In Photolithography, masks carry the pattern that is transferred onto a wafer.

Extreme Ultraviolet (EUV) lithography relies multilayer-coated mirrors and 13-nm-wavelength light. An EUV mask, or reticle, is made from a nearly perfect mirror coated with a patterned absorber layer. A mask is a highly complex optical system made from more than five different materials. The structure and material properties give the mask a wavelength-specific response and make testing with EUV light essential during development.

We have created a world-leading high-resolution EUV microscope capable of measuring the reflective properties of a mask on a 100-nm length scale, or smaller. Working in close collaboration with semiconductor industry sponsors, the project's goals include:

1. Understand and quantify EUV defects and repair strategies
2. Qualify other inspection tools becoming available world-wide
3. Validate and improve modeling
4. Evaluate the performance of actinic inspection

The AIT endstation on Advanced Light Source (ALS) Beamline 11.3.2. a bending-magnet beamline that provides clean, monochromatic EUV light.

Located on bending magnet beamline 11.3.2 at the ALS, the AIT provides monochromatic EUV light (λ = 13.4 nm) to the mask. The microscope operates in two modes. In scanning mode, the mask moves under the focused beam, while the reflected light is measured. In imaging mode, a tiny Fresnel zoneplate lens projects an image of the mask surface onto an EUV CCD camera.

Two Modes of Operation—Details
SCANNING MODE	IMAGING MODE
Bright-field Reflectivity testing → ≥ 1-μm spot → R measurements to ±0.1% Dark-field Scattering → Finds defects not seen by non-actinic     inspection tools. Calibrated Reflectivity → Measuring the incident and reflected     signal sequentially. → Region-of-interest identification → Scans a larger area quickly.	Exposure Time → ~0.5 alignment & navigation → ~10–20 s for highest resolution → As many as 450 images per 8-hour shift. Resolution(4x pixel over-sampling) → < 130 nm, Mask → ~32 nm, 4x Wafer equivalent Magnification → ~700x, direct to EUV CCD NA = 0.0625 → emulating an 0.25 NA, 4x stepper

The AIT provides detailed studies of important classes of defects. Ultimately, a "defect" is any unwanted mask feature that adversely affects the printed patterns. The sensitivity of the AIT allows it to quantify the strengths of measured defects in ways that printing cannot—i.e. without the complication of photoresist.

The table below shows several of the critical issues the AIT was designed to address, and the nature of AIT investigations in progress.

Particulate contamination is a serious problem for all lithography and steps are taken to protect the mask at every phase. For EUV masks, particles can fall on to the mirror surface before or after the multilayer coating is applied. Particles on the surface primary reduce the local reflectivity, while particles below the surface, depending on their size, may cause a noticable bump on the surface referred to as a phase defect. Other issues of concern are carbon contamination which can build up on the mirror surface when the mask is exposed to EUV light in an environment that is not clean, thermal damage or oxidation which degrades sharp multilayer interfaces and can reduce reflectivity.

Mirrors affected by these issues may appear perfect under inspection with other wavelengths of light besides EUV. So EUV actinic inspection with cross-comparisons is required during this pre-commercial, learning period. The AIT has been used in a number of such experiments. We have performed cross-correlation experiments with bump-type and pit-type phase defects. Since high-power UV laser light can damage the multilayer mirror surface, we have used AIT inspection to help set acceptable power levels for non-actinic inspection techniques. Operating as a high-resolution EUV microscope, the AIT detects changes in line patterns affected by absorber or phase defects, providing more detail about the aerial image than is readily available from printed images.

Page 2: Recent Upgrades and Progress
Page 3: Results; Funding