128 test tubes on Infineon biochip
06/01/2002
128 test tubes on Infineon biochip
Corporate Research at Infineon Technologies, Munich, Germany, has demonstrated the combination of electronic circuitry for test analyses with biochemical test sites on a single silicon chip — a so-called biochip. This breakthrough achievement is a significant step in research to accelerate and optimize molecular-level analysis.
The vision for biochip technology is simplified test procedures and cost advantages for testing nucleic acids and proteins in clinical diagnosis and individual patient medication. Infineon engineers expect that medical laboratory tests of their biochip will begin in about one year. Continued development and production plans are under discussion with Infineon's partners in this work.
In effect, the biochip is 128 test tubes on a 0.25cm2 silicon chip (see figure). Each biochip contains 128 homogeneous 100μm-diameter wells. Biological mini-probes can be linked to these wells, effectively allowing 128 different tests on a chip. The electronic circuits fabricated on the same chip replace optical analysis of samples with measurements of electric currents depending on the test samples' properties. The result is a test apparatus that is smaller and less complex than conventional optical systems.
 Infineon's experimental biochip for analysing nuleic acids and proteins. |
The Infineon biochip is fabricated using standard CMOS technology extended with additional process steps to form sensor electrodes made of gold. Infineon's success in integrating the gold electrodes on chip, without affecting the CMOS structures, is a process technology breakthrough.
Roland Thewes, with Corporate Research and head of the biochip activities within Infineon, said, "Gold is one of the few materials that satisfies requirements to be electrically conducting and bio-compatible while providing a robust attachment chemistry to immobilize or bind bio-molecules on it. Moreover, an extensive knowledge exists in the biochemical community concerning this material. While gold does not 'fit' with CMOS, it is used in bipolar IC fabrication. The challenge in our application was to marry these different process worlds, which are both available within Infineon's technology platforms, without affecting the performance of both processes."
"Our core competency in all aspects related to the silicon basic material — such as microstructures, circuit development, and process technologies — is leading to the opening of a new market area," says Soenke Mehrgardt, CTO at Infineon. "Development of these technologies leads to low-cost semiconductor-based biochip solutions to help realize the potential to simplify, improve, make cheaper, and speed up medical diagnosis."
Briefly describing the biochip's operation, Mehrgardt says, "The on-chip electronic analysis testing method starts with specific enzymes added to the probes. These enzymes split another substance, para-aminophenylphosphate (PAPP), added in a separate step, into electronically active components. This chemical procedure creates an electrical current of 1 pA to 100 nA at the sensor's gold electrodes, which is measured with highly sensitive circuits. The timeline and intensity of the electrical current identifies the composition and concentration of the tested substance."
In comparison with today's optical techniques, unknown gene sequences are loaded onto a biochip after first being tagged with a fluorescent dye emitting light of a particular wavelength when irradiated. During the tests, a specific CCD reads the light patterns emitted. These light patterns explain the composition of the substances tested. — P.B.
The HARI side of defect inspection
Among the 2001 ITRS grand challenges for yield enhancement, detecting defects associated with high-aspect-ratio trenches, contacts, and vias continues to be difficult, complicated by simultaneous needs for high sensitivity and high throughput with cost-effective detection tools. The ITRS specifically names high-aspect-ratio inspection (HARI, defined as defect detection into structures with an aspect ratio >3) as one of the five difficult inspection challenges for the 100nm node and beyond. The technical problems are poor transmission of energy into and back out of such structures and the large number of contacts and vias/wafer.
Conventional inspection technologies used in wafer fabs typically use dark field (e.g., laser scattering), bright field, or electron beam technologies that are capable of maximum aspect ratio of 1:1, 3:1, and >7:1 (under certain conditions), respectively. The general consensus is that evolutionary improvement for these technologies cannot continue to meet HARI requirements. Conventional optical technologies analyze the intensity of light reflected or scattered from surfaces and fail when insufficient light penetrates high-aspect-ratio structures. While electron beam inspection has the deepest aspect ratio capabilities, it is capable of only ~0.2 wafer/hr.
