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Surfscan SP1TBI vs. Surfscan 6420: a Performance Comparison

by Dale Guidoux, KLA-Tencor

As the semiconductor industry continues to shrink critical design rule dimensions, the need for increased performance from laser-based surface defect inspection tools has intensified. Prior to 0.25 µm geometries, unpatterned wafer inspection uses focused primarily on Particle per Wafer Pass (PWP) measurements. KLA-Tencor’s Surfscan 6420 became an industry standard, based on its particle sensitivity performance on rough films, particularly metal layers. With the implementation of 0.18 µm and 0.13 µm process nodes, there is a need to detect and classify more defect types than just particles. The unique axi-symmetric collection optics and brightfield channels of the Surfscan SP1TBI, coupled with normal incident angle illumination and oblique angle illumination provide superior defect sensitivity and Real Time Defect Classification (RTDC). The tool can be used to differentiate crystal orginated particles or pits (COPs) from particles, classify EPI stacking faults, mounds and dimples, and detect CMP microscratches, chatter marks and slurry residue.

An additional benefit of the axi-symmetric collection optics on the Surfscan SP1TBI is uniform detection of scratches, regardless of orientation. Systems that use non-symmetric collectors can miss important defects such as slip lines and scratches in certain orientations.

An increasingly important issue in design rules less than 0.18 µm are the presence of COPs on silicon wafers. The Surfscan SP1TBI is able to detect and classify these COPs separately from surface particles by comparing the defect signal from the wide angle collector with the narrow angle collector signal. Previous generations of Surfscans were not able to differentiate COPs from particles. The ability to classify COPs has two advantages. First, certain device performance can be affected by the presence of COPs. IC manufacturers must add epitaxial layers to overcome this problem. Secondly, when performing tool contamination monitoring with test wafers, it is important to measure just the particles added during a process step, and the presence of COP defects in the total count can give false indications of the actual particle trend.

Surface Nanotopography (SNT) is the newest product feature on the Surfscan SP1TBI. It provides the ability to measure surface features with nanometer height variations across a lateral surface dimension in the 0.5 mm to 20 mm range. A primary application for this feature

Nitride

CVD Oxide

Nitride N-Well

Boron

P-Well

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Nitride

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Figure 1. A small deviation in the underlying silicon topography can lead to leakage or breakthrough, and ultimately device failure. Surface height variations can also lead to depth of focus lithography problems in advanced <0.18 µm processes.

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40nmTan 150nm Cu/40nm Tan 100nm ECD Cu ECD Cu CMP Cu W 400nm Wsi 55nm AI 200nm BPSG 300nm Nit 150nm 150nm Nit/35nm Ox 100nm Ox (B) Ox 100nm 100nm Ox (A) 100nm Poly/550nm Ox 200nm Poly/100nm Ox 240nm Poly/300nm Ox Resist C Polished Si 1300nm TEOS 1000nm TEOS 720nm TEOS Tin 70nm Tin/Ti 500/200A

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Figure 2. SP1 sensitivity compared to 6420 on various films.

is to control non-uniform film thickness variation in shallow trench isolation (STI) CMP. Even with all of the new defect classification capabilities on the SP1, sensitivity to particles on rough films still remains a primary requirement of unpatterned wafer inspection tools, and the performance of the Surfscan SP1 is commensurate with the requirements of 0.13 µm processes. The graph in Figure 2 shows that the SP1

A case study comparing defect capture on the Surfscan SP1TBI versus the Surfscan 6420 on electro-chemical deposition (ECD) copper was performed at a customer site. At equivalent thresholds of 0.25 µm, the SP1 captured more than twice as many defects (Figure 3). The defects that were missed by the Surfscan 6420 were reviewed on a CRS laser confocal review station. The size of the additional defects captured by the Surfscan SP1TBI were not all close to the detection threshold, but rather distributed across a wide size spectrum as exhibited in Figure 4.

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Figure 3. The Surfscan SP1

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has superior sensitivity compared to the Surfscan 6420 for all examples tested. (Note: The sensitivities shown in Figure 2 are obtained from random customer samples.

The performance can be better, depending on the surface quality of the film.)

The oblique angle incident illumination, coupled with S, P or Circular polarization and the high efficiency ellipsoid collector are key to optimizing sensitivity on rough films. This same collector is also effective at capturing surface haze data over a wide spatial frequency range. Haze measurements have proven useful in monitoring film uniformity and surface roughness.

captured more than twice as many defects as the Surfscan 6420.

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Figure 4. Two types of defects that the Surfscan 6420 missed but the Surfscan SP1 TBI captured easily: a 0.51 µm high spike and a 1.10 µm particle.

This means the SP1 captures more defects (and defect types) than the Surfscan 6420 at equivalent sensitivities.

Another defect type captured by the Surfscan SP1 using the brightfield channel are the “copper swirl” marks shown in Figure 5. As can be seen in the microview close-up, these swirl marks have significant topography contours.

When adopting new tool sets, it is always advantageous to be able to correlate existing baseline data sets with the new tool data. Correlation studies performed between the SP1 and the 6420 show good correlation with the existing 6420 installed base. For example, a recipe was created on the SP1TBI that exhibited a count A

correlation R2 value of 0.979 on a typical oxide layer compared to the Surfscan 6420 count. No changes were made to the existing Surfscan 6420 recipe.

In summary, the Surfscan SP1TBI has better sensitivity and higher throughput (up to 125 wafers per hour), is able to capture a wider variety of defect types and perform real time defect classification on many defect types.

It has proven matching between tools and demonstrated correlation to the previous generation Surfscan 6420 series. It has already achieved a large acceptance at all the top IC, equipment and wafer manufacturers worldwide and has become identified as the tool of choice for both 200 mm and 300 mm installations.

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Figure 5. Copper swirl marks are another defect type detected using the brighfield channel of the Surfscan SP1 TBI .

5A: Brightfield map of ECD Cu wafer with swirl marks. 5B: Swirl marks captured by SP1 TBI .

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