Moore’s Law—which says that IC complexity doubles about every 18 months and that IC geometries shrink by approximately 10% a year—still holds true today, more than three decades after being postulated. During most of that time, it was relatively easy to construct the wafer and device measurement facilities needed by semiconductor suppliers and users.
But as IC manufacturing technology continues to overcome the increasingly daunting physics-related processing limitations of devices, the semiconductor wafer prober and test industry is being confronted by a new range of novel challenges. These extend from contacting near-invisible IC features to providing a femtoamp measurement environment.
Impact of Submicron Geometries
As semiconductor geometries shrink into the sub-quarter-micron region, more devices are containing multimillion transistors with several hundred I/O ports. These factors combine to shrink pad sizes and pitches and increase pad quantity and density. Establishing reliable interconnections between today’s minuscule device access points and the appropriate test instruments requires new, or improved, probes, probe cards, and probe stations.
Along with shrinking device geometries come thinner oxide layers, shorter transistor channels, and higher power density. To evaluate process parameters and functionality of devices endowed with these characteristics requires high-resolution measurements of gate leakage, subthreshold drain currents, and mobile ionic defect density, commented Larry Dangremond, marketing manager for the Probing Systems Unit at Cascade Microtech. Consequently, modern probe stations must make extremely low-level voltage and current measurements over a wide temperature range.
High-density devices also are faster which requires high-frequency measurement capabilities. This can be achieved by minimizing lead length and the control of transmission-path impedance. Of course, with millions of transistors per device, more tests must be performed, necessitating higher test throughput.
While probes, cards, and stations are individually affected by these diverse and severe requirements, there are many interdependencies among them that must be addressed.
Probing Issues
“One dramatic way that shrinking geometries have affected probes is evidenced by the smaller diameters now in use,” said Michelle Gesse, president of Advanced Probing Systems. “Five years ago, probably 80% of our sales were probes 10 mils or larger in diameter. This year, I would estimate that more than 50% of the sales will be probes with a diameter from 5 to 8 mils.”
Smaller probe bodies feature smaller tip diameters. While smaller tip surfaces are needed to contact today’s minuscule targets, they wear out faster, shortening the life of the probe card.
Fine line width and tight interconnection spacing make probing difficult in several ways. “As the pitch between the needles shrinks, illumination and visibility through the optical system become limited,” said Mr. Dangremond at Cascade Microtech. “Lines with a width below 0.2 micron are extremely difficult to see and probe.”
Smaller pad-to-pad spacing also presents a problem when testing at extended temperatures. “SEMATECH is planning to study the effects of temperature on probe-tip drift,” commented Michael Bonham, senior vice president sales and marketing at Cerprobe. “At 125ºC, the temperature-related drift for a 70-micron pitch may short out probes.”
The shorting problem may be avoided by using specially treated probes. “Coatings can be applied to insulate the probes,” explained Ms. Gesse. “Similarly, probe performance can be enhanced by employing a plating technique that reduces the resistivity of tungsten and tungsten-rhenium probes.”
Probe Cards
The increased number of pads and probes often requires a multilayer construction approach, with some epoxy-ring probe cards supporting as many as nine layers of needles. It is becoming very time-consuming and costly to produce and test such cards. “For instance, a 1,200-point card requires up to 8 h to test on an analyzer,” said Mr. Bonham of Cerprobe. “And because of their complexity, the cards cannot be repaired.”
As the number of pads to be probed increases, it also becomes more difficult to position the needles accurately in all three axes. Also, the higher speed and lower operating levels common in today’s dense-geometry devices pose additional performance-related challenges to epoxy-ring cards.
“At the higher operating frequencies, the inductance and capacitance of the needles can adversely affect measurement accuracy,” said Mr. Dangremond. “And shrinking geometries mean that testing is performed at much lower voltage and current levels. As a result, it is necessary to use fully guarded ceramic-blade needle cards, an implementation which, by itself, adds bulk to an already crowded probe card.”
To overcome these problems, several manufacturers produce non-needle-bearing probe cards in various forms. For example, the Cascade Pyramid Probe-Card uses a photolithographic process to define probe contact bumps and metal layers on a flexible membrane. Frequency performance can be as high as 20 GHz and inductance <0.2 nH, according to Mr. Dangremond.
But just as lead length and its accompanying inductance are detrimental to measurement accuracy, so are they to device performance. For fast high-pin-out devices, the trend is away from wire bonding and providing connections only around the perimeter of the die. Instead, a matrix array of bumps is placed on the die itself. These bumps make access to the innards of the device easier and minimize the length of interconnection leads since the bumps are bonded directly to the substrate package.
