Technology news
12/01/2003
Surface prep/wafer bonding process enables low anneal temps
A new surface activation and wafer-to-wafer bonding process that uses atmospheric pressure plasma has been developed by SÜSS MicroTec, eliminating the need for a vacuum chamber and associated process time and opening the door for new materials integration (see figure).
The process, called nanoPREP, results in post-bond anneal temperatures below 350°C — particularly useful for creating SOI and strained-Si using direct wafer bonding. Typical fusion bonding anneal temperatures for these materials are in the range of 1000–1100°C, a factor that limits the choice of materials that can withstand such temperatures.
Limiting the post-bond annealing temperature to <350°C also is useful for direct bond applications in MEMS and MOEMS devices where integration into wafer-level packaging is used.
"Low temperature annealing is important for developments of materials integration, including the use of wafer bonding for III-V semiconductor hetero-integration and the integration of III-V semiconductors with Si," notes SÜSS MicroTec CEO Franz Richter. He also points out that plasma surface treatment might eliminate the need for wet chemistry treatments for direct bonding applications in some cases.
According to Richter, there are three active elements to the process: 1) removal of adsorbed contaminants, 2) creation of appropriate end caps to the surface molecules, and 3) creation of reactive sites.
"The plasma changes the surface by modifying the final bond group," explains Richter. "For example, creating a surface rich in (-OH) on a Si wafer. The plasma may leave a higher level of dangling or unreacted atoms which, under the proper conditions, allows easy reaction with the target wafer." — D.V
'Nanosprings' may work as actuators and transducers in microsystems
A new zinc oxide nanostructure that spontaneously forms helical shapes with useful piezoelectric properties could be the basis for a new class of small-scale transducers and actuators useful in sensing and microsystem applications.
Researchers Zhong L. Wang and Xiang Yang Kong at the Georgia Institute of Technology have dubbed these structures "nanosprings," which are just 10–60nm wide and 5–20nm thick, but up to several millimeters long (see figure), said a Georgia Tech report.
Nanosprings are similar to but smaller than the "nanobelts" first reported by the institute's scientists two years ago in the journal Science. "These structures are a major step toward a new system of nanostructures," said Wang, the director of Georgia Tech's Center for Nanoscience and Nanotechnology.
The piezoelectric properties of the new structures could make them useful in detecting and measuring very small fluid flows, tiny strain/stress forces, high-frequency acoustical waves, and even air flows that would otherwise be imperceptible. When deflected by the flow of air or fluids, the nanosprings would produce small but measurable electrical voltages.
Semiconductor-based nanostructures that rely on electrostatic forces have gained widespread research interest, but Wang said development of nanomaterials for piezoelectric actuators have lagged. The new nanosprings could thus give designers of future nanoscale systems more options, the report said.
The new structures also display unusual electrostatic polarization, with positively and negatively charged surfaces across the thickness of the nanoribbon. This electrical charge could be used to attract specific molecules, potentially making the nanosprings into biosensors to detect single molecules or cells.
Many challenges remain before the nanosprings can find application, however.
"We can cut this material into specific lengths and manipulate it, but that's only the first step," Wang noted. "We need to know how to integrate this into existing technology."
The research was supported by the National Science Foundation and NASA.
Philips AMS expands applications range for low-k dielectrics
Philips AMS, in partnership with IMEC, the independent research center in Leuven, Belgium, has recently extended the capabilities of its Series 1200 tool to include measurement of low-k dielectric material properties (modulus and density). The forerunner to the noncontact, nondestructive tool has been used for copper applications including PVD, ECD, and CMP for more than five years.
Philips AMS worked jointly with both International Sematech (since 1999) and IMEC (since 2000) to develop metals and low-k applications for the tool. Recently, Philips AMS and IMEC jointly presented a paper at the VMIC conference describing the use of surface acoustic waves (SAW) to measure modulus and density of low-k films as well as making rapid contour maps (Fig. 1) of WIW modulus uniformity [1].
According to Philips GM Bill Gately, the copper market has solidified to the point where there is now a high-volume market that the company can target. "Our systems are fab-ready now," states Gately. "IMEC has our latest generation system and will continue to develop applications jointly with Philips AMS."
The joint work will include examining new low-k materials as well as the effects of various film-processing conditions on dielectric material properties. "In the metals area, we are working on basic applications such as post-CMP copper thickness measurements up to more advanced possibilities such as grain-size and resistivity variations in electroplated copper."
The SurfaceWave technology described by the researchers [1] is based on the transient laser-induced gratings method. Dispersion curves are generated when two beams from a sub-nanosecond laser pulse are overlapped on the sample surface and the interference between the two beams generates a grating pattern.
"When radiation is absorbed under each bright fringe of the excitation spot pattern, there is a thermal expansion of the dielectric film along each fringe," explains Michael Gostein, technology development manager.
"The result is the launching of surface acoustic waves that in turn cause the time-dependent diffraction of a continuous-probe laser beam directed at the sample," continues Gostein. "By monitoring the diffracted beam intensity, the wave motion can be detected. For a given acoustic wavelength, there are multiple possible oscillation modes of the film stack." Figure 2 shows different frequency components to the signal.
Because the method is noncontact, it avoids the problem of harming fragile low-k films or the substrate beneath, which can occur with the standard nanoindentation method that uses a stylus pressed into the film. This problem becomes prevalent as low-k dielectric thicknesses get below 0.5µm, typically starting at the 0.13µm technology node.
Reference
1."Measurement of Elastic Moduli of Low-k Dielectric Films Using Laser-Induced Surface Acoustic Waves," VLSI Multilevel Integration Conference, September 22–25, 2003.
New 193nm tool offers 70nm resolution for 200mm fabs
Any guru who had predicted this some years ago would have been laughed out of the industry — a 193nm stepper that makes incremental improvements to get ordinary old optical lithography down to 70nm half-pitch line resolution with ordinary old binary marks, for ordinary old 200mm fabs as well as 300mm.
ASML's latest 193nm Twinscan XT:1250 stepper also showed not-so-bad 85nm resolution on contact holes with alternating phase-shift masks, and ASML said development work with MaskTools's chromeless phase lithography masks showed potential for 65nm contact holes.
"We now see more and more interest from 200mm fabs for the most advanced technology too, so this is especially for 200mm fabs," said Paul van Attekum, senior VP of marketing and technology, in ASML's conference call announcing the new tool. "For some customers, it's a matter of survival. Or some just see the opportunity for added life for their old fabs." He said the company had initial orders for the tool for both 200mm and 300mm fabs for shipment in 2Q04. Lead-time is about six months.
The stepper's footprint is reduced 25% to meet 200mm fab space requirements by putting a section of the tool in a separate module that can go outside the cleanroom. Facility requirements for water and gas are likewise reduced to levels available in older fabs.
Though the lens NA remains unchanged, ASML says better metrology improved its detailed specifications. It increased stage speed, and now includes its full ultra k1 portfolio of tweaking software enhancements as a commercial, not development, product. "Most of the work was for improving the overlay," said van Attekum. "Some 50–70 things had to be tuned and optimized." He says overlay is <10nm for the same machine, 14nm for different ones. — Dr. Paula Doe, European Editor