Probe Card Analysis and Probe Mark Analysiscase studies from the sort floor

Rudolph Technologies, Inc.

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REAL TIME PROCESS MONITORINGPMA provides a wealth of information about the probing/sort process making it easy to identify and isolate errors from multiple sources. Figure 1 shows two wafer maps plotting “left distance”, the minimum distance between the left edge of the test pad and the probe mark. The right map was acquired four hours after the left map. The probe card in this case was smaller than the wafer and required six touch downs to cover the whole wafer surface. A number of observations can be made. Left distance differs by approximately 4 µm between the first (upper left) and third (middle left) touchdowns, a wafer load error. Left distance differs by approximately 7 µm between touchdowns on the left and right halves of the wafer, a prober error. Left distance differs by approximately 5 µm

INTRODUCTIONProbe card analysis (PCA) and probe mark analysis (PMA) allow engineers to optimize the performance of the equipment and procedures they use to test the electrical functionality and measure the performance of integrated circuits after the devices have completed the wafer fabrication process but before they have been diced and packaged. PCA tools examine the physical probes used to make mechanical/electrical contact with test pads on circuits. The probes are carried by a probe card and there may be anywhere from a few tens to tens of thousands of probes on a single card. PCA can determine the precise location, size, shape, overtravel deflection and more for each probe in the array.

Probes are designed to “scrub” across the test pad surface as the probe card is moved from the nominal contact z position to the overtravel z position. The scrubbing action breaks through any surface oxide or contamination that may be present, thereby ensuring good electrical contact. The scrubbing action also creates a probe mark which carries important information about the probing process. Probe mark inspection (PMI) has long been used by semiconductor manufacturers to detect defective marks that may interfere with device operation. Probe mark analysis differs from probe mark inspection in its intent to derive from the marks statistically valid information that can be used to improve the performance of the probing process. It does this primarily by detecting patterns in the data that would be difficult or impossible to find manually. PMA might identify a specific probe in the array that is creating defective marks, or determine from the pattern of position errors that the probe card is tilted with respect to the wafer, or even that the screws holding the wafer chuck have been loosened by temperature changes and probing forces.

Together PCA and PMA provide an extremely powerful toolset for investigating and optimizing almost every aspect of probing operations. They can be used to monitor and detect excursions in the process; to validate new processes, such as testing at temperature, pad shrinks, new pad materials; to qualify new or reconditioned probe cards; to compare the performance of alternative probe technologies; to qualify new equipment; to make tools-to-tool comparisons, to monitor maintenance requirements; to evaluate and select new tools; and more.

This paper offers PCA and PMA examples drawn directly from practical experience on the sort floor.

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between the left and right sides of the probe array, a probe card error. Left distance changes by approximately 7 µm between the left map and the right map, due to thermal drift over the 4 hour elapsed time.

Wafer Fabrication Process Monitor

Figure 2 plots variations in probe mark size (scrub area) over the wafer surface and (right) displays the detected extent of one mark within the outline of the probe pad. The process and error components had been well characterized and an increase in probe mark area was quickly identified as an out-of- tolerance condition, which triggered further review and evaluation. The increase in probe mark size was traced to a discoloration of the pad, which was in turn linked to an error in the fabrication process.

Controlling the Sort Process

Plots of continuity vs overtravel (Figure 3) demonstrated a significant overtravel needed to make full electrical contact with 2 different probe cards, PC10 and PC11. The scrub signature corresponds to a flatness issue - the reference surface is not flat. The plots at bottom left and right demonstrate excessive pad damage. The bottom middle shows the flatness

of the reference surface. The parallelism of the chuck to the reference surface and the response of all components to the compressive forces generated by overtravel are important parameters routinely characterized in overtravel optimization. In this case, the discrepancy in performance was ultimately traced to differences in the flatness of the card rings in the two testers.

VALIDATING NEW PROCESSESThermal Movement of Probe Card

Metrology is a key tool when testing devices at operating temperature (test-at-temperature). Since everything moves as it heats, it is critical to isolate and measure all contributing components: needle movement, array movement, pad movement, etc. What happens to the support material, flatness and planarity of the array, the reference surface, or the prober chuck? Do they all move in the same direction or does overall planarity change? With metrology all of these variables can be measured and a steady state or optimal process defined. Figure 4 (left) shows probe array z-movement of up to 110µm in response to heating. The images on the right show marks made by individual probes in nine different areas of the wafer. The marks drift within the pad as a result of different thermal scaling coefficients between the wafer, the probeneedle/card and the prober. Individual probe drift varies from 3 to 15 µm.

Thermal Movement of Probe Card

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Figure 5 compares “Y Scale” measurements made on two wafers at different temperatures 88 °C (left) and 150°C (right). Y scale measures the accuracy of the die-to-die step size in the y direction. This test used an 8 X 1 probe card array (outlined in black). The 88°C measurement shows a Y scale error range of 7 µm. The range increases to 33 µm at 150°C. Process modifications will be needed to support test at different temperatures.

Thermal Drift

Probe marks were found to be drifting off the pads (Fig. 6)during the probing process because of thermal scaling. PMA identified a thermal drift signature. The standard serpentine test pattern did not allow for temperature stabilization. A different pattern provided more stable thermal performance of the probe card.

