The relatively insignificant flaw in Pentium performance was detectable only by happenstance or ultraextensive, and therefore expensive, functional testing. But verification of device characteristics and process integrity at the wafer level are more deterministic and can yield impressive quality improvement vs test cost results.
As has often been said, the earlier a design flaw or process defect is detected, the less is its negative impact. As a result, many tests and verifications are–and more will be–performed at the earliest stage: the wafer level. These tests include device characterization over wide temperature ranges, and at-speed, leakage and other parametric measurements of individual die.
Hot and Cold Wafer Tests
If performance evaluations–rather than rudimental are-you-alive tests–are needed, wafer probing must take place at temperatures that typify ultimate usage conditions. Many of today’s probing stations can be equipped to test wafers over wide temperature ranges, some extending from -65(degree)C to +300(degree)C. But even when a smaller hot-cold temperature range is to be covered, special measures for each temperature extreme must be taken.
At the cold end of the range, it is necessary to moisture and frost formation on the DUT, probe card, probe, system chuck and prober stage drives. To accomplish this feat, a variety of special environmental enclosures has been developed.
For example, Cascade Microtech recently introduced a controlled environmental microenclosure for its probe stations. “This enclosure surrounds the immediate area around the thermal chuck without interfering with the probing operation,” explained Don Miller, Director of Sales. “The microchamber is purged with dry air or nitrogen to lower the dew point, permitting probing at subzero temperature in a dry environment.”
Similarly, Wentworth developed the ShieldMaster(TM) for the MP-2000 Programmable Analytical Probe Station. “It maintains a selected temperature at the wafer as low as -55(degree)C, and may be used with a probe card, individual manipulators, or both,” said Don Brown, Operations Manager at the company.
The 8800 Tempseal System for the Micromanipulator 8800 series probers also produces a localized dry environment at the prober chuck. “It allows frost-free probing to -65(degree) with probe cards or probes,” said Michael Jackson, Director of Sales and Marketing at Micromanipulator Co.
A set of different problems must be overcome at the high- temperature end. “Automatic compensation must be provided for the various expansion characteristics encountered at elevated temperatures,” said Phillip Truckle, Director of Marketing at Electroglas. “Careful selection of materials must be employed in conjunction with designs that compensate for the thermal expansion created.”
“IC manufacturers now test at 85(degree)C and 125(degree)C in production, but several factors are providing difficulties,” said Mike Bonham, Vice President at Cerprobe. “These include:
(o) FR-4 PCB bow and twist (warping) of the probe card.
(o) Expansion of the aluminum support ring.
(o) Softening of the epoxy, allowing needles to move. This is evidenced by location differences of scrub marks between ambient and high temperature (>100(degree)C) probing.”
“A probe card with the same thermal coefficient of expansion as the wafer would be ideal. Several concepts have been proposed to accomplish this but have not yet been accepted,” he continued.
“In the meantime, five approaches are being used to manufacture cards for high-temperature probing:
(o) Change PCB material from FR-4 to polyamide or ceramic.
(o) Use a ceramic support ring in place of aluminum.
(o) Increase the tip depth so the PCB is farther from the wafer, lowering the temperature on the board.
(o) Use a different, high-temperature epoxy or adhesive.
(o) Align the card at the required probe temperature to account for the drift due to temperature. This, however, can cause discrepancies at incoming inspection if the board is checked for alignment at ambient temperature,” Mr. Bonham said.
Another challenge is posed by new ICs with high power consumption. Their inherent temperature rise makes it difficult to accurately control die temperature. One solution is the Temptronic ThermoChuck product family, which offers a stable temperature platform for wafer test at DUT power levels up to 100 W and over a DUT (die) temperature range of 0(degree) to +130(degree)C.
Assuring Low-Level Measurement Fidelity
“Submicron device scaling and ever-thinner insulating gate oxides have increased interest in very low current, voltage and capacitance measurements,” said Larry Dangremond, Product Marketing Manager at Cascade Microtech. “Accurate low-level measurements require a low-noise, low-leakage and low-capacitance wafer probing environment.”
To provide such an environment, Cascade Microtech developed a low-noise -65(degree) to +200(degree) wafer probe station with a 1,000X reduction in wafer chuck (substrate) noise. The new wafer prober uses the company’s MicroChamber, a localized chuck enclosure which reduces dry-air purge time from hours to minutes and furnishes required noise shielding.