 30 phase map of surface scratch. |
One revolutionary technology that addresses HARI needs is patented direct digital holography (DDH), capable of aspect ratios of 12:1 and higher (the strong signal returned from the bottom of contacts tested so far indicates that higher aspect ratios are possible). DDH is commercially available from nLine Corp., Austin, TX, in the form of its Fathom Tool for defect detection on patterned wafers. The underlying research and development was undertaken by Dr. C.E. Thomas at Oak Ridge National Laboratory.
Briefly described, DDH creates a hologram by combining reference and object beams of coherent light reflected off the surface under inspection. Combination of the two beams creates a complex pattern of interference fringes that contains information about the intensity and phase of the reflected light. The DDH technique digitally captures the hologram with a CCD then separates it into a clear, crisp intensity image (basically a brightfield photograph) and a phase image. The latter is a graphic representation of the phase of the light falling at every pixel of the CCD. Since the phase of light at the CCD is a function of the path length between the inspected surface and the camera, the phase image can be converted to a topographic map of the inspected surface (Fig. 1). Because phase measurement resolution is conservatively 1/100th the illumination wavelength, the topographic map has remarkably high vertical resolution as well.
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Potential defect detection sensitivity for a DDH tool is about 1/13th the illumination wavelength (e.g., ~20nm for 266nm illumination) and is limited only by optical noise within the system. This is five times greater than the resolution of conventional, diffraction-limited intensity-based systems. DDH can detect defects that are 1/5th the size of those found by an intensity-based system operating at the same wavelength and camera pixel.
Among the advantages of DDH for wafer fabrication:
- It detects only defects caused by a change in surface topography. Buried defects that may cause a color change but do not create a surface bump or pit, and color variations caused by nonuniform film thickness, can be removed from the inspection data.
- It is applicable to high-aspect-ratio structures because the holographic image is created using a heterodyne signal amplification. The object signal is strengthened by combination with the reference beam. A very weak signal returned from the bottom of a deep, narrow surface structure is enough to create a usable signal. Deep surface features that are dark to an intensity-based inspection system (e.g., thin etch residues) are light and detail-rich with DDH.
- The heterodyne signal amplification technique also results in detection of surface defects in easily damaged films (e.g., low-k dielectrics) using very low illumination intensity.
Because phase detection is based on a difference in height measured by a camera pixel, the camera pixel corresponds to a certain projected area on the wafer determined by the magnification of the optical system (i.e., "pixel-area" multiplied by "phase-height" equals "phase-volume").
"This means that the minimum detectable signal for a given tool setup corresponds to a minimum detectable volume such as a bump or a pit," says Bob Owen, nLine CEO. "The aspect ratio of the minimum detectable volume does not matter in DDH for small defects; the defect can be short and broad, tall and thin, cubic, etc. Equal volumes give equal signals independent of their aspect ratios." — P.B.
Environmental Protection Agency tentatively OKs PFOS for semiconductor industry
A proposed ruling has been issued by the US Environmental Protection Agency concerning the importing and use of perfluorooctyl sulfonates (PFOS). These chemicals serve as photoacid generators in the acceleration of the resist process for the semiconductor industry.
In a separate but related decision, the EPA released a final rule requiring manufacturers and importers of PFOS to notify the EPA at least 90 days before manufacturing or importing the chemicals for significant new uses.
The good news for the chip industry, however, is that the low-volume, controlled-exposure uses of PFOS within the semiconductor industry are not considered a significant new use, and therefore are excluded from the final rule.
If the semiconductor industry had not been excluded from the rule, companies would have had to notify the EPA at least 90 days prior to manufacturing or importing PFOS, which would be nearly impossible in the fast-paced semiconductor environment.
The EPA has extended public comment until July 9 for the PFOS proposed significant new use rule. Comments can be submitted via mail, in person, or electronically. Any comment should identify docket control number OPPTS-50639C in the subject line on the first page. Submit comments to: Document Control Office (7407M), Office of Pollution Prevention and Toxics (OPPT), Environmental Protection Agency, 1200 Pennsylvania Ave., NW, Washington, DC 20460; [email protected]. — Staff Reports