Probe Card Verification at Temperature

Figure 7 shows hot (left) and cold (right) data collected while verifying a probe card at temperature. The Scrub X/Y position error plots (upper) use a vector display mode that clearly shows patterns in the magnitude and direction of the error. This card cannot be qualified to run at multiple temperatures because the scaling is not linear across the temperature ranges.

Validating Pad Shrink

Supporting a pad shrink can be a daunting problem. First, the total error of the process must be broken down into its components. The left side of Figure 8 shows the total error (Cpk) of the process. The right side shows the error associated with just the probe card. In this case the probe card is performing well, so it can be eliminated from the investigation and other error sources can be pursued.

Probe Card Verification

The key to a stable process is consistency among tool sets. Getting all of the tools sets to match some standard is the key. The standard can be somewhat arbitrary and may not necessarily be the same standard used by the tool supplier. Being able to quickly adjust incoming probe cards to an appropriate standard is key to maintaining the overall process. Figure 9 illustrates a typical verification process flow, in this case resulting in a significant improvement in probing process stability.

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PROBE CARD QUALIFICATIONValidating New Probe Technology

Figure 10 shows the probe card signature (vector view of scrub X-Y position) acquired for a new card using a blank, unpatterned wafer. The plot shows a strong thermal scaling effect. This card did not pass incoming inspection.

Validating Probe Card Design

In Figure 11, the probe card showed inconsistent results at temperature. Analysis determined that the change occurred after the prober stopped to check probe marks. This allowed the probe card to cool down causing a shift in scrub performance. The root cause for the rapid cool down was found to be the probe card stiffener, which was acting as a heat sink. The left image is the original stiffener; the right image is the same card with a new stiffener design. Changing the design improved the overall performance.

Automated Deflection Test

Modeling or understanding how the probe card performs under load is key to maintaining a uniform process. Using metrology equipment it is possible to model probe card deflection and optimize the programmed overtravel appropriately. Incorrect overtravel leads to a variety of sort related issues, including low Cres, under-pad metal cracking, punch through, excessive probe wear, excessive cleaning, excessive debris, etc. Figure 12 is a deflection plot, showing high probes moving up with the fixture as load increases even at this low overtravel (1 mil), this reduces the applied Z on high probes by 5µm. This will affect the scrub and CRES performance of the high probes.

Automated Deflection Test

Figure 13 plots (Probe Card from Figure 12) the expected planarity versus the actual planarity at load with probes sorted from lowest to highest. High needles translate up as increasing load causes card deflection.

PROBE TECHNOLOGYComparing Probe Card Technologies

Gauge studies provide a means to quantify measurement capability when comparing different technologies (Fig. 14)

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Comparing Probe Card Technologies

Figure 15 compares the scrub lengths measured for two different probe card technologies, cantilever Dual DUT (left) and MEMS Quad DUT (right).

Comparing Probe Card Technologies

Figure 16 compares probe mark area measured for two different tip types of cantilever probe cards using the same prober, prober settings, time and temperature. The probe tip on the right clearly leaves significantly smaller marks. Tip shape has a large impact on scrub performance for different types of applications.

EQUIPMENT COMPARISONProber to Prober

Prober to prober comparisons can help to standardize tools across the sort floor. Figure 17 shows the results of a test run to determine prober to prober variance. The test used the same card, prober settings, time and temperature. Errors in chuck tilt and head stage to chuck parallelism were found and fixed in the left tool.

Translation Effects

Some marks near the edges of the die were failing probe mark inspection. In particular, the area, shape and direction of marks left by the same probe were changing from touchdown to touchdown. In Fig. 18, the lower images show probe marks from the same needle in a touchdown on the left and right side of the wafer. These changes were found to be effects of probe card translation and deflection.

MONITORING MAINTENANCEEquipment Problems

The wafer map in Figure 19 plots “left distance”, the minimum distance from the scrub mark edge to the left pad edge. The observed pattern is typical of wafer scaling errors, which result from a mismatch between prober step size and die spacing on the wafer, in this case in the x direction. The error was immediately apparent from the prober report chart.

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Validating Maintenance Problems

In Figure 20, the left wafer map of scrub X/Y position shows a pattern typically caused by loosening of the screws used to adjust the chuck planarity the wafer chuck to the prober. Retesting after tightening the screws (right map) confirmed the source of the problem and the efficacy of the corrective action.

Stage Stepping Accuracy

The pattern shown in Figure 21 indicates scaling and offsets not applied correctly by the prober.

Setup Validation

The “swirl” pattern apparent in Figure 22 (left) indicates errors in prober setup and alignment. Adjustments to the prober setup resulted in a 50% improvement (right).

Probe Card Setup Errors

Incorrect setup by the operator caused an X-axis offset error.

Probe Card Setup Errors

Prober stage error resulted in theta alignment issues

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TOOL EVALUATIONProber vs Probe Card

The wafer plot on the left shows inconsistent pad damage which will ultimately result in higher failure rates. The data were acquired using a standard vertical probe card on which all needles scrub in the same direction. The resulting force caused stage translation. A new vertical head was designed on which half of the needles scrub in the opposite direction, thus balancing out the translational force applied to the stage. The wafer map on the left shows the improved performance (reduced and more consistent damage) achieved with the new design.

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