Several other companies also use enclosures not only for temperature environment control but also for noise shielding. For example, a variation of Cascade Microtech’s microenclosure provides an electrically shielded environment which enables performance of low-level measurements down to the femtoamp range, said Mr. Miller.
While shielding minimizes noise pickup, low-current parametric measurements can also be significantly affected by extrinsic current flow. Most low-current measurements are performed with a source-measure unit (SMU), an instrument which sources and measures voltage and current–with some SMUs having a sensitivity as low as 10-15 A.
In the path between the SMU and the DUT, current leakage will occur between the cable conductor and its shield. In addition, current will flow from the probe to any adjacent ground on the probe station. Since these current flows are in parallel with the DUT measurement path, they can cause erroneous readings.
More measurement errors may be produced by the incidental capacitance that exists between the cables, probes, DUT and chuck. These undesired capacitors will charge and discharge when a varying voltage is applied. During swept measurements, or subsequent to switching if there is inadequate allowance for settling time, the SMU will measure these charging currents along with the DUT current, causing errors.
To minimize leakage- and capacitive-induced errors, a series of triaxial products has been developed. Triaxial cables contain a conductor surrounded by a shield, which, in turn, is surrounded by another shield. For low-noise applications, the outer shield is connected to ground and the inner (guard) shield is driven by a separate low-impedance source to a voltage level equal to that of the inner conductor.
Since there is no voltage differential between the conductor and the inner shield, there will be no leakage current. Similarly, deleterious capacitive effects are mitigated.
“You can extend this procedure one step by using a triax chuck and bias the chuck guard,” said Craig Rahn, Western Regional Sales Manager at Cascade Microtech. “For the hot chuck, Temptronic offers an optional add-on surface that includes a copper-shield layer. This layer becomes the guard shield. If probe cards are used, a ground plane on the card is connected to the cable shield,” he continued.
Performance advantages are similarly realized for individual triax probe assemblies. “One of our customers recently switched from standard DC probes to triax probes and increased his ability to measure current by 500 times,” Mr. Rahn added. “This means he reduced the noise level by 99.98%.”2
Temptronic offers several new ultra-isolated/shielded temperature-controlled vacuum chucks. “This family of ThermoChucks contributes no electrical noise to the DUT and, with the proprietary guarding/shielding provided, allows reliable test of sensitive devices (at-temperature) with signal leakage in the femtoamp range,” said Ken Cole, Director of Customer Support at the company.
For many sensitive measurement tasks, such as C-V tests (capacitance variation as a function of voltage), it is necessary to minimize current leakage and capacitance between the device substrate and ground. To meet this need Temptronic, in cooperation with SEMATECH, has developed a ThermoChuck system for C-V testing.
“This new system provides very fast transition times between temperature extremes, 0(degree) to +300(degree)C, and has exceptional electrical isolation characteristics,” explained Mr. Cole. “This family of ThermoChucks has isolation of greater than 1 T(ohm) and is available with less than 200-pf shunt capacitance.”
Many established probe-system suppliers have expanded their product offerings to include low-noise components and systems. For instance, “Karl Suss now offers current measurement capabilities into the femtoamp region with prober shielding and a line of triaxial probes and guarded chucks,” said Daryll Smith, Product Manager of the company.
Signatone offers the QuieTemp series of hot and cold chucks and triaxial probes which provides 10-15(ohm), said Brian Dickson, Vice President Marketing at Signatone.
For many of the newer systems on the market, help is available. Informative application notes address low-noise probing systems and helpful guidelines focus on SMUs.3,4,5
References
1. The Wall Street Journal, Jan. 18, 1995, p. B6.
2. Rahn, C., Memorandum on “Low Noise Measurements,” Oct. 21 1994, Alessi, Inc.
3. “Basics of Low Current Probing,” Application Note, Micromanipulator Co.
4. Low-Level Measurements Handbook, Keithley Instruments, Inc.
5. Keithley 236/237 Source Measure Units Applications Overview, Keithley Instruments, Inc.
Products
Probe Card Bandwith
Extends to 5 GHz
Probe cards manufactured with the patent-pending Photolithographic Pattern Plated Probe (P4) process are one-piece assemblies offering stability and probe planarity of typically (+)12 micron. They provide scrub action and long life (300,000 touchdowns typical), and are available with up to 400 pins with 100-micron minimum spacing in customer-defined PCB layouts. Impedance is 50(ohm) to within 1 mm of the pad. The temperature range is -25(degree)C to -90(degree)C standard, -55(degree)C to +150(degree)C optional. The bandwidth of the P4 RFM Probe Card extends to 5 GHz. Fresh Quest Corp., (602) 497-4200.
