High resolution lithography method

Abstract: This is a method for forming patterned features. The method comprises: forming a single layer of resist 12 on a substrate 10, the layer 12 having a thickness; patterning the resist by selective exposure to a first energy source 16 to modify the developing properties of portions of the resist, leaving an amount of the thickness unexposed; and developing the resist. This is also a device which comprises: a substrate; a layer of resist over the substrate; and an energy absorbing dye in the resist. Other methods and structures are also disclosed.

Description: FIELD OF THE INVENTION This invention generally relates to a method for forming patterned features in the field of semiconductor devices and in particular in the field of lithography. BACKGROUND OF THE INVENTION The semiconductor industry is constantly striving to achieve higher density electronic devices. As the industry has moved into micron, submicron and even sub-half-micron sized features to achieve higher densities, the need for improved lithography methods to create such minute features has increased. Among the problems associated with conventional lithography techniques are the lack of uniformity of exposure of resist through a thick layer of resist and scattered light within the layer of resist due to reflective metallized surfaces under the resist. These problems tend to compound the loss of resolution problem by creating ill-defined patterns at the onset. Standard methods for image development, in the exposed etch resist, fall short of the requirements for sub-half-micron feature generation. Wet development of the etch resist often produces a positive-grade slope on the feature sidewall that degrades the contrast of image transfer into the underlying thin film during the dry etch process. This is due to the less than infinite etch rate selectivity between the resist and the film material. Thus, there is a need for a method for forming high resolution submicron and sub-half-micron sized features on a semiconductor device. In EP-A-204 253 and in IBM Technical Disclosures Bulletin, vol. 32, Nr. 7, December 1989, page 266: "Laser Development of Latent Images" and in EP-A-184567 processes are disclosed where a single resist layer covering a substrate is image-wise exposed to a very limited depth in order to modify the physical and/or chemical properties. These processes comprise a step of forming a resist layer, surface-patterning the resist and developing the resist. In Journal of The Electrochemical Society, vol. 133, No. 1, pages 192-196; T.R. Pampalone et al.: "Improving Linewidth Control over Reflective Surfaces Using Heavily Dyed Resists" dyed single resist layers preventing backscattering and interference patterns are described. SUMMARY OF THE INVENTION According to the invention the method for forming patterned features comprises: forming a single layer of resist on a substrate, the layer having a thickness; patterning the resist by selective exposure to a first energy source to modify the developing properties of portions of the resist, leaving an amount of the thickness unexposed; and developing the resist by simultaneously using a second energy source and a developing reagent. The first and second energy sources being both ultraviolet light. The resist may be acrylic-based. A dye may be incorporated into the resist to prevent exposure of the amount of the thickness to be left unexposed. The developing reagent may include oxygen or hydrogen. The patterning step may occur in an oxygen-free environment. A nitrogen purge may be used to create the oxygen-free environment and the substrate may have multiple layers. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: FIG. 1 is a cross-sectional view of forming a resist layer on a substrate; FIG. 2 is a cross-sectional view of formation of crosslinked regions; FIG. 3 is a cross-sectional view of patterning a preferred embodiment of the present invention. Corresponding numerals and symbols in the different figures refer to corresponding parts unless otherwise indicated. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Methods to achieve high resolution of a submicron sized feature include increasing the numerical aperture of the imaging tool and/or decreasing the wavelength of light used to expose the etch resist. In both instances, the image depth of field is diminished to less than the dimension of the thickness of the etch mask film and the surface topography. Resultantly, the projected image transfer from the master reticle is unacceptably distorted in various areas of the exposure field. Methods to overcome these problems, such as multi-layer resists, have been proposed. This involves "sensitizing" the surface layer of the etch mask, in some fashion, so the imaging tool only needs to project an accurate image onto the surface of the resist. Hence, the depth of field only needs to be greater than the characteristic surface topography. However, this has not been well-accepted in production environments. Process complexity, particle generation and poor critical dimension control and uniformity have been cited as shortcomings. A surface-imaging technique, that is compatible with an image development method, is needed to successfully pattern submicron and sub-half-micron features. Surface-imaging is needed to overcome fundamental depth-of-field limitations, associated with the optical