Applied Materials Ready with Tetra EUV Mask Etcher
By David Lammers
With customer interest in EUV lithography accelerating, Applied Materials will introduce its Tetra EUV mask etcher at the SPIE Bacus mask technology conference this week in Monterey, Calif.
Last September, Applied introduced the Tetra X etcher for conventional masks, aimed partly at the market for transmissive masks used in double and triple patterning with immersion 193 scanners.
“As soon as we finished Tetra X, our customers were pushing us extremely hard to make sure we were ready for EUV within the next year,” said Amitabh Sabharwal, general manager of the mask etch products division at Applied. The current hurry-up atmosphere contrasts with several years of delays to EUV readiness, he added.
“At this stage, all the customers doing EUV are accelerating at the same pace. Until last year I was hesitant to release a EUV product, to invest a bunch of dollars into it. Now, we see a strong customer pull from all sides,” he said.
EUV masks are patterned on the same MoSi multi-layer reflective surfaces seen on the EUV optics. The reflective mask blanks introduce certain end point control challenges, and the much higher cost of the blanks puts severe requirements on the mask etcher in terms of defects and contamination.
With EUV, there are “significant changes in the photomask itself, how it is going to be handled and cleaned. For the first time, we are dealing with a non-pellicle mask, and to a mask with more than three to four layers on it,” he said.
The biggest challenge is the switch from the well-understood chrome absorber material used for decades with transmissive masks to a tantalum absorber for EUV masks. Applied has some experience etching tantalum in IC applications, and it began its EUV mask development program six years ago, in 2005. Nevertheless, the industry knowledge base with etching chrome is deep and wide, while the experience with tantalum etch is relatively recent.
Chrome is a difficult material to etch, with relatively small differences between the etch rates of the absorber and the resist pattern. While tantalum is easier to etch, tantalum tends to generate more debris than chrome.
Banqiu Wu, chief technology officer of Applied’s mask and TSV etch division, said the chemistries for tantalum and chrome are quite different. The use of tantalum-based absorber materials required a complete redesign of the reactor in order to achieve a uniform etch rate. Improving control of the byproducts coming off the mask was another key objective in order to keep defects to a minimum.
Optimizing the substrate movement, the RF power, the rate of turning the plasma on and off, the line edge roughness (LER), all had to be “optimized very carefully so we get minimum defects,” Sabharwal said.
In a temporary sense, EUV masks present fewer OPC challenges. Today’s transmissive masks have lines at the relatively relaxed dimensions of 200, 400, or even 600 nm. But the OPC features include hammerheads and serifs with features in the range of 40-50-60nm, critical dimensions which require tight control by the etching system. EUV lithography is likely to come in with relatively relaxed OPC features, but as design rules move to 11nm and beyond even EUV will require complex OPC features.
Sabharwal said the biggest challenge facing mask makers is the high cost of the substrates. The mask etcher must work nearly perfectly.
Today, it is “fairly well known how to build chrome transmissive masks. With the EUV reflective masks, to produce a defect-free mask is extremely complicated,” he said. A particle on one layer of the Bragg reflector can be encapsulated by subsequent layers, resulting in a “big hump” at the substrate’s surface which is often difficult to repair.
“From our side, our customers are very tough on us for defects,” he said.
A transmissive mask coming out of the Tetra mask etcher. (Source: Applied Materials)
Tags: Applied Materials, Bacus, EUV lithography, masks, SPIE