Will Conley

Negative photoresists for optical lithography

Negative photoresists are materials that become insoluble in developing solutions when exposed to optical radiation. They were the first systems used to pattern semiconductor devices, and still comprise the largest segment of the photoresist industry because they are widely used to define the circuitry in printed wiring boards. However, the current use of negative resists in the semiconductor industry has been limited by past difficulties in achieving high-resolution patterns. Recent advances in the chemistry of negative-resist systems, however, have provided materials with wide processing latitude and high resolution that are used to manufacture IBMs advanced CMOS devices and to achieve high-aspect-ratio patterns for micromachining applications. This paper provides an overview of the history and chemistry of negative-resist systems and their development in IBM.

Lithographic performance of an environmentally stable chemically amplified photoresist (ESCAP)

Improved stabilization of chemically amplified photoresist images can be achieved through reduction of free volume by film densification. When the host polymer has good thermal stability, the softbake temperature can be above or near the glass transition temperature (Tg) of the polymer. Annealing (film densification) can significantly improve the environmental stability of the photoresist system. Improvements in the photoacid generator, processing conditions, and overall formulation coupled with high NA (numerical aperture) exposure systems afford 200 nm linear resolution with excellent post-exposure delay stability. In this paper, lithographic data is shown for the improved ESCAP photoresist system (now called UVIIHS) currently under development for DRAM and logic device technology. We review the photoresist system, along with process- and formulation-related experiments on device levels and substrates demonstrating excellent 250 nm and sub-250 nm process windows.

New ESCAP-type resist with enhanced etch resistance and its application to future DRAM and logic devices

This new photoresist system extends the capability of the ESCAP platform previously discussed. (1) This resist material features a modified ESCAP type 4-hydroxystyrene-t-butyl acrylate polymer system which is capable of annealing due to the increased stability of the t-butyl ester blocking group. The resist based on this polymer system exhibits excellent delay stability and enhanced etch resistance versus previous DUV resists, APEX and UV2HS. Improved stabilization of chemically amplified photoresist images can be achieved through reduction of film volume by film densification. When the host polymer provides good thermal stability the soft bake conditions can be above or near the Tg (glass transition) temperature of the polymer. The concept of annealing (film densification) can significantly improve the environmental stability of the photoresist system. Improvements in the photoacid generator, processing conditions and overall formulation coupled with high NA (numerical aperture) exposure systems, affords linear lithography down to 0.15 micrometer for isolated lines with excellent post exposure delay stability. In this paper, we discuss the UV4 and UV5 photoresist systems based on the ESCAP materials platform. The resist based on this polymer system exhibits excellent delay stability and enhanced etch resistance versus APEX-E and UV2HS. Due to lower acrylate content, the Rmax for this system can be tuned for feature-type optimization. We demonstrate sub-0.25 micrometer process window for isolated lines using these resists on a conventional exposure tool with chrome on glass masks. We also discuss current use for various device levels including gate structures for advanced microprocessor designs. Additional data will be provided on advanced DRAM applications for 0.25 micrometer and sub-0.25 micrometer programs.

High-speed aqueous-developing negative resist based on triflic-acid-catalyzed epoxy polymerization

The need for higher resolution is a continuing driving force in the development of new lithographic materials. In this paper we discuss a new high speed, high resolution negative photoresist based on acid catalyzed epoxy polymerization. These materials are copolymers of two monomers that each provide a separate function in the photoresist. This combination provides a unique new material with aqueous processability in metal ion-free developers and high sensitivity to photogenerated triflic acid. Imaging characteristics in electron beam and i- line exposure systems are discussed.

Negative DUV photoresist for 16Mb-DRAM production and future generations

This paper discusses a new negative tone aqueous base developable photoresist that has demonstrated excellent sub-half micron resolution with commercially available DUV (deep ultraviolet) exposure systems. This system which consists of a phenolic resin (pHOST), a glycoluril crosslinker (TMMGU), and a triflic acid generating material is currently in use for the manufacturing of 16 M b-DRAM and related CMOS logic technology. We provide supporting manufacturing data relating to our experiences with this program, along with the benefits realized by the implementation of a negative tone photoresist system.

Negative tone aqueous developable resist for photon, electron, and x-ray lithography

The use of negative acting photoresists has become a integral part of device fabrication strategy. In this paper we will. discuss a phenolic based photoresist which incorporates a crosslinkable resin and an acid generating sensitizer. When exposed and thermally treated, the resist forms a negative tone image which is developable in an alkaline medium. We will discuss the materials, processes and results from photon, electron and X-ray lithographic evaluations.