The pitch of these arrays is currently 250 microns, limited primarily by the size of the bumps. The SEMATECH IC development roadmap anticipates that pitch will decrease to 100 microns by the year 2006.
“To test devices with area array bumps located across the entire die, not just on the periphery, requires vertical probe cards such as the Cobra,” said Mr. Bonham of Cerprobe. Cobra probe cards for 250-micron pitch-size implementations, now are available from several vendors.1 Cobra cards physically transform a 250-micron pitch array into a larger array, such as one with a 2,500-micron pitch conforming to the dimensional limitations of the lands on a PCB.
“But today, the space transformer still limits probing the area array die at speed,” commented Mr. Bonham. “Multilayer ceramic space transformers are being used, but the price and production cycle time of the multilayer ceramic are generally unacceptable.”
Probe Stations
When evaluating process-dependent physical parameters or performing device failure analysis, it often is necessary to probe internal IC features. But placing a needle probe on 0.25-micron lines with adequate repeatability is virtually impossible for all but the most sophisticated probe stations.
Several problems must be addressed in the design of such probe stations, explained Mr. Dangremond of Cascade Microtech. First, it is necessary to create a totally vibration-free environment. Second, enough illumination has to be provided through the optics system to allow the operator to see where to position the probes.
Many solutions are available. For instance, the Alessi REL-6100 Series Stations are mounted on an antivibration table in an airtight, dark box. Programmable probe positioners are guided to the features to be contacted.
“The entire probing system must be designed for small geometry probing, ranging from providing stability of stage, manipulator, and microscope to the resolution of the manipulator accessories,” commented Mike Jackson, director of sales and marketing at The Micromanipulator Company. “At one time, an engineer could equip an inexpensive probe station with high-resolution manipulators to probe small geometries. But this approach doesn’t work any more.
“The small geometries have caused a move toward hands-off automatic probing,” Mr. Jackson continued. “A motorized, programmable manipulator can have a resolution of 13 nm with present (albeit top-end) technology. Users must depend on this technology since the hand’s-on approach just causes too much disruption in position to hit the small targets.”
The high pin-out and pin density of today’s devices also put a substantial mechanical strain on the prober systems, commented Mr. Bonham of Cerprobe. For example, a wafer with 4,000 bumps, each contacted with a force of 9 grams, requires an overall force of 36 kg or 79.2 lb. This calls for a new Z stage for the probers since most can handle only 50 lb today.
“The vertical forces exert an enormous impact on the chuck surface,” confirmed Dr. Ashwin Ballal, product marketing manager at Electroglas. “New materials are needed to absorb the high Z forces to minimize chuck deflections and maintain continuity. Shrinking geometries, smaller pads, and tighter pitches also will require probing in an ultra-clean environment, which will affect the design and manufacture of the next-generation probers.”
Another environment-related issue affecting probe-station and wafer-chuck design is the need to test wafers over extended temperature ranges. “Testing chips at- temperature at the wafer stage enhances the possibility to detect failures before chips are processed into devices,” said Tom Gerendas, president of Temptronic. “This results in significant cost savings which accounts for the dramatic increase in the number of manufacturers who now probe at-temperature on the production floor.”
High-density devices often have high power dissipation, which may result in a substantial rise of chip temperature. If excessive heat is not conducted away from the device during probing, measurements may be inaccurate. This could result in setting erroneous guard-band limits which would reduce yield. A cooling chuck often is used to eliminate this problem, even if probing is specified only at room temperature.
Temperature chucks also are used to determine proper device operation over an entire operating range, which may extend from -65ºC to +200ºC. “Temperature specifications influence the size and weight of thermal chucks, and new designs minimize these as well as limit horizontal and vertical movements due to temperature changes,” explained Mr. Gerendas.
Need for Integrated Solutions
Not only do small geometry devices place special burdens on the design of individual elements required for probing, but also on their compatibility. A typical high-end analytical—not even automatic or end-of-line production—probing system, according to Mr. Jackson of The Micromanipulator Company, may consist of:
An ATE test head mounted on a vibration isolation table, a low-femtoamp-resolution probe card, cabling to the ATE, a thermal chuck, and a wafer-handling robot.
Software that navigates by using the device CAD files. The software communicates with the tester, thermal chuck, and prober to set test parameters; imports a wafer map; sets the chuck temperature; controls movement to the correct die location; activates tests and then files the test data according to the die location, and does it all over again without operator intervention.
“While these hardware items are available from the vendor who designed it, someone must make it all play together. And it won’t unless all vendors involved share the information needed to make it happen.” Mr. Jackson emphasized.
