UV Microscopes Inspect Submiron Features

Here's how microscope manufacturers developed UV microscopes for use in imaging submicron semiconductor devices.

C.G. Masi, Contributing Editor -- Test & Measurement World, 12/1/1998

Traditional high-resolution optical microscopes—the kind semiconductor manufacturers have always relied on to verify the quality of their lithography processes—run out of steam for device feature sizes below 0.25 microns. Yet, design rules at the world’s most advanced fabs have already passed below that limit, and according to the 1997 technology road map put out by the Semiconductor Industry Association, design rules should drop below 0.10 microns by about 2005.1 Clearly, the industry needs new inspection tools.

A number of microscopic imaging techniques are available, such as confocal microscopy, scanning-electron microscopy (SEM), and atomic-force microscopy (AFM). But these methods are expensive and don’t lend themselves to basic inspection tasks. According to John Rowan, assistant manager of the Microelectronics Systems Department at Nikon (Melville, NY), instruments such as SEMs lend themselves more to use in R&D labs. They’re expensive and require a special environment, such as a high vacuum. Whereas, with deep-ultraviolet light, you can have real-time imaging in an ordinary production process.

The most straightforward way to extend the limits of high-resolution microscopy is to redesign the standard compound microscope to use ultraviolet (UV) light at the same wavelengths steppers use to lay down circuit features in the first place. Microscope manufacturers are starting to offer UV microscopes. Here’s how they developed and designed them.

Microscopes Require a New Design
Compound microscopes operating at the deep-ultraviolet (DUV) wavelength of 248 nm can resolve features as small as 0.08 microns. Designing a microscope to operate at such a wavelength, however, required vendors to completely rethink the instrument’s design.

According to Werner Hunn, director of product and market development at Leica Microsystems (Chicago, IL), optical glasses, which have been optimized for visible-light (VIS) applications, transmit UV radiation poorly. For that reason, the first UV microscopes used reflection optics made of multiple paraboloid first-surface mirrors instead of lenses. Such designs are large and clumsy, which makes them inappropriate for high-resolution semiconductor inspection applications.

Getting UV light through standard optical glass is made more difficult by the fact that available UV light sources tend to be less intense. In addition, the materials of interest, especially photoresists, absorb more UV than visible light. The astronomer’s trick of simply making the objective larger to gather more light won’t work for these UV microscope designs. To maximize resolution, optical designs have already brought microscope-objective numerical apertures (the microscopic measure of a “fast” optical system) to near their limit of 1.0 for nonimmersion objectives. Immersion lenses, where a drop of oil or glycerin is placed between the objective and sample, allow higher numerical apertures, but would cause contamination problems for production wafers.

Thus, UV microscope manufacturers use special glasses formulated to transmit radiation for the i-line (365 nm) and G-line (436 nm) UV emission bands in the output of a mercury lamp and by using quartz or calcium fluorite glasses for DUV. The optical components manufactured from these materials need anti-reflection coatings optimized for the wavelength at which they will be used.

Anti-reflection coatings are thin-film interference filters deposited on the lens surfaces. Film thicknesses are optimized so light waves traveling through the system see constructive interference and pass through unimpeded, while light waves reflecting back from the lens surface suffer destructive interference and are suppressed. Because the wavelengths of UV light are shorter than those of visible light, film thicknesses must be proportionately thinner.

Because UV-transparent lens materials, UV wavelengths, and UV bandwidths differ from those used in visible-light microscopes, the entire optical system must be reevaluated. For optical designers, the new constraints bring both good and bad news.

The good news is that color correction is practically trivial for UV optics. UV microscopes cover the same illumination bands that steppers use for exposing photoresists, as shown in Table 1. In comparison, the table also shows the wavelength range for a VIS microscope. Note that all the UV bands are less than 20 nm wide, which is more than an order of magnitude narrower than the visible band.

Figure 1 shows how the narrowness of UV illumination bands affects the color correction needed for a UV microscope optical system. In VIS optics, red light focuses closer to an uncorrected lens than blue light. This effect is called chromatic aberration. To overcome the focusing problem, the lenses use color-corrected optics.

Figure 1. (a) Chromatic aberration in the wide visible-light band makes red (long wavelength) light focus much closer to the lens than blue (short wavelength) light. (b) The much narrower UV illumination bands make color correction much easier.

Color-corrected optics use two or more elements made of materials that possess different chromatic-aberration properties. Through careful calculation and construction, the chromatic aberrations of the different optical components almost cancel each other, so the lens focuses all of the light in the wavelength band at the same point. The wider the band, the more difficult it is to perform color correction. The fact that UV illumination bands are so much narrower than the VIS band makes the color correcting in UV lenses vastly simpler.

The bad news for UV microscope designers is that the only way to capture a UV image is by using a CCD camera. CCD cameras, however, are far less forgiving of geometric aberrations, such as coma and field curvature, than are human eyes—which cannot detect UV light. Thus, while less work needs to go into color correcting UV lenses, more work has to go into removing geometric aberrations.

Switch to Visible Light, Too
Things look different under UV light, says Werner Hunn. For that reason, you need some way to quickly switch between UV and visible wavelengths so you can compare what you are seeing with UV light with what you can see under visible light.

Considering that all of the optics as well as the illumination source have to change, a “quick” switch can be a tall order. Figure 2 shows the elements that need to move in a dual-light microscope to make the switch.

Figure 2. A UV microscope actually contains two entirely different optical systems, one for UV and the other for visible light, built into the same microscope.

Illumination for the VIS inspection usually comes from a tungsten incandescent lamp. The UV source uses either a mercury-arc lamp or a UV laser. The beam combiner, which provides on-axis illumination, must also be able to switch illumination sources.

Switching between UV and VIS objective lenses is as straightforward as changing magnifications by using an objective turret. Most users would like to have the light source change simultaneously with the objective lenses .

Similarly, when changing from UV to VIS objectives and sources, the microscope must insert a right-angle prism to deflect the beam from the UV-sensitive CCD camera to the VIS ocular. The same mechanism should remove the prism when the microscope switches back to UV.

Individual microscope designers choose how to lay out these optical components and whether the user switches them back and forth via a mechanical linkage or through the use of electric motors. These details differ depending on a microscope’s manufacturer. Instruments also differ in the illumination bands they offer (G-line, i-line, or DUV), and in the magnifications they make available.

Despite these differences, all of today’s UV microscopes for semiconductor inspection provide simple, convenient operation in an ambient atmosphere. Their operation is familiar to visible-light microscope users. They work with exactly the same handling equipment that feeds visible-light microscopes, but they are capable of resolutions at least twice as great as visible-light microscopes. T&MW

FOOTNOTE
1. The National Technology Roadmap for Semiconductors, Semiconductor Industry Association, San Jose, CA. 1997.

 
C.G. Masi works as a freelance technical journalist. He is the former chief editor of Test & Measurement World.