imaging tool. The surface-imaging should preferably not be complex or sensitive to small variations in process conditions. A compatible image development technique is needed to ensure the exposure image is faithfully transferred to the remainder of the resist. In turn, this is preferably done in a manner that creates a structure that will faithfully transfer the image into the underlying substrate. Disclosed is a surface-imaging exposure method in conjunction with a dry development method to service submicron and sub-half-micron lithography requirements. A layer of resist 12 is deposited on a substrate 10 that is to be patterned, shown in FIG. 1. The resist 12 does not need to contain a sensitizer, also referred to as a PAC (Photo Active Compound). Referring to FIG. 2, a mask 14 may be used to selectively expose the resist 12 to an energy source 16, such as a deep UV exposure tool, to crosslink the resist 12 on selected regions 18 of the resist 12 surface. The crosslinking is preferably done in an essentially oxygen-free environment. One method which may be used to realize a oxygen-free environment is a nitrogen purge. The process steps shown in FIG. 1 and FIG. 2, are followed to produce crosslinked regions 18 in the surface or the resist 12. The structure is not exposed to a diffusion reagent. Then the structure goes directly to the develop stage, shown in FIG. 3. The resist 12 is preferably anisotropically dry developed with high contrast by a gentle, photo-assisted etch process (or other "gentle," low-energy etch processes, such as ECR), involving, for example, an oxygen-containing source as a reagent and a light source 22 as an energy source that is capable of disrupting the surface bonding. The "dangling bonds" that are generated by the disruptive light 24 (or other energy source), react with the oxygen-containing reagent (and/or its products), to remove the resist in the regions 20 that are not crosslinked. The crosslinking serves as an etch mask and the resultant pattern in this embodiment is negative tone. Again, since such low-energy processing may exhibit excellent etch rate selectivity between the crosslinked regions and the uncrosslinked regions, acceptable CD control and uniformity of the feature may be achieved and vertical sidewalls, for crisp image transfer into the underlying film, may be generated. The resists which may be used for the embodiments described above are numerous. The choice is limited only by the energy source 22 used and the embodiment selected. In the embodiment described above, the crosslinked regions 18 serve as an etch mask and must therefore be more resistant to etching. As an example, an acrylic-based resist may be used, which when crosslinked becomes very hard and resistant to etching. Dyes may be incorporated into the resist to limit the depth of the conversion caused by the crosslinking and still avoid depth of field problems even with a relatively intense exposure. Preferably the dye is a photon absorbing dye. The strength and intensity of the energy source chosen directly affects the degree of resistance to etching required in the crosslinked regions 18 of the resist 12. This embodiment takes advantage of surface-imaging by crosslinking the surface of the resist 12 in the regions 18 exposed, for example, by a UV optical imaging tool. The depth of the crosslinking is not expected to be more than several hundred nm and could, as an example be in the vicinity of 10 nm (100 Å). This embodiment also takes advantage of dry development, to produce high image contrast vertical sidewalls for good CD control and uniformity, and to generate an etch resist feature that will accommodate good image transfer into the underlying film. A preferred embodiment has been described in detail hereinabove. It is to be understood that the scope of the invention also comprehends embodiments different from those described, yet within the scope of the claims. The oxygen-containing reagent used in the develop step may be replaced with, for example, hydrogen or a hydrogen liberating source such as ammonia.

Claims: A method for forming patterned features, said method comprising: a. forming a single layer of resist on a substrate, said layer having a thickness; b. patterning said resist by selective exposure to a first energy source to modify the developing properties of portions of said resist, leaving an amount of said thickness unexposed; c. developing said resist by simultaneously using a second energy source and a developing reagent; wherein said first and second energy sources are both ultraviolet light. The method of claim 1, wherein said resist is acrylic-based. The method of claim 1, wherein said resist does not contain a sensitizer. The method of claim 1, wherein a dye is incorporated into said resist to prevent exposure of said amount of said thickness to be left unexposed. The method of claim 1, wherein said developing reagent. includes oxygen. The method of claim 1, wherein said developing reagent includes hydrogen. The method of claim 1, wherein said patterning step occurs in an oxygen-free environment. The method of claim 7, wherein a nitrogen purge is used to create said oxygen-free environment. The method of claim 1, wherein said substrate has multiple layers.

Inventors: DOUGLAS MONTE A, DOUGLAS, MONTE A.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1992
Main classification: G03F7
IPC codes: H01L21, G03F7