Fortunately, such sharing of information is becoming commonplace, and total test solutions are being developed by cross-industry teams. “The teams consist of representatives from ATE, interface, probe-card, and prober suppliers,” said John Hope, director of sales and marketing at SVTR, a Cerprobe subsidiary. “This makes it possible to establish and verify performance against a system specification.”
For instance, rise time, propagation delay, capacitance, and leakage can be specified on a composite basis and designed into the system. Mechanical issues such as probe-card deflection from pogo force or chuck deflection due to probe-tip force also can be accounted for and minimized.
“Customers obviously benefit from an integrated, well-functioning product that can deal with today’s small geometry devices. But the biggest benefit from the team approach might very well be reduced time to market,” concluded Mr. Hope.
References
1. Bates, R. D., “The Search for the Universal Probe Card Solution,” 1997 IEEE International Test Conference Proceedings, pp. 533-538.
Wafer Test Products
Desktop Prober Provides
Electrically Quiet Environment
The Model S8 Prober is an 8″ analytical prober/test station. It is designed for applications that require a dark and electrically quiet environment such as for low-current and CV measurements or MOS and CCD-elements evaluations. The station can be configured with ambient coaxial or triaxial 6″ or 8″ chucks. For at-temperature probing, the company’s thermal chuck series covers temperatures ranging from -65º C to +400º C. All desktop prober versions with the thermal chuck include an ambient cooling option. The Micromanipulator Company, (702) 882-2400.
Prober Provides Automatic,
Unattended Parametric Testing
The PS21 Parametric Series Autoprober provides automatic, unattended, on-wafer DC/CV parametric measurements when interfaced with an HP 4062, an HP 4071, or a Keithley S600 test system. The autoprober is equipped with the company’s patented MicroChamber™ which ensures light-tight, noise-protected measurements. With the thermal-guarded chuck, low noise measurements over a -55°C to 200°C range can be made. A low-capacitance, low-noise probe card offers fast measurement settling times with low leakage. Up to 25 wafers can be loaded in the standard cassette. Cascade Microtech, (503) 626-8245.
Wafer Prober Handles
Flip-Chip Technology
The Horizon 4090µ Wafer Probing System features an integrated mini-environment and a standard mechanical interface. It supports the Class 1 test and high load at sort requirements of 0.25-micron and smaller designs as well as flip-chip and controlled-collapse chip connection technologies. The system performs high-force probing (70 kg) as needed for high pin-count devices. Software automatically tracks work-in-progress and process equipment setup. Electroglas, (408) 727-6500.
System Facilitates Vision-
Controlled Probe Alignment
The PC-based Probe-to-Pad-Alignment (PTPA) System is retrofittable to the company’s 2001X and 2080S series probers and uses the 2001 auto-align camera, monitor, and keyboard. It mounts to the side of an existing chuck and assists in aligning the tips of the probe card to the bond pads of the wafer. It provides automatic best-probe-to-pad fit and achieves repeatable results without intervention. PTPA reduces the time and effort typically required for setting up a prober for a production run. Silicon Valley Test & Repair, A Cerprobe Co., (602) 967-0090.
Wafer Test-Probe Needles
Offered With Special Coatings
Probe needles designed for wafer testing are being offered in tungsten-rhenium, tungsten, and beryllium copper. They are available in diameters of 0.005″ to 0.015″ and in lengths of 0.75″ to 2.0″. Probe tapers, whether isolinear or parabolic, match customer specifications. Needles may be supplied with special coatings to reduce needle resistance or to provide insulation. Advanced Probing Systems, (303) 939-9384.
Analytical Probe Stations
Are Field Upgradable
The ChekMate Series of Analytical Probe Stations comes in a variety of configurations including the manual 8″ CM-100, the CM-200 with a joystick control and manual override, the CM-300 with a joystick and precision thumb wheels, and the CM-400. The CM-400 includes software control of the microscope stage, wafer stage, and probes. All systems are field upgradable. The ChekMate CM112, CM212, CM312, and CM412 offer features similar to the CM-100 through CM-400 Series but are designed for probing 300-mm wafers. Lucas/Signatone, (408) 848-2851.
Thermal Chuck System Complies
With International Standards
The Model TP03200 ThermoChuck Series Systems for testing wafers, chips, and hybrids from -650C to +1500C comply with international and EC standards. The systems offer enhancements in ease of use and performance and feature color touchscreen control and graphic capabilities. A Controlled Environment Enclosure accessory ensures moisture-free probing at low temperatures. The TP03200 is self-contained and CFC-free and does not require liquid nitrogen or CO2 for cooling. It interfaces to any major wafer-probing station. Temptronic, (617) 969-2501.
Copyright 1998 Nelson Publishing Inc.
March 1998
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