Cesar M. Garza

Manufacturability Issues Of The DESIRE Process

Surface-imaging schemes are an attractive alternative to overcome many of the limitations optical microlithography is presently facing. A good example of this type of approach is the so-called DESIRE process. A preliminary performance characterization of the DESIRE process showed a significant increase in resolution and process latitude. However, new process variables must be understood and technical challenges overcome before this process can be successfully implemented in a manufacturing environment. The purpose of this paper is to explore these new process variables and suggest solutions for the implementation of the DESIRE process in high-volume production of semiconductor devices.

Silicon diffusion characteristics of different surface imaging resists

This paper describes a study of the silylation characteristics of different resists that are suitable for single-layer, surface-imaging patterning applications. In particular, the effect of different process parameters on the silicon diffusion in UCBs Plasmask®resist is discussed. The diffusion profile of silicon in the resist is decorated by a staining technique followed by SEM analysis. This allows for two-dimensional resolution of the diffusion profiles and the observation of other process attributes. Links are established among exposure, silylation and etch by observing silylated profiles. It is shown that the silylation profile characteristics are dominated by the resist image created during exposure. Also, the effects of post-exposure bake and silylating agent temperature are presented. Diffusion profiles for MacDermids PR1024 are also shown.

A Practical Approach To Submicron Lithography

This paper will describe the characterization and optimization of a high resolution image reversal process using dissolution rate data and the development simulation computer program, PROSIM, and compare the results with experimental data. AZ®5214 E photoresist was chosen for its simplicity of processing in negative tone and the potential to print submicron lines and spaces with good latitude using a 0.35 NA I-line stepper. This resist is unique in the sense that the image reversal process is thermally induced after exposure and requires no pre- or post-exposure chemical treatment. Without the post-exposure bake this resist is positive working, and in either tone is developed with standard aqueous alkali developers. The key variables in the optimization are the temperature of post-exposure bake, the concentration of developer, type of developer, and the time of development. The driving force in the optimization was to achieve a high resolution process with good contrast and process latitude. Development rate data and the development simulation computer program, PROSIM, were used to assist in determining the process conditions with the best contrast and broadest process latitude. Using a process which included a flood exposure after the post-exposure bake it is possible to use a 110°C to 120°C post-exposure bake and developer concentration of 0.21N TMAH. Such a process produced the best resolution with excellent side wall profiles and wider process latitude, but with some sacrifice of photospeed. Also the effect of defocus was examined near the limits of resolution and superior latitude to a comparable positive tone system was found. Thin film interference effects were examined for their impact on linewidth control and process consistency and image reversal was found to suppress linewidth variation as compared to conventional positive tone resists. The AZ 5214 E in image reversal is sensitive over a broad spectrum of exposure energy. Data for image reversed patterns produced from g-line steppers and from a 248 nm excimer laser will be presented.

Resists For Use In 248 Nm Excimer Laser Lithography

The future use of deep UV sources such as excimer lasers in optical projection lithography will push resolution limits down to 0.5 micron and possibly even beyond. Looking ahead to deep UV lithography, questions arise as to whether any current resists can be used with deep UV exposure and whether there is any difference in resist processing between the conventional blue/near UV and deep UV lithography. Using an excimer laser source and contact printing we have undertaken an evaluation of several novolak resists and their processing for future use in 248 nm lithography. The 0.8 micron pitch features on the lx masks can be replicated using 0.5 micron thick resists. Studies of the development process using real time interferometric resist thickness measurements indicate differences in the behavior of the resists to the deep UV exposure compared to near UV exposures. For the novolak based resists the principal difference is reduced contrast due to poor light transmission and due to the presence of crosslinking reactions which lead to decreased development rates. The slower development rates become significant in terms of processing only when the novolak resists are post-exposure baked to minimize standing waves.

Resist Dissolution Kinetics And Submicron Process Control

The details of photoresist dissolution kinetics play a major role in image quality and process control, especially at sub-micron geometries. Kinetic parameters for photoresist dissolution are obtained using methods familiar to most lithography engineers. For example, activation energies can be extracted from the temperature dependence of dissolution rate while chemical reaction orders are obtained from the variation of dissolution rate with developer concentration. Taken together, these data provide valuable insight, not only about the mechanism of dissolution but also about the details of resist performance. Large chemical reaction orders and negative activation energies are observed at low to medium exposure doses, which prevail at the edges of lithographic features. The decrease in exposure selectivity at lower temperatures might suggest that higher resolution can be obtained at higher developer temperatures. Furthermore, a resist-developer combination which exhibits a strong dependence of reaction order on exposure energy is expected to show higher selectivity than a system with a lower reaction order-exposure dependence. Other desirable lithographic properties such as sidewall angle, latitude and resolution will follow the same trend. In this paper, the results of experiments in dissolution kinetics are tied to submicron lithography through semiempirical Prosim modeling. Activation energies are measured for various exposures and developer concentrations. Dissolution reaction orders are obtained for various developer temperatures and exposure energies. The computer modeling program, Prosim, is then used to model resist characteristics under various conditions. These include exposure, focus and mask-matching latitudes as well as resist profiles. Prosim calculations are verified by selected applications results which illustrate the influence of kinetic parameters on lithographic performance.