Probe Card Features
Controlled Impedance
The Transmission Line Probe Assembly (TLPA) Card is a controlled-impedance version of the CerCard series 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.
Triaxial Probe Facilitates
Low-Current Measurements
The triaxial TRX Probe permits low-current measurements into the femtoamp range. Used with the Signatone QuieTemp Triaxial series of hot and cold chucks, this guarded probe provides extremely low leakage to ground: <20 fA at ambient and 1015(ohms) and floor noise is (+)1 fA, even at chuck temperatures of 250(degree). Lucas/Signatone Corp., (408) 848-2851.Integrated Option Facilitates
Rapid Changing of Probe Cards
The Bottom Probe Card Changer (BPCC) option for the Horizon 4060X(R)/4080X(R) Wafer Probers facilitates front loading and unloading of probe cards without undocking the test head. It features automatic latching for safe, reliable contact to the spring pins. Planarity repeatability is better than 0.0005(“) and angular repeatability is greater than 0.016(degree). Available for most ATE, the BPCC can accommodate probe cards up to 260 mm dia. It is retrofittable to existing systems. Electroglas, Inc., (408) 727-6500.
Enclosure Offers
Dry Probing Environment
The Controlled Environment Enclosure (CEE) allows low-temperature probing without risk of contaminating the wafers with moisture. The CEE is lightweight and mounts directly to the wafer probing station. It contains a dry gas purge manifold, wafer cassette load/unload chambers, a front door for X-Y forcer access and a sealed bulkhead for routing fluid and electrical lines. Access to the probe ring is provided through the top of the enclosure. Temptronic Corp., (617) 969-2501.
Stations Enable
Safe Probe Placement
The PPMS Probing Station family includes the PPMS100 high-resolution probing station and the PPMS200 fine-pitch probing system. The PPMS enables engineers to guide microwave and standard probes onto small-geometry targets, such as fine-pitch interconnects, SMT packages, MCMs, hybrids and ICs. The PPMS100 positions microwave probes and is an optional accessory for the Tektronix IPA 310 interconnect parameter analyzer, which is used to characterize and extract modeling parameters from packages, interconnects, MCMs and ICs. Tektronix, Inc., (800) 426-2200.
Enclosure Facilitates
Below-Ambient Tests
The ShieldMaster(TM) Enclosure is used in conjunction with the MP-2000 Semi-Automatic Probing System. Operating under the LabMaster probing environment, ShieldMaster prevents moisture from forming around the wafer chuck when probing at below-ambient temperatures. While operating with thermal chuck systems, ShieldMaster facilitates wafer tests from -65(degree) to +200(degree). Purge time is typically <10 minutes. Dry-air or nitrogen-purge hookups support cooling. A wafer-loading entry/exit door provides access to DUTs. Wentworth Laboratories, Inc., (203) 775-0448.Station Provides Fast
Low-Noise Measurements
The Summit 12000-Series Semiautomatic Probe Station creates a protected measurement environment inside a low-noise, shielded and light-tight MicroChamber(TM) for testing 8(“). wafers. The FemtoGuard(TM) chuck and guarded coaxial probes offer an ultra-low, 0.2-pF capacitance for swept measurements without capacitive error currents. A guarded version of the thermal chuck reduces chuck noise below +200 fA while maintaining temperatures from -65(degree) to 200(degree)C. Closed-loop position feedback provides for accurate submicron moves. The software is MS Windows-based. Cascade Microtech, Inc., (503) 626-8245.
Probe Station Combines
Semiautomatic/Manual Features
The 8800 Series of semiautomatic probe stations now can be equipped to provide a manual mode, retaining the accuracy and the capabilities of its semiautomatic system. 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., Inc., (702) 882-2400.
Analytical Probe Station
Converts to Automatic System
A robotic wafer handler for the Alessi Model REL-5500 Probe Station provides fully automatic wafer probing capabilities on an analytical probe station. The Pattern Recognition System is used for wafer alignment and controlling probe placement. Up to three cassettes with wafer sizes from 3(“) to 8(“) can be accommodated. Optional bar code or optical character recognition is available. The REL-5500 is controlled from any PC-compatible running MS Windows(R). Cascade Microtech, Inc., (714) 830-0660.
Copyright 1995 Nelson Publishing Inc.
March 1995