Advances in metrology tools combat nanometer contamination

When measuring semiconductor features, recent industry developments illustrate that the linkage between metrology and contamination control is getting tighter as features shrink.

Therma-Wave Inc. (Fremont, Calif.; www.therma-wave.com) has rolled out a thin-film and critical dimension metrology tool that features an improved, built-in version of the company’s molecular contamination cleaning system. This desorption unit removes the airborne molecular contamination build-up on wafers so that thin-film measurements can be made.

KLA-Tencor Corp. (San Jose, Calif.; www.kla-tencor.com) also includes a desorption module in its optical thin-film measurement products. According to KLA-Tencor staff technologist Arun Srivatsa, “This is used to clean and measure selectively on metrology targets on scribe lines.” He adds that the technique, which involves heating up a small area on the wafer so that organics boil off, is of most value when measuring the gate oxides and other ultrathin films for 65-nanometer (nm) and below processes.

These thin films are less than 50 angstroms (Å)-or 5 nm-thick. Studies by KLA-Tencor have revealed that the build-up of airborne molecular contaminants can exceed 0.5 Å within two hours after a gate oxide is grown. That’s large enough to throw off optical measurements of the thin film by more than the few tenths of an angstrom that the measurement tools have in their error budget.

An extreme magnification of a Xidex Corp. carbon nanotube (fine line) grown atop a larger silicon tip structure used in a scanning probe microscope. Such surface sensors are seen as providing important measurement capability for producing advanced semiconductors, where contamination control issues will become increasingly crucial. (Source: Xidex Corp.) Click here to enlarge image

So, metrology companies must introduce point-of-use cleaning for contamination control. Only by removing contaminants just prior to measurement can the tools ensure accurate results.

Recently, Xidex Corp. (Austin, Texas; www.xidex.com) revealed that its carbon nanotubes would be used in semiconductor metrology tools. The industry wants to use carbon nanotubes to form the tips of scanning probe microscopes. These tips act like a microstylus, tracing out molecular-sized features. Scanning probe microscopes, which are also known as atomic force microscopes, can profile deep vias, high aspect ratio trenches and critical dimensions in advanced semiconductor processes (like the upcoming 32-nm process node).

The research consortium SEMATECH (Austin, Texas; www.sematech.org), which is interested in carbon nanotubes because of their characteristics, is working with Xidex in developing its technology. Nanotubes, unlike other materials, are very resistant to wear and can be made with diameters that measure in nanometers. That radius of curvature is tighter and the resulting tips smaller than are possible with other materials.

Even though carbon nanotube tools have yet to be deployed, scanning probe microscopes face measurement challenges that are contamination-driven. The Xidex solution involves the use of what’s known as tapping mode. “The tip only momentarily contacts the surface,” says Paul McClure, president and CEO of Xidex. “It drives itself in and out of any surface layers so quickly that they have no effect.”

At present, the tips are not made in a cleanroom but that may change as production ramps up. “A cleanroom will be required to increase yield and to protect the tips prior to packaging,” predicts John Allgair, project manager for lithography metrology at SEMATECH. III

Engineers, lab planners and scientists developed unique lab layouts with groundbreaking temperature and vibration controls

By Dan Hahn, HDR Architecture

In the world of nanoscience, as the size of the material for evaluation diminishes, the challenge to control ambient conditions increases. Therefore, you can imagine the tightly controlled environmental conditions required at the metrology laboratory at the National Institute of Standards and Technology (NIST), where the seismic fountain clock measures time accurate to one second over the life of the entire universe.

To meet these requirements, engineers, lab planners and scientists developed lab layouts with temperature and vibration control not found anywhere else.

Vibration criteria

The vibration criteria for NIST is stated as four vibration amplitude functions given in terms of frequency. Numbers are represented in a vibration curve, in descending order from an extremely quiet standard to that of typical construction.

Criterion Type A1: Velocity amplitude of 3 micrometers/sec or 125 microinches/sec at frequencies below 4 Hz; velocity amplitude of .75 micrometers/sec or 30 microinches/sec at frequencies between 4 Hz and 100 Hz. These criteria could only be achieved with engineered vibration isolation slabs.