Exploring the limits of phase-shift lithography: Part I--the alternating shifter

Phase shift technology shows promise to extend the useful resolution and focus latitude to contemporary optical steppers. If successful in application, this represents significant cost savings to the manufacturing wafer fobs provided that the steppers can be used or modified to take advantage of phase-shift techniques. In this paper we explore the limits of phase-shift lithography, particularly at i-line. We do this following a two-fold approach: a) using simulations and b) collecting experimental data using different resist processes and phase-shift techniques. We conclude that using state-of-the-art photoresist processes and phase-shift techniques, i-line optical lithography can be extended to the 0.25 ?m regime.

Surface imaging lithography at 248 nm

This paper discusses the use of surface imaging lithography for deep UV step and repeat applications. In particular, results obtained using the DESIRE process are presented. It is shown that surface imaging is a very attractive option for 248 nm lithography given the small depth of focus of steppers working at this wavelength and the lack of standard, commercially available, photoresists able to fulfill the requirements of deep UV lithography. First, the concept of surface imaging lithography and the DESIRE process are reviewed. Next, results obtained using g‐line resists exposed at 248 nm are presented. Although the results are acceptable, the exposure requirements are prohibitive. A deep UV formulation of the resist allows for higher silylation temperatures and the consequent reduction in exposure requirements to the 100 mJ/cm2 range. The focus latitude for 0.5 μm lines is ∼1.5 μm, while the uniformity across the wafer, for the same lines, is 5%. The etch is done using a highly selective process in a pa...

Applying deep ultraviolet lithography

This paper discusses the application of a deep wafer stepper and associated photoresist systems to advanced semiconductor processing. It is shown that, even with the limited number of photoresists available, deep UV lithography is a viable candidate for advanced processes requiring half micron and smaller features. First, the performance of the exposure tool is discussed; lens performance data including image field curvature, and distortion is shown. Next, results obtained using two different approaches for photoresist processing are discussed. Single layer wet developed photoresist processing is shown to be useable in those cases where topography is not severe. Surface imaging photoresist approaches using the DESIRE process are shown to be applicable in cases where topography is an issue or broader process latitude is required.

Preliminary Performance Characterization Of The DESIRE Process

The performance benefits of multilayer resist processes are well known, but because of the additional processing complexity their use has been restricted mainly to lab scale or pilot line environments. More recently a surface sensitive scheme [1], the so-called DESIRE process, has been developed. It has the potential of reaping the benefits of multilayer resist processes without their process complexity. In this paper a preliminary characterization of the lithographic performance of this process is described. In particular the issues of resolution, focus latitude, exposure latitude and uniformity are examined. When the appropriate silylation and development conditions are used, the data clearly show that a significant improvement in the working stepper resolution can be achieved while maintaining the uniformity across the wafer. In a 0.28 numerical aperture lens stepper, the exposure latitude first becomes the resolution limitation. Using the DESIRE process a five-fold increase in the exposure latitude is achieved as compared to an optimized process using conventional development. As the stepper numerical aperture increases, the focus latitude becomes the limiting factor to resolution. Using the DESIRE process the focus latitude increases nearly two-fold in a 0.35 numerical aperture stepper as compared to conventional processing. This preliminary characterization study shows that the DESIRE process has the potential of extending the useful life of optical projection printers.

Resist Characterization And Optimization Using A Hevelonment Rimulation Romnuter Nrooram, Prostm.

Computer simulation is a well established tool in the design and processing of integrated circuits. The best known such simulation program in microlithography is SAMPLE (1), which is based on the Dill model (2). A new computer simulation program, PROSIM, has been developed (3). It uses the aerial image calculated by SAMPLE and the string development algorithm of SAMPLE, but replaces calculated photoresist development rates with rates measured using in-situ interferometry. Three different positive photoresist systems were investigated under varying process conditions typically examined during the characterization process. PROSIM simulated profiles and exposure latitudes are in good agreement with experimental measurements, showing that PROSIM can be used as an effective tool to aid in the characterization and optimization of positive resist processes.