Michael F. Cronin

The relationship between critical dimension shift and diffusion in negative chemically amplified resist systems

Reports of latent image instability in several positive acid catalyzed resists have stimulated interest in the image stability of advanced negative resists (ANR). The critical dimension stability of the chemically amplified ANR resist SAL603 with respect to delay time between the exposure and post‐exposure bake (PEB) steps was determined. Both dense and isolated 1.0 μm features were stable during a 24 h delay period and delay times from 24 h to 48 h resulted in a decrease in linewidth. The method of loss in linewidth as a function of delay time was not due to diffusion of the acid within the film. The threshold crosslink density model was introduced based on several simple assumptions concerning the effect of crosslink density (Θ) on image formation and the relationship of the crosslink density to resist acid concentrations. A method was described whereby the threshold crosslink density model can be used to determine the relative crosslink density of the resist as a function of distance along the feature ...

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.

Resist application effects on chemically amplified resist response

Chemically amplified negative resist films of Shipley SAL 605 at 1.0 μm thickness show a statistically optimized sensitivity that is twice that observed with 0.5 μm films when exposed to x rays from an electron storage ring. Because of the large ‘‘depth of focus’’ of proximity x‐ray lithography, the differences between thick and thin films are due to resist chemical and physical effects and processing conditions, not to the exposure process. Using identical processing conditions, 1.0‐μm‐thick films showed 34% larger linewidths than 0.5 μm films for isolated lines from one mask with a target linewidth of 0.215 μm. Gel permeation chromatography results indicated that there was no dependence of resin or crosslinker concentration on thickness of the applied film. The photogenerated acid and residual solvent were quantified prior to postexposure bake, and neither was able to explain the apparent sensitivity dependence on thickness. The work suggests that some of the differences arises in the resist application process because of the ability of the thicker film to retain small molecular weight species which facilitate the linewidth production. This implies that thinner films should show better resolution purely because of chemical and physical factors in the resist film.

Process optimization of the advanced negative electron beam resist SAL605

Electron beam lithography requires a high resolution resist system capable of maintaining tight linewidth tolerances. These tolerances require a resist process which yields the greatest process latitude. SAL601‐ER7 is an example of an electron beam resist which has demonstrated the ability to provide submicron resolution coupled with high resist sensitivity. This paper will focus on developing an optimized process for a new advanced negative electron beam resist (ANR), SAL605 which also utilizes a chemically amplified crosslinking system. The SAL605 electron beam resist is a next generation version of SAL601‐ER7, and as such has many of the same processing requirements. The effect of developer normality and development time on sensitivity and linewidth control with MF‐312 based developers will be presented. The effect of post‐exposure bake (PEB) temperature and time with respect to sensitivity and linewidth control will also be presented. We will show that there are processing windows for submicron resolu...

Ultrathin DUV resists for logic applications

Ultrathin ESCAP resists have been examined for logic applications in our paper. Lithographic studies have been carried out with these resists at the thickness of 60, 120 and 400 nm for contacts, and at the thickness of 250 vs 400nm for gate layer applications. A clear relationship between optimization of resist profile and film thickness through design of inhibited resists is shown. As the resist thickness is reduced to the UTR regime, more significantly inhibited resists are necessary. This effect is shown through the low Rmin values for XP1335 and the extreme surface inhibition built into the resist film. The benefits of UTR resists are multiple: larger overall process windows particularly with respect to exposure latitude, profile optimization through inhibition, and reduced pattern collapse effects. We can see dramatic differences in dissolution properties and solvent distribution below 120nm film thickness for the UTRs in this study. We attribute most of these differences due to solvent distribution effects which are more severe as the resist is thinned. It is well known from previous studies, that photogenerated acid and volatilized base profiles may be also affected by solvent depletion. There also is no dramatic change in photospeed as resist thickness is reduced.

Development of DUV resists for zero angle and angled implant applications

As the semiconductor industry continues to follow Moores Law by continually shrinking linewidths, DUV lithographic capacity is increasing. This greater capacity has increased the use of 248nm DUV lithography for all levels particularly applications such as metal and implant layers. Smaller features have required that more advanced implantation techniques be employed. These include greater control of implant depth, gradient, and lateral ion movement. These tighter requirements on the implant process naturally necessitate advanced requirements on the photoresists used in these processes. This paper will discuss the design criteria necessary to develop an advanced DUV resist for a variety of implant layer sand will show resist performance for these applications.

DUV positive resist system designed for Micrascan use

The development of a new DUV positive resist, XP-9493, is reported. XP-9493 was designed for the Micrascan II exposure system. Its absorbance over the 245 - 252 nm wavelength range is 0.48/micrometers , substantially higher than that of APEX-E, which is approximately 0.2/micrometers . The dissolution selectivity, tan ((phi) ), of XP-9493, 7.7, is much higher than that of APEX-E, 4.1. Thus, XP-9493 is capable of higher resolution than APEX-E with better swing curve control. It is shown that the deprotection rate of this resist is proportional to the pKa of the photogenerated acid. This result leads to the prediction that protonation of the protected polymer may determine the rate of the deprotection reaction. Further study of the photospeed vs PEB temperature showed a two-tier activation energy for the deprotection reaction. At high PEB temperatures, >= 95 degree(s)C, the deprotection reaction is diffusion-limited, where the protonation step is rate limiting. The activation energy, Ea, under diffusion limited conditions is only 7.6 kJ/mole. At lower PEB temperatures, < 95 degree(s)C, the deprotection reaction is reaction-limited, where the deprotection of the protonated protected polymer is rate-limiting. Ea under these conditions is 26.3 kJ/mole. This behavior leads to lower linewidth shifts per unit PEB temperature change, (Delta) CD/(Delta) T, at the recommended process conditions. For XP-9493, (Delta) CD/(Delta) T is approximately 6 nm/ degree(s)C for 300 nm line/space pairs. XP- 9493 resolves 0.225 micrometers line/space pairs at 19 mJ, and 0.25 micrometers contact holes at 27 mJ. The 0.25 micrometers process window for XP-9493 is >= 1.0 micrometers depth-of-focus, with 25% exposure latitude on a MSII exposure tool.

Film life enhancement of chemically amplified electron-beam resists

A key resist requirement for electron beam sensitive resists is an extended film life of up to 12 weeks, the demands of which far exceed the reported film life of current high resolution acid catalyzed electron beam sensitive resists. Simple techniques are described which increase the coated film life of acid catalyzed electron beam resists, specifically MICROPOSITTM SALTM603 E-Beam Resist, to greater than 12 weeks. Evidence shows that water in the resist film is the likely cause of decreasing resist sensitivity. Finally, it is demonstrated that electron beam sensitive negative acid catalyzed resists have sufficient film life stability to be a viable choice for routine mask making applications.

Advanced chemically amplified resist

This paper describes a set of empirical correlations between bulk resist properties and fine line iniaging quality in an Advanced Negative e-beam Resist (ANR) . At the on set of this study, it was not clear whether bulk properties, such as contrast, could be used to predict image quality. Consequently, several definitions of contrast have been examined and compared with imaged structures at four unique processes. These processes were selected based on the results of a modified Taguchi1 L9 experimental design. The slope of the line intersecting the thickness response curve at zero percent and ninety percent retention provides the best correlation with image quality. This bulk parameter is recommended as a suitable nionitor for sub-half micron e-beam ANR image quality in 2.38% TMAH.