Abstract:
A prober for measuring the light output of digital devices integrally formed on a single wafer. The prober includes a light-integrating sphere sequentially aligned with selected devices. Each time that a device is aligned with the sphere, the device aligned with the sphere is activated, so that the light output of each device is individually measured. In the disclosed embodiment, the devices are vertical cavity surface emitting lasers (VCSELs) and light emitting diodes (LEDs).

Description:
BACKGROUND OF THE INVENTION 
     This nonprovisional application claims priority from provisional application No. 60/304,989 filed Jul. 12, 2001 and entitled “VCSEL/LED Wafer Probe System”. 
    
    
     The present invention relates to instruments and methods for probing wafers of digital devices, and more particularly to probing wafers of digital light-emitting devices. 
     Digital light-emitting devices are well known and widely used in many applications. Two such devices are vertical cavity surface emitting lasers (VCSELs) and light-emitting diodes (LEDs). These devices are fabricated on wafers with counts of over 20,000 individual devices on a three-inch diameter wafer being common. After fabrication on the wafer, the devices are cut into individual devices (called die) and packaged for use. 
     Manufacturers of light-emitting devices desire to make optical measurements of the individual devices before the wafers are cut into individual dies. The motivation for such measurements is that the characteristics of the devices can vary significantly across a single wafer. By mapping a wafer, a manufacturer can bin the die after cutting, separating the good from the bad and segregating the good ones by power output, wavelength, or other parameters. Performing this testing while the devices are still in wafer form permits the manufacturer to eliminate the bad ones before spending money to package them. Also, some manufacturers sell bare die to packagers, who want specifications on what they are buying. 
     Semiconductor manufacturers have been probing devices and circuits on wafers for many years, for example, measuring the electrical properties and performance of the integrated circuits on wafers. Instruments for probing digital devices are known as “probers” and are manufactured and sold, for example, by Karl Suss America, Inc. of Waterbury Center, Vt. (www.suss.com). 
     Usually, information on two optical parameters is desired—the total power and the spectral distribution. Currently, the spectral distribution of the light is measured using a fiber optic spectrometer mounted on the prober. The device can be used in conjunction with the microscope which is mounted on the prober for initial alignment of the wafer on the prober bench. The prober moves the wafer to sequentially align each light-emitting device with the fiber optic pick-up; the light-emitting device is activated; and the light is captured by the fiber optic pick-up. Unfortunately, problems arise in conjunction with the use of fiber optic pick-ups. First, the device to be tested must be precisely aligned with the pick-up. If not, a portion of the light output is lost (i.e. does not enter the pick-up), negatively impacting the accuracy of the measurement. Second, the fiber optic pick-up receives only a portion of the light even under optimal circumstances, because the light emitted from light-emitting devices diverges. This further detracts from the accuracy of the measurement. 
     SUMMARY OF THE INVENTION 
     The aforementioned problems are overcome in the present invention in which a light-integrating sphere is included in a wafer prober for more completely capturing the light outputted by each device. The sphere has a relatively large opening in conjunction with the previously used fiber optic pick-ups. Accordingly, a significantly higher percentage of the light from the device is captured by the sphere. 
     More specifically, the invention comprises a wafer prober on which an integrating sphere is mounted. In operation, the prober aligns at least one light-emitting device with the sphere, activates each device aligned with the sphere, and measures the light output of the activated device. 
     These and other objects, advantages, and features of the invention will be more readily understood and appreciated by reference to the detailed description of the preferred embodiment and the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of the prober of the present invention; and 
     FIG. 2 is a perspective, partially exploded view of the integrating sphere and its related mounting hardware. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A wafer prober constructed in accordance with a preferred embodiment of the invention is schematically illustrated in FIG.  1  and generally designated  10 . The prober includes a computer control  12 , a source system  14 , and a measuring system  16 , which in turn includes a light-integrating sphere  18 . 
     With the exception of the integrating sphere  18  and the related circuitry, the prober is generally well-known to those skilled in the prober art. For example, an appropriate prober is manufactured and sold by Karl Suss America, Inc. of Waterbury Center, Vt. as Model PA-200. This prober includes a wafer bench (not shown) for supporting a wafer W and wafer chucks (also not shown) for retaining the wafer W on the wafer bench. Other probers suitable for use in conjunction with the present invention will be readily recognized by those skilled in the art. 
     The conventional components of the prober  10  need be only briefly described. The control computer  12  can be any one of a wide variety of desktop or other suitable computers. Also the software for operating the computer  12  is well known by those skilled in the software art. The computer  12  communicates with the spectrograph  50  via an RS-232 connection, and with the source system  14  and the measurement system  16  via an IEEE 488 connection. 
     The source system  14  includes a Keithley 2400 source meter  51  and a pair of probing tips  52 . The probing tips  52  are in fixed physical location with respect to the sphere  18 . Under the control of computer  12 , the source meter  51  provides power to the prober tips  52  when a device D on the wafer W is properly aligned with the sphere  18 . The source meter also measures electrical properties at the probing tips. 
