by Debra Vogler, Senior Technical Editor, Solid State Technology

Oxford Instruments Plasma Technology recently announced a collaboration with NIL Technology to develop etch processes targeted specifically for the etching of nanoimprinting stamps, de-scumming of the NIL polymer, and nanoscale etching with the NIL polymer mask. The work was accomplished using Oxford's Plasmalab System 100, an inductively coupled plasma (ICP) tool. Data from the joint work was presented at the Plasma Etch & Strip in Microelectronics conference held at IMEC (Sept. 10-11).

Attributes of ICP that make it desirable for NIL applications are its high ion density rate (>10¹¹-10¹² cm^-3), low process pressures, and the ability to separately control the ion energy and ion density, according to Colin Welch, Oxford's principal applications engineer. "We're extending the ICP tool's use into a new area for us...it's an evolution of the same ICP technology," he said. Among the key requirements for NIL stamp etch: uniform depth and CD control, no negative slope (i.e., profile), and smooth sidewalls with no trenching (see image below).

"Etching of the stamp used in the imprint process -- typically made in silicon or fused silica -- is important to make high performance stamps with a long lifetime and good CD control," noted Brian Bilenberg, CTO and founder of NIL Technology, who collaborated on the work with Welch. "Especially sidewall slope and sidewall roughness are important factors to control in this step."

A negative profile occurs when the top of a trench is slightly smaller than the bottom -- when the stamp is pushed into the trench the edge of the polymer will be damaged, and when the stamp is retracted there will be further damage. "It will get snagged -- it's like a barb," Welch explained.

To avoid trenching, the DC bias is reduced, but since the ionizing energy is a function of the bias power, Welch notes that a fine balance must be maintained to avoid bowing or "bottling" of the profile, as well as tapering. However, too low an ionizing energy, and the ions will not reach the bottom of the trench.

The ICP process is ideal for the descum application because it can be done under low-pressure conditions, which ensures anisotropic etching -- meaning that little etching is done sideways (i.e., isotropic etching is avoided), thereby avoiding a loss of CD control and losing the shape of the profile. "You can run a very low pressure process and the plasma will still be stable [and] viable," said Welch. Running the etch at higher pressures will result in "polymer etching," i.e., the pattern will etch sideways and the CDs of the trenches will get wider. Being able to run at lower temperatures will also reduce lateral etching. -- D.V.

IMAGE CAPTION:
SEM image of optimized fused silica stamp etch: 30nm features to 200nm depth. Source: Oxford Instruments.