Criterion Type A: Velocity amplitude of .025 micrometer or 1 microinch at frequencies between 1 and 20 Hz; velocity amplitude of 125 microinches/sec at frequencies between 20 Hz and 100 Hz. Site studies showed these limits were achievable at grade with slab-on-grade design and special vibration detailing.

Criterion Type B: Velocity amplitude of 6 micrometers/sec or 250 microinches/sec at frequencies between 1 Hz and 100 Hz.

Criterion Type C1: Velocity amplitude of 12.5 micrometers/sec or 500 microinches/sec at frequencies between 4 and 100 Hz. Criteria B and C1 were achievable in conventional building frame design with a consideration given to frame stiffness and mass.

Designing accordingly

At NIST, as with any lab, different portions of the building were programmed with different uses, each required different levels of vibration control. General-purpose labs and a cleanroom had more conventional requirements, while areas of the metrology lab required the A1 criterion mentioned above.

Ambient conditions produce vibration-these run the gamut from drinking fountains and vending machines to air compressors and air handling units. Paradoxically, while the stringent environmental demands of a state-of-the-art lab require proximity of mechanical systems to produce close-tolerance environmental control, these systems are vibration inducing. To meet the NIST metrology lab’s requirement of maintaining temperature to +/- .01˚C, the vast quantities of air-handling equipment needed special attention paid to control of vibration through design. More importantly, to achieve these incredibly precise vibration controls, designers placed the NIST metrology wings underground with no superstructure directly above it, only two to three feet of soil and open area. Because wind does no blow directly upon the shell of the structure, the dynamic forces that could excite the building shell are reduced or even eliminated.

To further reduce vibration, and to allow for the exceptional quantity of ductwork required, the metrology floor was moved to a greater depth, reducing the amplitude of surface waves that naturally decrease with depth. Collaborating with scientists, and based on the prototype studies, engineers also placed giant masses of concrete in pits on air springs under the lab floors. It was found that data measured on the prototype T-shaped concrete slab showed that A1 criterion can be met on the mass in the pit when isolated with air springs down to 1.5 Hz, the lower frequency limit of the measurements.

A walk-on floor isolated from the pit and the mass where experiments take place allows for the experiment to be isolated from the vibrations introduced by the researcher himself (see Fig. 1). Study of the prototype led to a separate support system to isolate each component of the lab floor, pit and mass.

Figure 1. Design of the isolation slab included a walk-on floor, allowing the experiment to be isolated from researcher-induced vibrations. Click here to enlarge image

Certainly, the most stringent of vibration controls is achieved only through layer upon layer of isolation techniques. For example, the mechanical air handling equipment is isolated internally within the unit using mechanical isolation devices. Isolators are also located within ductwork and supports. Then, equipment is isolated externally. The lab floor is further isolated from surrounding structural elements by vibration isolation joints.

Additional vibration control was studied for potential future use. Active control systems that would read vibration input and adjust spring response to eliminate the vibration input before it travels to the supported mass was evaluated. Variation in the design of the mass was also evaluated for potential improved response.

These controls are moving researchers and engineers into the realm of the experimental, forging virgin ground that can only be determined by meticulous testing and trial and error.

A final word

Lab designers need to fit the vibration control needed to the type of experiments being conducted. It can be a costly mistake to “over design” a lab. The best and most cost-efficient lab is one that is designed “dead on”; one in which the planners fully identify and understand the needs-and constraints-of the researcher.

When construction is completed on the NIST labs, they will contain a range of about a dozen concrete slab masses of various weights and heights, some 100,000 pounds and 3 meters tall, and others 10,000 pounds and 1 meter tall. All meet A1 criterion but vary to accommodate the size of an experiment. Other spaces in the lab will simply provide the conventional slab-on-grade construction, fitting the design to the use. All the vibration features of the NIST AML add to the flexibility, reliability, and redundancy of the whole facility.

In the end, understanding vibration from the source to the experiment led to the right choices. The best-designed lab is simply one that performs as intended. And, in this case, the quietest lab is the one that makes the most impact. III

Dan Hahn, senior vice president with HDR in Omaha, Neb., is the lead structural engineer on the NIST project.