News Column

Researchers Submit Patent Application, "Sample Inspection System Detector", for Approval

May 21, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- From Washington, D.C., VerticalNews journalists report that a patent application by the inventors Kavaldjiev, Daniel Ivanov (San Jose, CA); Biellak, Stephen (Sunnyvale, CA); Zhao, Guoheng (Palo Alto, CA); Vaez-Iravani, Mehdi (Los Gatos, CA), filed on October 24, 2013, was made available online on May 8, 2014.

The patent's assignee is KLA-Tencor Corporation.

News editors obtained the following quote from the background information supplied by the inventors: "Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a substrate or wafer. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.

"Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. As design rules and process windows continue to shrink in size, inspection systems are required to capture a wider range of physical defects on wafer surfaces while maintaining high throughput.

"One such inspection system is a scanning inspection system that illuminates and inspects a wafer surface. Light collected from the wafer surface is directed to a detector, or an array of detectors, for conversion to electrical signals useful for storage and analysis. Typical detector arrays are limited in their sensitivity due to significant detector noise. Often, this results in inspection systems that operate in a detector noise limited regime, rather than a photon limited, or surface limited regime. In some examples, detector noise is overcome by increasing the amount of illumination power. However, in high-power, laser-based inspection systems, increasing the power density of the incident laser beam may cause damage to the wafer surface. In addition, increasing the illumination power, particularly at short wavelengths, increases cost and may introduce reliability risks.

"Previous inspection systems have relied on a variety of detectors, each with different advantages and disadvantages in specific applications. Exemplary detectors include photo-multiplier tubes (PMTs), charge-coupled devices (CCDs), PIN diodes, photodiodes, etc. Each of these detectors presents its own challenges and shortcomings. PMTs, for example, are typically bulky, and require high drive voltage. PMTs are also not available in large arrays. CCDs suffer from internal read-out noise mechanisms that limit their ultimate sensitivity compared with PMTs.

"Avalanche photodiodes (APDs) are small sensors that provide significant gain and require lower drive voltage than PMTs. APDs may be configured in one of two operational modes. In linear mode, the voltage across the APD is set at a value below the break-down voltage. The output of the APD in this mode is a signal that is proportional to the amount of light detected. The gain of the APD may be set at relatively low values (e.g. 100.times.). In Geiger mode, the voltage across the APD is set at a value above the break-down voltage. In this mode, the gain of the APD becomes very large. Absorption of a single photon may give rise to a large pulse at the output that may be passed through a comparator to generate a clean TTL-like pulse. Thus, very high sensitivity may be achieved by APDs operating in Geiger mode.

"However, once a Geiger pulse is trigged the APD is not responsive (i.e., 'blind') to the arrival of another photon until a period of time (i.e., the 'quench time' associated with the APD) has passed. Once the APD pulse is 'quenched', the APD is again able to detect another photon. A typical quench time associated with an APD operating in Geiger mode is a few hundred picoseconds. Unfortunately, this period of blindness limits the dynamic range of APDs operating in Geiger mode, and thus limits their utility in current wafer inspection systems.

"Improvements to the sensitivity and dynamic range of array based detectors employed in surface inspection systems are desired to detect defects on a wafer surface with greater sensitivity while avoiding thermal damage to the wafer surface."

As a supplement to the background information on this patent application, VerticalNews correspondents also obtained the inventors' summary information for this patent application: "Methods and systems for enhancing the dynamic range of a high sensitivity inspection system are presented.

"In one aspect, the dynamic range of the inspection system is increased by directing a portion of the light collected from each pixel of the wafer inspection area toward an array of avalanche photodiodes operating in Geiger mode and directing another portion of the light collected from each pixel of the wafer inspection area toward another array of photodetectors (e.g., an array of avalanche photodiodes operating in linear mode, PIN photodiodes, PMTs, CCDs, etc.). The array of avalanche photodiodes operating in Geiger mode is useful for inspection of surfaces that generate extremely low photon counts. The other array of photodetectors is useful for inspection of larger defects that generate larger numbers of scattered photons. The array of avalanche photodiodes operating in Geiger mode has a different resolution than the other array of photodetectors to optimize the dynamic range of the overall detector system.

"In one embodiment, light scattered from each pixel of the inspection area of the surface of a wafer is collected and directed to a beam splitter. The beam splitter directs a portion of the collected light to an array detector that includes a number of avalanche photodiodes (APDs) operating in a Geiger mode. Similarly, the beam splitter directs another portion of the collected light to another array of photodetectors. Both detectors generate output signals usable in combination to determine the presence of anomalies and their characteristics with high sensitivity and large dynamic range.

"In another embodiment, light scattered from each pixel of the inspection area of the surface of a wafer is collected and directed to an array detector that includes a number of avalanche photodiodes (APDs) operating in a Geiger mode. A portion of collected light is absorbed by the array detector. Another portion of the collected light is reflected from the surface of the array detector and is directed toward another array of photodetectors. Both detectors generate output signals usable in combination to determine the presence of anomalies and their characteristics with high sensitivity and large dynamic range.

"In yet another embodiment, light scattered from each pixel of the inspection area of the surface of a wafer is collected and directed to an array detector configured in a stacked layer arrangement. Incoming light passes through a first array of photodetectors disposed in a first layer at the top surface of detector and a second array of photodetectors are disposed in a second layer of detector 160, below the first layer. The first array of photodetectors includes a number of avalanche photodiodes (APDs) operating in a Geiger mode.