PHS with inert blocking groups for DUV negative resist

The synthesis, characterization, and lithographic evaluation of a polyhydroxystyrene (PHS) modified with isopropyloxycarbonate groups is described. The inert blocking group is attached to the hydroxyl sites on PHS resin to slow the dissolution rate and make the resin useful in resists designed for 0.263 N TMAH developers. A negative tone resist (CGR-IP) that is formulated with the modified polymer is compatible with the industry standard 0.263 N TMAH developer and is capable of resolving 0.22 micrometer L/S features and 0.14 micrometer isolated lines on a 0.50 NA imaging system. Reaction with PHS resin occurs primarily at the phenolic sites as shown by carbon-13 NMR and 10% protection is sufficient to lower the dissolution rate to an acceptable level so that there is less than 50 angstrom film loss in exposed areas. The blocking group described here is not acid labile and reaming intact after the resist film is baked at 150 degrees Celsius.

Challenge of 1-Gb DRAM development when using optical lithography

The traditional lithographic approach employed by the semiconductor industry has been to pursue use of advanced prototype optical exposure tools and resists. The benefits of doing so have been: (1) The lithographic process that is used in development more closely resembles the process that will in fact be used to manufacture the chip. (2) The cost of low K1 imaging (phase-masks, off-axis illumination, and surface imaging resist) can be avoided. However with the introduction of 1Gb-dynamic random access memory (DRAM) development, a paradigm shift is being experienced within the optical lithographic community. With 1Gb-DRAMs, the minimum feature size falls irreversibly below the optical wavelength used to image the feature. Such a situation will make low K1 factor imaging unavoidable. With 175 nm groundrules typical for first generation 1G-DRAMs, K1 factors near 0.4 will be common with 0.5 as an upper limit on advanced systems currently in development irrespective of optical wavelength. This paper will cover the selection process, experimental data, and problems encountered in defining and integrating the lithographic process used to support the critical mask levels on 1Gb-DRAM development. Factors considered include: resist, masks, and illuminations via both simulation and experiment. The simulations were conducted with both internal and externally developed software. The experimental data to be reviewed was generated using an experimental 0.6 NA KrF step and scan system provided by Nikon. The resist used is commercially available from the Shipley corporation.

Photosensitization in dyed and undyed APEX-E DUV resist

One of the principle concerns in the design of deep-ultraviolet (DUV) photoresist systems is optimization of the optical absorbance of the resist at 248 nm. Conventional novolak resists absorb strongly (OD greater than 1/micrometer), and are therefore not useful in the DUV. Most DUV formulations consist of a poly(hydroxystyrene)-based resin, a photoacid generator (PAG), and perhaps an additional component (crosslinker, dissolution inhibitor, dye, contamination stabilizer). There are multiple available PAGs, such as aryl onium salts, which have been designed to undergo efficient direct photolysis upon irradiation at 248 nm. Certain PAGs, however, are nearly completely transparent at this wavelength, yet are seen to function well in acid-catalyzed DUV resist systems. This is attributed to photosensitization of the PAG by the resin. Steady-state and dynamic fluorescence quenching and acid generation measurements were used to study this phenomenon and data for several systems are presented. It is concluded that both electronic energy migration and photoinduced electron transfer can play important roles in acid generation and that where possible, these functions should be incorporated into the design of high photospeed resist systems. Additional data is presented for a positive-tone photoresist with a photosensitizing dye component.

Further improvements in CGR formulation and process

In previous papers, we described initial evaluations of CGR 248 negative resist using a variety of exposure tools. During subsequent studies, the emphasis has been placed on optimizing material and process for Micrascan and Micrascan II pilot line and manufacturing operations. The formulation is based on polyhydroxystyrene (PHS), tetramethoxymethyl glycoluril, and a sulfonate ester of an N-hydroxy compound. We will discuss image stability as a function of delay time between post apply bake (PA) and expose and as a function of delay time between expose and the post expose bake (PEB). Further, data will show that immersion or puddle development provides a larger process window than spray development for features in the 0.30 to 0.35 micrometers range. The thermal stability of the imaged resist will be discussed as well as the shelf life which is at least 6 months at 23 degree(s) C. Finally, additional data is available concerning image uniformity and how print bias and etch bias contribute to the overall nested-isolated line offset for positive tone (APEX-M) and negative tone (CGR) resists. Data obtained from Micrascan II exposures with test reticles will demonstrate an acceptable process latitude for 0.30 and 0.35 micrometers features and a wafer to wafer image uniformity similar to that observed for APEX.