The two-pronged advancements in wafer technology—smaller device geometries and larger wafers—have made the cost of die an ever smaller part of the total IC cost. With die packaging and handling costs becoming more dominant, comprehensive testing at the wafer level makes sense since it avoids further processing steps and wasting money on bad die. But simultaneously, dealing with larger wafers has made the handling, interconnecting and testing tasks more arduous.
Comprehensive testing usually involves low-current and at-speed functional tests. Dealing with ultra-low currents requires special chuck and probe-station implementations.1,2 At-speed functional die testing places another set of severe demands on the probe card, the probe-station structure and the test-system interface.
Probe Cards for At-Speed Testing
Minimizing crosstalk, transmission delays and reflections is essential for at-speed testing of today’s fast devices. Ideally, interconnections between test equipment and die are accomplished via controlled-impedance transmission lines.
While a transmission line-type environment can be created for most of the interconnecting path, a single point of contact still is required for accessing each die pad. Extending the transmission-line environment as far as possible has been the aim of many designers, resulting in several ingenious implementations such as membrane probe cards and Cerprobe’s patented Transmission Line Probe Assembly (TLPA) option for CerCard epoxy-ring probe cards.3
The Cerprobe TLPA provides controlled impedance of the card and needle by combining three factors. The probe card contains a surface ground plane inside of the power rings, the needle support ring is grounded, and the card needles are assembled with a constant clearance over the ground plane and ring.
“This wire-over-ground technique is analogous to manufacturing a controlled- impedance PCB, but air—not FR-4—serves as the dielectric,” explained Michael Bonham, Vice President at Cerprobe.” The result is a controlled impedance and a very short needle contacting the die pad, allowing the probe card to be transparent to the device and test system. Using the TLPA technique, Cerprobe has manufactured high pin-count epoxy-ring probe cards with analog bandwidths exceeding 4 GHz.”
Not only must special attention be paid to the signal transmission path, but also to power and ground lines. “RF test engineers have found that many new high-performance ICs oscillate during wafer probe, yielding erroneous results or even damaging themselves,” stated Larry Dangremond, Marketing Manager for the Probing Systems Business Unit at Cascade Microtech. “Excess inductance in the ground and power supply lines is the culprit and can cause oscillations even if devices are not being tested at full speed.
“Conventional needle or blade probes have more than 10 times the required subnanohenry inductance required to stop these oscillations,” Mr. Dangremond continued. “Cascade Microtech’s Pyramid Probe, a second-generation membrane probe, can provide a few tenths of a nanohenry of inductance and is being used successfully to avoid oscillations during test, even if performed at-speed.”
The need for providing known-good die for use in multichip modules, especially for RF or mixed-signal applications, creates more special demands. Matching networks for low impedance power outputs or emulation of package parasitics such as wire-bond inductances may be required. “For one application, Cascade Microtech provided a card accommodating 65 multi-gigahertz signals and a total I/O count of 256. Neither epoxy- ring nor microwave-coax probes are capable of providing this combination of speed and complexity,” added Mr. Dangremond.
Dealing With Larger Wafers
The move to larger-diameter wafers presents new challenges to designers of wafer chucks, probe stations and handling equipment. “Current temperature-controlled wafer vacuum chucks must maintain flatness to a few microns over wide test temperatures (-65°C to >+200°C) and be minimally affected by the increased probing forces required for many of today’s wafer products,” advised Ken Cole, Director of Customer Support at Temptronic.
The chuck also must be capable of accelerating and decelerating larger amounts of mass. In many cases, contact must be provided to more than 2,000 pads faster and more accurately than ever before, commented David Durham, Prober Product Director at Electroglas. With pad sizes and pitches continually shrinking, overall prober system accuracy becomes much more difficult to achieve and sustain over temperature and time.
The progression from 6″ to 8″ wafer probing occurred without major mechanical implementation bottlenecks. But the transition to 12″ wafers is a formidable task.
“By more than doubling the area to be probed, demands for dimensional accuracy increase geometrically,” said Don Feuerstein, Marketing Services Manager at Wentworth Laboratories. “New transport mechanisms must be devised to provide the required accuracy and repeatability and to ensure system stability.”
Safe transfer and handling of the 12″ wafers pose other challenges, probably requiring mechanical transfer systems, not just for automatic but also for semiautomatic and even manual probe stations. “Gripping wafers on the edge with tweezers may not be practical due to the weight of the wafer. And the cost of destroying large, expensive wafers due to human handling error could be catastrophic,” added Mr. Feuerstein.
Numerous wafer-fabrication facilities are still processing 6″ wafers, many have converted to 8″ wafers and a few are planning to produce 12″ wafers within a few years. In spite of the expected difficulties in processing, handling and testing these large wafers, the progression toward larger wafers will go on unabated and will continue to result in price reductions for the consumers.
References
1. “Basics of Low Current Probing,” Application Note, The Micromanipulator Co., A1006287.
2. Jacob, G., “Probing Systems Offer Low-Noise, At-Temperature Environment,” EE- Evaluation Engineering, March 1995, pp. 32-40.
3. Fisher, T. and Kister, J., “Reducing Test Costs for High-Speed and High Pin-Count Devices,” EE-Evaluation Engineering, February 1992, pp. 96-101.
Wafer Test Products
Analytical Probe Station
Supported by Comprehensive Software
The MP-2000 Programmable Analytical Probe Station, supported by MS-Windows based WL-LABMASTER software, provides an integrated failure-analysis environment. It accommodates devices 1/2″ or larger and 3″ to 8″ wafers. A motorized microscope lift offers 7″ vertical travel and the gold-plated 8″ quick-load chuck facilitates loading/unloading. ML-LABMASTER enables mouse-directed or programmed movements of the stage or micropositioners. Live video images from the system’s CCTV camera may be displayed on the computer monitor. Wentworth Laboratories, (203) 775-0448.
