The Imaging Technology Laboratory, with funding from the National Science Foundation, has developed a cold CCD wafer probing system to quantitatively characterize scientific CCDs at the wafer and die level.  The goals of this system are to evaluate devices for front and back illuminated packaging, to provide rapid feedback to CCD manufacturers concerning device performance, particularly for new devices and device technologies, and to allow cryogenic and imaging characterization of other devices and device structures.  We have adapted our system to handle many die and wafer sizes.  At right are probe cards for a 156 pin 4kx4k CCD (left) and a 21 pin 2kx2k CCD (right).

The Prober Hardware

The probing system consists of an Electroglas 2001x automated wafer prober equipped  with a Temptronic 6" cold chuck, a controlled environmental enclosure, a Keithley matrix switching system, DC test equipment, and a San Diego State University CCD controller connected to a workstation running IRAF under Linux.

The Electroglas 2001X prober has an automated material handling unit capable of testing 25 wafers per cycle.  This is a high volume, high reliability prober used throughout the semiconductor industry.

The Temptronic cold chuck can handle wafers as large as 150 mm in diamater.  Testing can be performed at any temperature between +40 C and -65C using the closed cycle refrigeration unit (shown at at right) .

The Controlled Environment Enclosure (CEE) is used to maintain light tight and low humidity conditions near the wafers and other cold components in the system.  The purging of the CEE is done with filtered air passing through a drier capable of reaching a dew point as low as -100C (see picture at right, background, mounted on wall).  A cold testing cycle is defined by the dry air purging requirement and cooling and heating cycle of the chuck and wafers.  Purging is typically a few hours before each cold probing cycle.

DC Test System (shorts testing)

The DC test system consists of a Keithley matrix switching system and Keithley test meters capable of measuring resistance, voltage, and current between all probe contacts.  The DC test system is under the control of a National Instruments Labview program written in-house.  This allows downloading of any number of test files for specific characterization requirements. All data are written to an Excel spreadsheet for easy analysis, display, and archiving.  The system is easily customized for different applications.

A particularly important aspect of the DC probing system is to fully test CCDs for shorts at operating conditions.  We have the ability to set voltage levels under software control and test current flow to determine breakdown points of the clocks and other  structures.  This allows a much better characterization of the devices than standard probe testing, which is done at considerably lower voltages than used in actual operation.  We have found that proper DC testing is critical to determine which devices will have long term survivability, both for the thinning operation, and during the device's operational lifetime.

AC Testing (image testing)

The SDSU CCD controller is used because it is identical to the controllers we use at our telescopes for actual scientific observations.  This allows accurate calibration of the data with scientific results.  Our current controller operates at relatively slow speed (<50 kHz pixel rate) and is well suited for low light level characterization. The controller allows fully programmable voltages and waveforms which are stored on-line for easy switching from one device type to another.  The prober controller currently uses one readout channel, but can be upgrade to a 4 output system if necessary.  One of up to four amplifiers can be selected for readout with our current system.

Data are acquired with the ICE package in IRAF (Image Reduction and Analysis Facility) and displayed, stored, and analyzed on a workstation in the Lab or accessible on the Internet. IRAF, created at NOAO, is a sophisticated program used primarily by astronomers.  It allows full scientific image processing and analysis directly at the probe station computer.  Data can be archived using any media attached to our computer system.

Imaging tests include flat field illumination, Fe-55 gain and CTE calibrations, dark images, and read noise analysis.  We have the capabilities to test, in addition to the front-illuminated die and wafers, back-illuminated die prior to packaging.  Both DC and AC tests may be performed on die after thinning, after oxidation, and/or after coating before placed into a package.

At left, the fiber optic screen used for flat field illumination is visible.  This screen, residing on the underside of a cover, is illuminated by an LED that is under computer control.  The cover is part of the light-tight enclosure system, so that testing may be completed with the lab lights on, without affecting test results.