Optical Micrometer Measurement System Product Description

Size: px

Start display at page:

Download "Optical Micrometer Measurement System Product Description"

Vernon Potter
5 years ago
Views:

Optical Micrometer Measurement System Product Description Virginia Semiconductor Incorporated Fredericksburg, VA 22401 www.virginiasemi.com; www.opticalmicrometer.com (540) 373-2900.

1 Optical Micrometer Measurement System Product Description Virginia Semiconductor Incorporated Fredericksburg, VA (540) OMMS Engineering and Scientific Background The Optical Micrometer Measurement System (OMMS) is based on correlating silicon thickness to percentage of optical absorption. Our technique is patented [1,2] and generally proven. A basic diagram of the system set-up is shown in Figure 1. Figure 1 Block diagram and method of operation for OMMS. A stable and coherent optical beam is expanded and columnated to have as uniform beam intensity as possible. A diffusing wheel and optical component set (see figure 2 imaging optics) are then used to modify the beam phase correlation and eliminate interference along the optical path. This optical component is critical to the overall method and application. The beam passes through the micromachined structure, a set of optics for magnification, and is imaged to a high performance CCD array. The wavelength of the beam should be relatively transparent, but at the same time absorbing to the silicon micromachined structure. By maintaining the power, frequency, and mode of the laser,

2 the percentage absorption of the beam pixel-by-pixel can be used to map the membrane thickness pixel-by-pixel. The beam must have reasonable, but not excessive, absorption in order to maintain a strong signal-to-noise relationship when correlating percentage absorption to thickness. As the thickness increases the signal decreases. VSI has determined that wavelengths near 600nm 1000nm are appropriate for thickness measurements ranging from 15um to 185 um. In order to span the entire thickness range, several different powers are used. Extensive and complex signal processing and calibration to wafers of know thickness is needed to reliably map the absorption information to absolute thickness. Also, proper lens and mirror design is needed to eliminate interference phenomena along the optical path. The following equations are used as the primary transfer functions to translate absorption information to thickness information. The response of the camera, relative to the thickness of the wafer and the laser intensity is given by the equations, Rpx = {IL(Spx)(Rf 1)(Rb 1)EXP( at)} + Rpxnf and after rearranging, solving for t, 1 ( Rpx Rpxnf ) t = ln a IL( Spx)( Rf 1)( Rb 1) where it is assumed that the majority of the beam is received after one internal reflection, and t=thickness of Si, Rpx=pixel optical response, Rpxnf=pixel optical response noise floor, IL= laser intensity, Spx= pixel optical sensitivity, a=optical absorption coefficient. Therefore a calibrated and stable system can directly relate t, thickness, to the pixel-bypixel optical response (Rpx ) of the CCD. The absorption coefficient at a given wavelength is generally known [30], and can be mathematically expressed as a polynomial with the variable temperature (T). The front and back reflection coefficients Rb,Rf are determined by, Rf,b= [(n1 2 (n0-n2) 2 cos 2 kh+(n0n2-n1 2 ) 2 sin 2 kh]/[n1 2 (n0+n2) 2 cos 2 kh+(n0n2+n1 2 ) 2 sin 2 kh] where n is the index of refraction, h=oxide or nitride coating thickness, and k=laserwavelength-dependent propagation constant. The index of refractions (n0,n1,n2) are also known for Si, Silicon Dioxide, Silicon Nitride, etc. The laser power, frequency, and mode must be maintained constant as well as the camera noise floor and response. Extensive signal processing is completed using LabView and a thickness map for the membrane is generated from the percentage of absorption in the beam read from the CCD image referenced to calibrated, wafer standards and absorption. The algorithms are complex and beyond the scope of this proposal, and fully explained in [1,2]. LabView, National Instruments I/O boards, and a PC are used for signal processing, system control, user input, data and analysis display, and data storage. Although the technique seems relatively simple, in order to achieve less than 1% repeatability error and a large range of thicknesses from 0um-400um significant scientific and engineering problems must be resolved.

3 Figure 2 is taken directly from the US Patent and further illustrates the OMMS configuration. Figure 2 OMMS illustration from US Patent Figure 3 is taken from the US Patent and illustrates some of the patented image processing and software technology used with the OMMS. Figure 3 OMMS software and algorithm illustration from US Patent

4 Figure 4 is also taken from the US Patent and shows other potential embodiments of the technology. Figure 4 OMMS illustration from US Patent showing alternate embodiments

5 OMMS Hardware and Software Layout Figures 5,6 below show the basic hardware and software configurations for the OMMS. Figure 5 OMMS Hardware Photos Figure 6 OMMS integrated into manufacturing

Figure 7 shows the LabView front panel for the OMMS that is used for system operation. Figure 7 OMMS operating software interface The analysis area is controlled by the upper left-hand controls.

