Lithography is a field in which advances proceed at a swift pace. This book was written to address several needs, and the revisions for the second edition were made with those original objectives in mind. Many new topics have been included in this text commensurate with the progress that has taken place during the past few years, and several subjects are discussed in more detail. This book is intended to serve as an introduction to the science of microlithography for people who are unfamiliar with the subject. Topics directly related to the tools used to manufacture integrated circuits are addressed in depth, including such topics as overlay, the stages of exposure, tools, and light sources. This text also contains numerous references for students who want to investigate particular topics in more detail, and they provide the experienced lithographer with lists of references by topic as well. It is expected that the reader of this book will have a foundation in basic physics and chemistry. No topics will require knowledge of mathematics beyond elementary calculus.

Principles of Lithography

An object to be processed has a chromium-based thin film made of a material containing chromium. The thin film is etched using a resist pattern as a mask. The thin film is etched by the use of a chemical species produced by preparing a dry etching gas containing a halogen-containing gas and an oxygen-containing gas and supplying a plasma excitation power to the dry etching gas to thereby excite plasma. The thin film is etched using, as the plasma excitation power, a power lower than a plasma excitation power at which plasma density jump occurs.

Method for etching chromium thin film and method for producing photomask

Aspects of the present invention provide for a novel photomask for patterning features for an integrated circuit, the photomask including masked features having interior nonprinting windows. In some embodiments, the interior nonprinting window is an alternating phase shifter, while the area surrounding the masked features transmits light unshifted. In other embodiments, the interior nonprinting window transmits light unshifted, while the area surrounding the masked features is an alternating phase shifter. Thus any arrangement of features can be patterned with no phase conflict.

Photomask features with interior nonprinting window using alternating phase shifting

Exposure optimization in high-resolution e-beam lithography

Method for reducing the fogging effect in an electron beam lithography system wherein the exposure is controlled in order to obtain resulting pattern after processing which are conform to design data. A model for the fogging effect is fitted by individually changing at least the basic input parameters of the control function, the function type is chosen in accordance to the Kernel type used in the proximity corrector. The proximity effect is considered as well and an optimised set of parameters is obtained in order to gain a common control function for the proximity and fogging effect. The pattern writing with an e-beam lithographic system is controlled by the single combined proximity effect control function and the fogging effect control function in only one data-processing step using the same algorithms as are implemented in a standard proximity corrector.

Method for reducing the fogging effect

The Cr etch rate is affected by Cr density to be etched at the photomask. Different mask-to-mask and within-mask pattern densities have made difficult to control the final CD. We have tested loading effect using binary Cr mask with ZEP7000 photoresist. The loading effect was evaluated for the masks fabricated at the various dry etch conditions with different within-mask Cr loading. The Cr etch rate and selectivity was observed at various process conditions and relations between parameters of dry etch process and Cr loading were evaluated. The horizontal and the vertical Cr etch rates were investigated and the process parameter dependence on the Cr loading was analyzed. The horizontal and the vertical photoresist etch rates were evaluated for the photoresist loading effect. The cause of mask-to-mask loading and within-mask loading are mainly from Cr loading and photoresists loading, respectively. The Cr loading is mainly affected by source power, pressure, and Cl 2 /O 2 ratio. In our system, within-mask Cr loading is strongly dependent on the process parameters when the selectivity of Cr to PR is below 1. If uniformity and selectivity are acceptable, high DC bias, high ga flow, low pressure, and high Cl 2 /O 2 ratio are recommended to reduce loading effect.

Loading effect parameters of dry etcher system and their analysis in mask-to-mask loading and within-mask loading

The Cr etch rate is affected by Cr density to be etched at the photomask and the Cr loading effect has become main obstacles to overcome for the next generation photomask process. Different mask-to-mask and within-mask pattern densities have made difficult to control the final CD (critical dimension). We have tested loading effect using binary Cr mask with ZEP7000 (3000 angstroms) photoresist. The loading effect was evaluated for the masks fabricated at the same process condition with the different mask-to-mask Cr loading and different within-mask Cr loading. The CD variations of dark field and clear field were observed and the phenomenological approach was proposed for the loading effect by some simplified equations.

Dry etching of Cr layer and its loading effect

As critical dimensions of photomask extends into submicron range, critical dimension uniformity, edge roughness, macro loading effect, and pattern slope become tighter than before. Fabrication of photomask relies on the ability to pattern features with anisotropic profile. To improve critical dimension uniformity, dry etcher is one of the solution and inductively coupled plasma (ICP) sources have become one of promising high density plasma sources for dry etcher. In this paper, we have utilized dry etcher system with multi-pole ICP source for Cr etch and MoSi etch and have investigated critical dimension uniformity, slope, and defects. We will present dry etch process data by process optimization of newly designed dry etcher system. The designed pattern area is 132 by 132 mm 2 with 23 by 23 matrix test patterns. 3 (sigma) of critical dimension uniformity is below 12 nm at 0.8 - 3.0 micrometers . In most cases, we can obtain zero defect masks which is operated by face- down loading.

Applications of MICP source for next-generation photomask process

For high-voltage vector e-beam writing systems, solving the resist heating effect problem is one of the highest priorities because it is a major factor affecting localized critical dimension (CD) uniformity. In order to write patterns for 90nm node devices, the utilization of proximity effect correction (PEC) is essential for e-beam mask writers to achieve high CD performance.
In this study, the dependence of CD variation on e-beam write conditions was investigated under optimum PEC parameter conditions. Writing conditions such as current density, shot size, number of writing passes, and settling time were tested to see their affects on resist heating. Industry-standard Nippon Zeon ZEP 7000 resist was written by a Toshiba EBM-3500B 50KeV vector e-beam writer using patterns found in sub-130nm node devices. Results indicated that the main factor affecting resist heating CD variation for ZEP 7000 was in fact the e-beam writer shot size selected. Multi-pass writing was effective in reducing the CD variation, and the settling time of each shot in the EBM-3500B had very little influence.

Optimum PEC Conditions Under Resist Heating Effect Reduction for 90nm Node Mask Writing

We have manufactured dry etcher system for photomask process utilized the new plasma source and process optimizations have been done for CD (critical dimension) uniformity and loading effects. The 3 σ of CD uniformity(final CD - develop CD, point by point subtraction) of Cr pattern, with 132 × 132 mm 2 area and 11 x 11 pattern arrays, was obtained below 10 nm, where the target CD is 0.8 um clear pattern. Cr and MoSi slopes are 88° ∼ 90°, which shows highly anisotropic etch. The selectivity of PR to Cr was over 1.6 at the clear area ratios of < 50 % and the selectivity was mainly affected by oxygen partial pressure and clear area ratio. Phase uniformity for PSM was 180 ± 1° and transmittance uniformity is within 6.3 ± 0.02 %. Validity and probability of dry etcher system to produce next generation photomask were discussed.

Soo-Hong Jeong

Papers

Loading effect parameters of dry etcher system and their analysis in mask-to-mask loading and within-mask loading

Dry etching of Cr layer and its loading effect

Applications of MICP source for next-generation photomask process

Optimum PEC Conditions Under Resist Heating Effect Reduction for 90nm Node Mask Writing

Dry etching technology of Cr and MoSi layers using high-density plasma source