Automatic Prober Provides
Integrated Class 1 Environment
The Horizon 4085X Automatic Wafer Prober facilitates high-speed, high-force probing of advanced devices on 8″ wafers. A multicassette wafer handler accommodates large lot sizes with minimal operator intervention. An optional internal integrated standard mechanical interface (SMIF) may be used in conjunction with the company’s clean air management system (CAMS). The SMIF-CAMS combination provides wafer transfer without exposure to ambient facility air, creating a Class 1 clean-room environment internal to the prober. Networking capability is included. Electroglas, (408) 727-6500.
Electrically Quiet Chuck
Allows Probing to 200°C
The REL-6100 Semiautomatic Analytical Probe Station features submicron resolution, stability and precise motion control. A new semiautomatic, guarded, thermal, triaxial chuck allows probing from 0°C to 200°C with noise levels below 200 fA in a 75-pF capacitive measurement environment. The Windows-based Galaxy software facilitates automatic station control, step and repeat operations, and wafer-map creation. Test results can be overlaid on the map, saved, printed or transferred to file. Manual control is via a joystick or IEEE 488 commands. Cascade Microtech Alessi, (503) 626-8245.
Probe Station Combines
Semiautomatic/Manual Features
The Model 6640 and 8800 series of analytical probing stations can be operated in a semiautomatic or a manual mode. Micro-touch knobs are standard on new stations and may be retrofitted to installed stations. Functioning in synchronism with the station controller, manual controls give the expected tactile feedback but don’t require semiautomatic setup overhead. Controls are tied into the computer at all times, avoiding positioning errors, mechanical adjustment and override problems associated with older combined systems. The Micromanipulator Co., (702) 882-2400.
Probe Card Features
Controlled Impedance
The Transmission Line Probe Assembly (TLPA) Card is a controlled-impedance version of the CerCard series of epoxy-style probe cards. Probes are arranged over an extended ground plane, which includes the support ring. A transmission-line environment is maintained to within 200 mils of the die pads. The remaining path exhibits an inductance of 5 to 10 nH, less than the bond wire and lead-frame inductance of packaged parts. TLPA cards have a bandwidth exceeding 2 GHz (4 GHz with low dielectric materials). Cerprobe Corp., (602) 967-7885.
Thermal Chuck Does Not
Use Liquid Coolants
The ultra low-noise TP03020B ThermoChuck® System probes and characterizes wafers, chips and hybrids at temperatures from +20°C to +200°C. No liquid coolants circulate to the chuck, and the system provides positive control for at-temperature probing from below-ambient through +200°C during continuous operation or while cycling. A new autotuning control adapts to various wafer masses and power dissipations. Operation is controlled from the front panel or via the IEEE 488 or RS-232 interface. Temptronic, (617) 969-2501.
Temperature Chuck System
Features Low Profile
The T-6100 series of temperature chuck systems is comprised of a refrigeration/heating unit with a microprocessor-based temperature controller and a vacuum wafer chuck. The passive chuck does not contain any electronic heating or cooling elements, preventing magnetic field interference. The temperature ranges from -65°C to +135°C with a transition time of 18 minutes from +125°C to -55°C. Refrigerants comply with the Montreal Protocol. Remote control is supported with RS-232 and IEEE 488 interfaces. Thermonics, (408) 496-6838.
Probing Systems Configured
For High Frequency, Low Noise
Application-specific analytical probing systems have been configured for process control and device development of low-current and high-frequency components. The systems consist of the PA 200 Probe Station, motorized motion control of optics, a CCTV camera and video window, a material handler with prealigner and scanner, a light-tight EMI shielded enclosure, a vibration isolation table and a thermal chuck. The Windows-based ProberBench® software interfaces with the thermal chuck controller, a laser cutter, calibration software, HP HF IC-CAP software and HP test instrumentation. Karl Suss America, (802) 244-5181.
Membrane Probe Card
Contacts 1,000+ Points
TestFrame™ Membrane Probe Cards employ multilayer thin-film technology. Copper traces are insulated by a polymide dielectric which combines a low dielectric constant (3.5) with the flexibility necessary to contact hundreds of die pads during at-speed testing. The membrane card features a single-chip probe area of 15 mm x 15 mm and a multisite probe area of 70 mm x 40 mm. The maximum number of probing points is 1,000+, including power, signal and ground. Contact size of 40 microns is standard. MicroModule Systems, (408) 864-7437.
Manual Probe Station
Offered With Accessories
The PSI 400TM Probe Station is a manual analytical prober for 8″ wafers (6″ optional). It accommodates submicron probing on wafers and packaged devices. The stage has an 8″ x 8″ X-Y travel and is available with Mitutoyo® FS60, A-Zoom®, MicroZoom® or StereoZoom® microscopes. Push-pull and coaxial X-Y controls enable one-hand operation. The PSI 400 may be used to perform DC and AC probing and electromigration, CV, low-current and microwave measurements. Probing Solutions, (702) 882-9554.
Parametric Test System Offers
Accuracy and Functionality
The S900B combines high-speed instruments, a low-noise switching matrix, a PC and the Keithley Test Environment software to form a powerful parametric test system. It includes a new integrating system voltmeter unit, based on a 16-bit A/D converter with line-cycle integration to support higher-accuracy measurements. The result is a combination for MOS and bipolar process monitoring or device-development testing. Keithley Instruments, (800) 552-1115.
Copyright 1996 Nelson Publishing Inc.
May 1996
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