable for providing the first layer of 
material employed in accordance with the present invention are AZ-1370 and 
AZ-1470 from Shipley; AZ-4110 and AZ-4210 from AZ Photoresistive Division 
of American Hoechst; HPR 204 from Phillip A. Hunt; Kodak 820 from Kodak, 
and OFPR 800 from Tokyo Ohka. 
The first layer according to the present invention must also contain an 
imidazole, triazole, indazoles, tetrazoles, or mixtures thereof. The 
preferred compounds are the imidazoles with the most preferred being 
benzimidazole. 
Examples of some additional compounds of the above types are imidazole, 
1-methyl-imidazole, 1-propyl-imidazole, 2, 4-di-methyl-imidazole, 
4-methyl-imidazole, 2-isopropyl-imidazole, 2-phenyl-imidazole, 
1-benzylimidazole, .beta.-imidazolopropionic acid, 1, 2-dimethylimidazole, 
1-methyl-2-hydroxymethyl-imidazole, 4-sulfo-imidazole, 
2-methyl-4-sulfo-imidazole, 2-(sulfophenyl)-imidazole, 
2-isopropyl-4-sulfo-imidazole, 1-n-propyl-5-sulfo-imidazole, 
1-n-propyl-4-sulfoimidazole, 1, 2-bis-(1'-imidazolyl)-ethane, 
1-(p-sulfophenyl)-imidazole, histidine, 2-(imidazolo-ethyl)-pyridine, 
1-(2'-aminoethyl)-imidazole-hydrochloride, 
1-(3'-aminopropyl)-imidazole-hydrochloride, 
1-methyl-2-carboxy-methyl-imidazole, 2-(p-sulfophenyl)-4-sulfoimidazole, 
1-methyl-2-sulfo-imidazole, 2-sulfoimidazole, 1, 
2-bis-(1'-methyl-5'-imidazolyl)-ethane, 5-sulfobenzimidazole, 5, 
7-disulfobenzimidazole, tetraxole, indaxole, triazol-(1, 2, 4), 
4-ethyl-triazole-(1 ,2, 4), 4-methyl-triazole-(1, 2, 4), 
4-phenyl-triazole-(1, 2, 4), 3, 4, 5-trimethyl-triazole-(1, 2, 4), 
4-(p-sulfophenyl)-triazole-(1, 2,4 ), 3-methyl-triazole-(1, 2, 4), 
3-ethyl-triazole-(1, 2, 4), 3, 5-dimethyl-triazole-(1, 2, 4), 
3-phenyl-triazole-(1, 2, 4), 1-methyl-triazole-(1, 2, 4), 
1-ethyl-triazole-(1, 2, 4), 1-phenyl-triazole-(1, 2, 4), 
3-sulfo-triazole-(1, 2, 4), 3-amino-triazole-(1, 2, 4), 3, 
5-diamino-triazole-(1, 2, 4), 1, 2-bis(5'-sulfo-3'-triazolyl)-ethane, 1, 
2-bis-(5'-amino-3'-triazolyl) ethane, 1, 2-bis-(3'-triazolyl)-ethane, 1, 
2-bis-(4'-methyl-3'-triazolyl)-ethane, bis-(3-triazolyl)-methane, 
bis-(5-sulfo-3-triazolyl)-methane, bis-(5-amino-3-triazolyl)-methane, 
bis-(3-triazolyl)-methane, bis-(5-sulfo- 3- triazolyl), bis-(5-amine- 
3-triazolyl), 3, 3'-bistriazolyl, 1, 2-bis-(1'-triazolyl)-ethane, 
3-(2'-aminoethyl)-triazole-(1, 2, 4), .beta.-(1-triazolyl)-propionic acid, 
1, 4-bis-(5'-sulfo-3'-triazolyl)-butane, 1, 
4-bis-(5-amino-3'-triazolyl)-butane, 1-(3-sulfopropyl)-triazole-(1, 2, 4), 
1, 2-bis-(4'-triazolyl)-ethane, 1-methyl-triazole-(1, 2, 3), 
1-ethyl-triazole-(1, 2, 3), 2-ethyl-triazole-(1, 2, 3), 
2-propyl-triazole-(1, 2, 3), 1-(2'-carboxyethyl)-triazole-(1, 2, 3), 
5-sulfo-benzotriazole, 5, 7-disulfo-benzotriazole, benzotriazole, 
4-methyl-triazole-(1, 2, 3), 4, 5-dimethyl-triazole-(1, 2, 3), 
4-butyl-triazole-(1, 2, 3), 4-phenyl-triazole-(1, 2, 3), 
1-(3'-aminopropyl)-triazole-,(1, 2, 3), 1-(2'-aminoethyl)-triazole-(1, 2, 
3), and 1, 2-bis-(5'-triazole)-ethane. 
These materials are employed in an amount sufficient to increase the 
solubility of the layer in aqueous alkaline developer after exposure to 
imaging radiation. These materials are usually present in amounts of about 
0.5% to about 4% and preferably about 1% to about 1.5% by weight of the 
photoresist. 
The first layer is usually about 3,000 to about 7,000 angstroms and 
preferably about 4,000 to about 5,000 angstroms. 
This first layer is commonly referred to as the planarizing layer and 
generally is thicker than the second layer atop it. 
Deposited on said first layer is a second and different positive 
photoresist layer which differs from the first layer. This second layer 
contains a phenolicformaldehyde type polymer and a diazo-sensitizer of the 
type discussed above for the first layer. The second layer must have a 
lower degree of solubility than the first layer in aqueous alkaline. The 
second layer is usually thinner than the first layer and usually about 
3,000 to about 5,000 angstroms and preferably about 3,000 to about 4,000 
angstroms. 
The structure is exposed to imaging radiation and developed. The resist 
image is typically produced by imaging radiation having a wavelength of 
about 365 to about 436 nanometers. The exposure is usually from about 60 
to about 100 millijoules/cm.sup.2 and preferably from about 80 to about 90 
millijoules/cm.sup.2. 
The exposure to the radiation can be achieved by employing an ultraviolet 
lamp source. 
In addition, if desired, electron beam radiation can be employed as the 
radiation source. In such a case, the dose of the electron beam imaging 
radiation is usually at least about 10 microcoulomb/cm.sup.2 and 
preferably about 15 microcoulomb/cm.sup.2. 
The exposed portions are removed with an aqueous alkaline solution such as 
a potassium hydroxide or sodium hydroxide aqueous solution containing 
about 0.2N% to about 0.28N% of the hydroxide, preferably potassium 
hydroxide.

The following examples are presented to further illustrate the present 
invention. 
EXAMPLE 1 
Onto a planar silicon substrate is deposited a first layer of about 5,000 
angstroms of a novolak resist containing about 17-20% diazonaphthoquinone 
and about 
1.5% benzimidazole and a second and top layer of about 3,000 angstroms of 
the same novolak resist containing about 17-20% diazonaphthoquinone as in 
the bottom layer, but containing about 0.75% tetrazole. The structure is 
exposed to electrons at 25 microcoulombs/cm.sup.2 at 25 KeV. the exposed 
films are developed in 0.25N KOH developer and 0.5 micron features are 
resolved with a controlled undercut profile for subsequent lift-off. 
EXAMPLE 2 
Example 1 is repeated, except the substrate employed is a silicon substrate 
with 0.4 .mu.m silicon dioxide steps. The submicron features are 
delineated with no evidence of linewidth distortion.