     The measurement system  16  includes a Keithley 2400 radiometer  62 , a power supply  64 , and the integrating sphere  18 . The power supply  64  provides power to the detector  40 . The radiometer  62  receives the measurement signals from the detector  40 . 
     The sphere  18  is known to those skilled in the light and color measurement art. For example, an appropriate sphere is manufactured and sold by Labsphere, Inc. of North Sutton, N.H. as Model VCSEL-0850, which is fabricated of a material identified by the trademark SPECTRALON®. The sphere  18  includes a sample port  23  through which sample light enters the sphere. The diameter of the sample port is larger, and preferably substantially larger than the lateral dimensions of the light-emitting portion of a device D on the wafer W to enhance the amount of light captured by the sphere during measurement. 
     The sphere support assembly  20  is illustrated in FIG.  2 . While the hardware has been specifically designed for the present application, the basic components will be recognized by those skilled in the art. The support  22  is a part of the prober. The Unislide  24  is connected to the support  22  through a conversion plate  26 . An Ausleger  28  is mounted on the Unislide  24  and in turn supports a post  30  for vertical adjustment. A verizontal translator bracket  32  is mounted on the lower end of the post  30 . The sphere support plate  34  is mounted on the translator bracket  32 . The plate  34  defines a aperture  36  within which the sphere  18  is mounted. A detector  40  of conventional design is mounted on the side of the sphere  18  to provide light measurement as is conventional in the light measurement art. The remaining but unnumbered elements included in FIG. 2 are conventional mounting elements and need not be described in detail. 
     The spectrograph  50  is utilized to measure the light in the sphere  18  as is customary in the art. This information is provided to the computer  12  for appropriate adjustment and/or correction of the light measurements. 
     OPERATION 
     The operation of the present prober is quite similar to previous probers. Under the control of the computer  12 , the prober positions the wafer so that a selected light-emitting device D on the wafer W is aligned with the sphere  18 , and more particularly with the sample port  23 . When the wafer is properly positioned, the prober tips  52  contact the electrical power leads of that device. Power is then supplied to the device through the prober tips  52  under the control of the computer  12 . The light emitted from the device radiates into the sphere  18  through the opening  23  and is detected by the detector  40 . The measurement information is transferred from the radiometer  62 , the source meter  51 , and the spectrograph  50  to the computer  12 . 
     After a measurement is taken, the wafer W is repositioned as necessary to align another device D with the sphere  18  for measurement. The sequence of positioning and measuring continues until all desired devices on the wafer have been sampled. 
     If the sample opening  23  has a diameter more than twice as large as a single device D, then two or more devices can be aligned with the sphere  18  at a single time, so that two or more devices can be measured without repositioning the wafer. Each device aligned with the opening is sequentially and individually activated and measured before the wafer is repositioned. This accelerates the measurement of all devices on the wafer. 
     The use of an integrating sphere maximizes the amount of light captured from the device, and thereby improves the accuracy of the resulting measurement. Ideally, the sphere  18  is positioned as close to the wafer as physically possible. When VCSELs are to be measured, the sphere is positioned four millimeters (4 mm) above the surface of the wafer and the opening is 6.3 mm in diameter. Because the VCSEL emits light in a 40-degree cone, this arrangement results in all of the emitted light being captured by the sphere. When LEDs are to be measured, it is anticipated that the sphere will need to be positioned more closely to the wafer because of the greater angle of divergence of the emitted light. Specifically, it is contemplated that the sphere will need to be positioned approximately one-half millimeter (0.5 mm) from the wafer. In this case, it is further contemplated that the prober tips  52  will need to be recessed into the sphere  18  to enable the sphere position to be as close to the wafer as desired. 
     The optical characteristics that can be measured by the present system include: 
     For VCSELs: 
     Radiant flux (optical power) 
     Power 
     LIV curves 
     Spectral properties 
     Peak wavelength 
     Full-width/half-max (FWHM) 
     Kink current 
     Kink voltage 
     Threshold power 
     Threshold current 
     Threshold voltage 
     Wall plug efficiency 
     Slope efficiency 
     External quantum efficiency 
     Forward current 
     Forward voltage 
     For LEDs: 
     Luminous flux 
     Radiant flux 
     Chromaticity 
     Dominant wavelength 
     Peak wavelength 
     Full-width/half-max (FWHM) 
     Centroid wavelength 
     Center wavelength 
     Purity 
     Leakage current 
     Reverse voltage 
     Forward current 
     Forward voltage 
     The above descriptions are those of preferred embodiments of the invention. Various alterations and changes can be made without departing from the spirit and broader aspects of the invention, which are to be interpreted in accordance with the principles of patent law, including the doctrine of equivalents.