Resist materials for 157-nm lithography

Details Resist materials for 157-nm lithography Resist materials for 157-nm lithography

2000-02-25

2002-10-22

Hamilton, Cynthia (Department: 1752)

Radiation imagery chemistry: process, composition, or product th

Imaging affecting physical property of radiation sensitive...

Radiation sensitive composition or product or process of making

C430S326000, C430S907000, C430S914000

Reexamination Certificate

active

06468712

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention generally relates to photoresist materials useful in micro-lithography and, particularly, to improved materials and methods for pattern formation on semiconductor wafers.
Processes for patterning semiconductor wafers typically rely on lithographic transfer of a desired image from a thin-film of radiation-sensitive resist material. The process entails the formation of a sacrificial layer, the “resist”, which is photo-lithographically patterned. Generally these resists are referred to as “photoresists”.
The patterning of the resist involves several steps, including exposing the resist to a selected light source through a suitable mask to record a latent image of the mask and then developing and removing selected regions of the resist. For a “positive” resist, the exposed regions are transformed to make such regions selectively removable; while for a “negative” resist, the unexposed regions are more readily removable.
The pattern can be transferred into surface texture in the wafer by etching with a reactive gas using the remaining, patterned resist as a protective masking layer. Alternatively, when a wafer is “masked” by the resist pattern, it can be processed to form active electronic devices and circuits by deposition of conductive or semiconductive materials or by implantation of dopants.
Materials used in single layer photoresists for optical lithography should meet several objectives. Low optical density at the exposure wavelength and resistance to image transfer processes, such as plasma etching, are two important objectives to be met by such a photoresist material. Shorter wavelengths of radiation permit greater resolution. The most common wavelengths currently used in semiconductor lithography are 365 nm and 248 nm. The desire for narrower linewidths and greater resolution has sparked interest in photoresist materials that can be patterned by even shorter wavelengths of light.
The high absorbance of many organic functional groups at 157 nm makes it difficult to develop an organic polymer that is both base soluble and has low absorbance at 157 nm. Traditional photoresist polymers contain either phenols or carboxylic acids to solubilize the base polymer. Both organic groups, phenols and carboxylic acids, impart an excess of absorbance to the polymeric resist material to allow the polymer to be an effective component of a single layer photoresist for 157 lithography.
For example, many polymeric photoresists generally incorporate aromatic groups within or are appended to the polymer backbone currently used in UV lithography. Aromatic groups within these polymeric photoresists absorb energy at about 250 nm and absorb so much energy at sub 200 nm ranges that they are ill-suited for use at 157 nm. Generally, the currently used UV photoresists include phenolic groups to facilitate dissolution in basic aqueous developing solutions, and it is the phenolic groups which absorb energy at shorter wavelengths. One approach to improve dissolution of the photoresists has been to incorporate carboxylic acid groups within the polymeric structure of the photoresist. However, carbonyl groups absorb energy in the 160 nm range, thereby posing a difficult problem towards preparing photoresistive materials which can be used at or below the 157 nm range.
SUMMARY OF THE INVENTION
Photoresist materials and methods of photolithography at very short ultraviolet wavelengths are disclosed employing photosensitive compositions which include a photo-acid generator and an aliphatic polymer that includes one or more protected hydroxyl groups. In addition, one or more electron withdrawing groups attached to or adjacent to a hydroxyl containing carbon of the polymer can be incorporated to improve the base solubility of the polymer after removal of the hydroxyl protecting group. In one embodiment, the present invention provides positive photosensitive compositions which do not absorb, or minimally absorb, radiant energy at wavelengths greater than 157 nm, thereby improving performance characteristics for lithography applications. The photosensitive compositions of the invention are particularly useful at 157 nm with fluoride excimer lasers that emit radiation and provide enhanced resolution and imaging over currently known photoresistive compositions.
In one aspect of the invention, the aliphatic polymer component of the photoresist includes one or more electron withdrawing groups adjacent to, or attached to, carbon atoms bearing protected hydroxyl groups, and the protecting groups are labile in the presence of in situ generated acid. Suitable electron withdrawing groups are those which do not interact with, or minimally absorb, visible or near ultraviolet radiation include carboxylate ion, fluorine, chlorine, bromine, iodine, N═NPh, carboxylic acid, carboxylic esters, e.g., t-butyl esters, ketone, trifluoromethyl, NH
3
+
, CN, SO
2
Me, nitro, NMe
3
+
, and N
2
+
, preferably, fluorine or chlorine groups. In one embodiment, the protected hydroxyl groups are covalently attached to carbon atoms bearing these electron withdrawing groups. In one embodiment, the protected hydroxyl groups are adjacent to carbon atoms bearing electron withdrawing groups. In another embodiment, the protected hydroxyl groups are both covalently attached to carbon atoms bearing electron withdrawing groups and are adjacent to carbon atoms bearing electron withdrawing groups. In another embodiment, halogen atoms attached to or adjacent to a hydroxyl containing carbon of the polymer improve base solubility of the polymer after in situ generated acid removal of the hydroxyl protecting groups. In a preferred embodiment, the halogen atoms are fluorine atoms.
The present invention also provides methods to prepare such photosensitive compositions, and methods to prepare circuits and/or devices with the compositions.
An interaction between an energy source, e.g. a source that generates 157 nm radiation, and the photo-acid generator results in the release of acid which facilitates selective cleavage of protecting groups from hydroxyl sites. As a consequence, the resultant unprotected hydroxyl groups are susceptible to solubilization under basic conditions, i.e., the regions of the photoresist material that are exposed to the far UV radiation are rendered alkali soluble, whereas the unexposed (protected hydroxyl) regions of the photoresist material remain alkali insoluble. Suitable protecting groups for the hydroxyl groups of the polymer include acetals, ketals, esters (including carbonates) and ethers.
In one general class of positive photosensitive compositions, according to the invention, polymer components can have the formula
where D is a carbon atom or a cyclic or bicyclic group. Y
1
, Y
2
and Y
3
, are each independently hydrogen atoms, electron withdrawing groups (e.g., halogen atoms), or a pendent group as described below, and a is a positive value from 1 to 100, inclusive, b is a value from 0 to 100, inclusive, and z is a positive value from 2 to 100,000 inclusive. P is a protecting group for a hydroxyl group selected from the group of acetals, ketals, esters (including carbonates) and ethers and T denotes a covalent bond between carbon atom Da and OP, or is a bridging group having the formula:
wherein Z
1
and Z
1
are each independently an-electron withdrawing group or a hydrogen atom and f is a value from 0 to 6.
Each K, independently, is an electron withdrawing group, or a pendent group having the formula:
where X
1
, X
2
, X
3
, X
4
and X
5
are each independently hydrogen atoms or electron withdrawing groups, preferably halogen atoms and g is a value from 0 to 4, inclusive. For example, when D is a carbon atom and Y
1
, Y
2
and Y
3
are hydrogen atoms then at least one, and preferably at least two, of X
1
, X
2
, X
3
, X
4
or X
5
of T are electron withdrawing groups, e.g., halogen atoms, and when only one of Y
1
, Y
2
and Y
3
are halogen atoms, then at least one of X
1
, X
2
, X
3
, X
4
and X
5
of T preferably are electron withdrawing groups, e.g., halogen

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