Excessive dark loss has been observed along the edge nearest the lid of aged chemically amplified resist blanks, which was traced to organic acid contamination evolving from the acrylic plastic lid of the shipping box. Thermal Gravimetric Analysis (TGA) combined with Fourier Transform Infrared Spectroscopy (FTIR) of the shipping box lid material have proven useful in identifying that organic acid evolves from the plastic at 110°C. An alternative plastic shipping material offered by the supplier was tested with the same analysis technique and no organic acid was evolved during the test. To accelerate the aging effect, both lid materials were baked in an oven for 4 days, and no excessive dark loss was observed with the new shipping material. An evaluation with chemically amplified resist comparing storage in the original shipping materials at ambient conditions vs. storage in dry nitrogen demonstrate that nitrogen storage improves, but does not eliminate, the excessive dark loss from the original plastic lid material.
Resist heating has been known to be one of the main contributors to local CD variation in mask patterning using variable shape e-beam tools. Increasingly complex mask patterns require increased number of shapes which drives the need for higher electron beam current densities to maintain reasonable write times. As beam current density is increased, CD error resulting from resist heating may become a dominating contributor to local CD variations.
In this experimental study, the IBM EL4+ mask writer with high voltage and high current density has been used to quantitatively investigate the effect of resist heating on the local CD uniformity. ZEP 7000 and several chemically amplified resists have been evaluated under various exposure conditions (single-pass, multi-pass, variable spot size) and pattern densities. Patterns were designed specifically to allow easy measurement of local CD variations with write strategies designed to maximize the effect of resist heating. Local CD variations as high as 15 nm in 18.75 × 18.75 μm sub-field size have been observed for ZEP 7000 in a single-pass writing with full 1000 nm spots at 50% pattern density. This number can be reduced by increasing the number of passes or by decreasing the maximum spot size. The local CD variation has been reduced to as low as 2 nm for ZEP 7000 for the same pattern under modified exposure conditions. The effectiveness of various writing strategies is discussed as well as their possible deficiencies. Minimal or no resist heating effects have been observed for the chemically amplified resists studied. The results suggest that the resist heating effect can be well controlled by careful selection of the resist/process system and/or writing strategy and that resist heating does not have to pose a problem for high throughput e-beam mask making that requires high voltage and high current densities.
The mask fabrication industry is slowly migrating to chemically amplified (CA) resists to take the advantages of their high contrast, resolution, and sensitivity. During this migration process, the industry has encountered several problems associated with CA resists such as baking homogeneity of thick mask plates on hot plates, footing on Cr masks, and storage stability of mask blanks. In addressing these issues, we have adopted a low Ea CA resist platform to overcome the bake latitude issue. The resist formulation has been reformulated to reduce the footing and a new package method has been introduced to extend the storage of the blanks. In addition, we will also discuss our studies on two major areas, such as sensitivity and etch resistance, which we think is extremely important for E-beam resists in the future. The mask industry started with 248nm DUV CA resist systems and then found out that there was a need for even higher sensitivity resist systems to address the throughput issue. In our early study, we have observed that by simply increasing photoacid generator loading in the resist formulation we were able to increase the sensitivity, but there was a significant reduction in the dose latitude. After studying the dissolution and inhibition properties of different PAGs, we have been able to optimize PAG and base loading in combination with proper choice of PAGs to achieve high sensitivity and large dose latitude. The new resist formulation exhibits a large dose latitude of 38% for 100 nm l/s images with high sensitivity of 4.4μC/cm<sup>2</sup> at 100 kV. Due to the electron scattering effect and the image collapse issues with thicker resists, thinner imaging layer is desirable. Sufficient etch selectivity is needed to compensate the insufficient resist thickness. Therefore, there is a need to develop a high Cl<sub>2</sub>/O<sub>2</sub> RIE (used in Cr etch process) etch resistant resist system for mask making. We have reported earlier that a resist formulation based on blending KRS-XE with SSQ polymer has resolved 50nm l/s resist images with etch rate 20% better than conventional novolak I-line resist systems. Since then, we have investigated a few new SSQ polymers and found some lithographic improvement in this new blending systems due to better compatibility of the SSQ polymer to the KRS-XE.
KRS-XE, a high performance chemically amplified photoresist designed specifically for e-beam mask making applications, has been enhanced to achieve reduced “footing” on chrome oxide surfaces while still maintaining the original lithographic characteristics that make KRS-XE a promising mask making candidate. These attributes include high resolution, superior bake latitudes, high vacuum stability, coated shelf life of greater than 2 months, and, most notably, the absence of a post exposure bake. In conjunction with the footing reduction the requisite sensitivity requirement of <10uC/cm2 with 50 keV exposure tools has been achieved while retaining the robust process latitude previously reported for this resist. Through a careful study of the photoresist formulation components a route to the ultra-high sensitivity of <2.5uC/cm2 at 50 keV has been elucidated which will further enhance throughput, decrease heating effects, and potentially be a suitable resist for e-beam projection lithography (EPL).
KRS-XE is a chemically amplified resist developed to enable electron-beam lithography for mask making at the 100nm node. This material has been shown to provide an excellent process window for mask manufacturing at this node. Characterization of this material using both 50keV raster and 75keV vector scan e-beam exposure systems will be presented. A higher sensitivity version of this material has been developed specifically for a vector, shaped beam 50keV application. Initial mask manufacturing results for this higher sensitivity version of KRS-XE will be presented for 75keV. In addition, recent developments using KRS-XE formulations modified to achieve high sensitivity and improved etch resistance will be discussed.
The traditional mask making process uses chain scission-type resists such as PBS, poly(butene-1-sulfone), and ZEP, poly(methyl a-chloroacrylate-co-a-methylstyrene) for making masks with dimensions greater than 180nm. PBS resist requires a wet etch process to produce patterns in chrome. ZEP was employed for dry etch processing to meet the requirements of shrinking dimensions, optical proximity corrections and phase shift masks. However, ZEP offers low contrast, marginal etch resistance, organic solvent development, and concerns regarding resist heating with its high dose requirements1. Chemically Amplified Resist (CAR) systems are a very good choice for dimensions less than 180nm because of their high sensitivity and contrast, high resolution, dry etch resistance, aqueous development, and process latitude2. KRS-XE was developed as a high contrast CA resist based on ketal protecting groups that eliminate the need for post exposure bake (PEB). This resist can be used for a variety of electron beam exposures, and improves the capability to fabricate masks for devices smaller than 180nm. Many factors influence the performance of resists in mask making such as post apply bake, exposure dose, resist develop, and post exposure bake. These items will be discussed as well as the use of reactive ion etching (RIE) selectivity and pattern transfer.