6 Figure 7 shows the LabView front panel for the OMMS that is used for system operation. Figure 7 OMMS operating software interface The analysis area is controlled by the upper left-hand controls. The yellow box in the lower image defines the region to be mapped for thickness. The image in the upper righthand area has four movable cursors that are positioned with the mouse to define regions for thickness line profiles and thickness data shown in the lower right corner.

7 Demonstrated OMMS Performance To date, VSI has been very successful with this technology for the 0um 180um range (see Figures below). For this band, two thermoelectrically cooled solid-state lasers, 10- bit Cohu Corporation Camera (1024 counts per pixel), and specialized optics have been used to build a reliable system. Figures 8 show a high performance commercial MEMS component with a 9.4 um thick microelectromechanical membrane being successfully analyzed by this OMMS technology. The entire membrane structure can be analyzed with 0.2 um resolution and accuracy by the OMMS as shown below. Also, basic inspection for defects and etch anomalies can be achieved while capturing thickness information. Figure 8 shows the front panel, LabView, display of an OMMS instrument operating from 0um-180um and 0.5um repeatability. The lower left hand corner is a lower magnification IR image of the device. The operator locates the region for analysis using this image. The upper right hand corner is a higher magnification image with cursors that can be moved using a PC mouse to generate thickness profiles within the image. The profile between the vertical bluepurple curses is given in the lower right hand corner. The system requires about 10 seconds to produce a complete thickness map of the region shown in the upper right hand corner.

Figure 8 LabView front panel display during measurement of a 9.4um thick Si Micromachined membrane using the proposed optical absorption technique.

8 Figure 8 LabView front panel display during measurement of a 9.4um thick Si Micromachined membrane using the proposed optical absorption technique. Figure 9 shown below is the actual calibrated transfer function that is embedded into the software system and algorithms for correlating camera signal to silicon thickness. The curve is given by the above equations with calibration standards being used to extract unknown coefficients for laser power and camera sensitivity. As the silicon thickness decreases, the signal increase at a fixed laser power. Proper determination of laser wavelength, optical path configuration, and laser power are needed to optimize the signal-to-noise ratio and achieve 0.2 um resolution. Extensive signal processing of the image is needed to achieve better than 0.5 um repeatability.

9 Graph Thickness Camera Pixel Signal no ox cali DATA Figure 9 Graph showing the theoretical relationship and calibration transfer function for the 0-15 um thickness range. Similar functions are used at greater laser powers to extend the measurement band. VSI OMMS Technology and relationship tovti As of May 2003, Virginia Semiconductor Incorporated has delivered three commercial OMMS systems to MEMS manufacturing companies. The first system was installed in Virginia Technologies Incorporated (VTI), is a separate company operating in Charlottesville, VA, and serves as the contract manufacturer to VSI for OMMS instrumentation. VTI and VSI are wholly separate companies both located in the state of Virginia. Virginia Semiconductor owns all Intellectual Property related to OMMS technology, and is the only company selling OMMS instrumentation. VSI and VTI enjoy many business collaborations. Employees from both companies are co-inventors of several electronic and optical systems having US Patents. Contact VSI today for more information on OMMS technology at or send to Stephen H. Jones, President, VSI at shjones@virginiasemi.com. For more information on Virginia Technologies Inc. visit or send to Bob Ross, President, Virginia Technologies Inc., at ross@vatechnologies.com. References [1] Stephen H. Jones, Optical Micrometer for Measuring Thickness of Transparent Substraes Based on Optical Absorption, US Patent Number 5,959,731, Issued September [2] Robert Ross, Stephen H. Jones, Optical System for Measuring and Inspecting partially Transparent Substrates, US Patent Number 6,057,924, Issued May [3] see further information on VSI optical micrometer technology at

A Laser-Based Thin-Film Growth Monitor

TECHNOLOGY by Charles Taylor, Darryl Barlett, Eric Chason, and Jerry Floro A Laser-Based Thin-Film Growth Monitor The Multi-beam Optical Sensor (MOS) was developed jointly by k-space Associates (Ann Arbor,