"In another aspect, an array detector may include other photodetectors in addition to APDs operating in a Geiger mode.

"In one embodiment a detector includes a linear array of macro-pixels. Each macro-pixel includes a number of APDs operating in Geiger mode and connected in parallel such that multiple photons arriving simultaneously are properly counted. In addition each macro-pixel includes a number of APDs operating in a linear mode. Moreover, each macro pixel may be configured to generate separate output signals; one indicative of the number of photons counted by the APDs operating in Geiger mode, and another indicative of the radiation flux detected by the APDs operating in a linear mode. In some embodiments, light collected from each pixel of an inspection area on the wafer surface is imaged onto a macro-pixel. Hence, a portion of light collected from each pixel of an inspection area of the wafer surface is detected by one or more APDs operating in a Geiger mode and another portion of light collected from the same pixel is detected by another photodetector within the same integrated detector.

"In another embodiment a detector includes a linear array of APDs operating in Geiger mode disposed adjacent to another linear array of photodetectors (e.g., APDs operating in linear mode). In some embodiments, light collected from each pixel of an inspection area on the wafer surface is imaged onto adjacent pixels of both linear arrays. Hence, a portion of light collected from each pixel of an inspection area of the wafer surface is detected by one or more APDs operating in a Geiger mode and another portion of light collected from the same pixel is detected by another photodetector within the same integrated detector.

"In yet another embodiment a detector includes a linear array of APDs operating in Geiger mode interleaved with another linear array of photodetectors (e.g., APDs operating in linear mode). In some embodiments, light collected from each pixel of an inspection area on the wafer surface is imaged onto adjacent pixels of both linear arrays. Hence, a portion of light collected from each pixel of an inspection area of the wafer surface is detected by one or more APDs operating in a Geiger mode and another portion of light collected from the same pixel is detected by another photodetector within the same integrated detector.

"In another aspect, the APDs of a detector array are configured to be switchable between a Geiger mode and a linear mode of operation. In one embodiment, a linear array detector includes drive electronics configured to switch APD elements between a Geiger mode of operation and a linear mode of operation in response to a control signal.

"In some embodiments, a number of APDs are switched between a Geiger mode of operation and a linear mode of operation at a particular switching frequency and duration (e.g., pulse width modulated signal). Based on output signals received from the detector array, either or both of the switching frequency and duration values may be adjusted to emphasize or deemphasize output data generated by APDs operating in a Geiger mode.

"In some other embodiments, a number of APDs are switched between a Geiger mode of operation and a linear mode of operation based on the saturation level of APDs operating in Geiger mode.

"The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not limiting in any way. Other aspects, inventive features, and advantages of the devices and/or processes described herein will become apparent in the non-limiting detailed description set forth herein.

BRIEF DESCRIPTION OF THE DRAWINGS

"FIG. 1 is a simplified diagram illustrative of one embodiment of an inspection system 100 including a first array of photodetectors including a number of avalanche photodiodes (APDs) operable in a Geiger mode and a second array of photodetectors.

"FIG. 2 is a simplified diagram illustrative of a first array of photodetectors including a number of avalanche photodiodes (APDs) operable in a Geiger mode and a second array of photodetectors in another embodiment.

"FIG. 3 is a simplified diagram illustrative of a first array of photodetectors including a number of avalanche photodiodes (APDs) operable in a Geiger mode and a second array of photodetectors in yet another embodiment.

"FIG. 4 is a simplified diagram illustrative of a detector employing both APDs operable in a Geiger mode and other photodetectors in one embodiment.

"FIG. 5 is a simplified diagram illustrative of a detector employing both APDs operable in a Geiger mode and other photodetectors in another embodiment.

"FIG. 6 is a simplified diagram illustrative of a detector employing both APDs operable in a Geiger mode and other photodetectors in yet another embodiment.

"FIG. 7 is a simplified diagram illustrative of a detector including APDs that are switchable between a Geiger mode and a linear mode of operation.

"FIG. 8 is a flowchart illustrative of a method 400 of enhancing the dynamic range of a high sensitivity inspection system.

"FIG. 9A is a diagram illustrative of a light pipe array 210 having light pipe elements 210A and 210B disposed in front of macro-pixels 211A and 211B, respectively.

"FIG. 9B is a diagram illustrative of a distribution 213 of incoming light 212A projected onto macro-pixel 211A depicted in FIG. 9A without mixing.

"FIG. 9C is a diagram illustrative of a distribution 214 of incoming light 212A projected onto macro-pixel 211A with mixing by light pipe 210A depicted in FIG. 9A.

"FIG. 10 is a diagram illustrative of a light pipe array 220 in another embodiment.

"FIG. 11 is a diagram illustrative of a two dimensional array of photodetectors employed to perform one-dimensional measurements with increased dynamic range."

For additional information on this patent application, see: Kavaldjiev, Daniel Ivanov; Biellak, Stephen; Zhao, Guoheng; Vaez-Iravani, Mehdi. Sample Inspection System Detector. Filed October 24, 2013 and posted May 8, 2014. Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=4629&p=93&f=G&l=50&d=PG01&S1=20140501.PD.&OS=PD/20140501&RS=PD/20140501

Keywords for this news article include: Electronics, Semiconductor, KLA-Tencor Corporation.

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Source: Electronics Newsweekly


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