Magazine summer00 cmp defect

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CMP Defect Detection and Process TBI Control using the Surfscan SP1

By Katia Devriendt, Paul Mertens, Wim Fyen, Karine Kenis, Marc Schaekers, IMEC, Dale Guidoux, Grant Sergeant, Stephane Robic, Rene Moirin, KLA-Tencor Corporation

The Chemical Mechanical Planarization (CMP) process is now widely used to provide global planarity of layers during the fabrication of integrated circuits. Successful yield management of CMP requires detection of all critical defects in the presence of noise sources such as film thickness non-uniformity within a wafer or process variation within a lot. CMP defects can be separated into two categories; residual slurry particles or other foreign material on the surface, and microscratches or pits in the surface. Both defect types are known to have a negative impact on device yield. In a joint study between IMEC and KLA-Tencor, an experiment was performed to show how the Surfscan SP1TBI unpatterned wafer inspection system can be used to monitor both types of critical defects. Electrical test patterns were generated on CMP wafers to study the correlation of device yield to defect types.

In our experiments, High Density Plasma (HDP) oxide layers were polished using IMEC’s standard oxide CMP process. After cleaning on a scrubber using ammonia on the brushes, the polished wafers were inspected on a Surfscan SP1TBI. As seen in the Surfscan SP1TBI optics layout (Figure 1), the tool has both a normal and oblique incident beam and two collection channels, wide and narrow. The wide and narrow channels were both calibrated to give similar defect counts using Polystyrene Latex (PSL) spheres. On a standard CMP polish and clean, the wafers also exhibited similar Light Point Defect (LPD) counts in both the wide and narrow collection channels.

We suspect that the LPDs detected in both channels are primarily surface particles, as we would expect particles to scatter into both collection channels, whereas microscratches or surface void defects should scatter preferentially into only one of the detectors. To confirm this hypothesis, we added or “spiked” the standard CMP slurry with 1.5 µm diameter alumina particles.

Another set of HDP oxide wafers were then polished with the contaminated slurry, 72

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Yield Management Solutions

cleaned and scanned on the Surfscan SP1TBI. The wafer scans for each channel are shown in Figure 2. Using an oblique incident “C” polarized beam, the wide channel exhibited a much higher LPD count than the narrow channel. Under review using a CRS confocal laser review microscope, we confirmed that the higher counts Normal Incidence Beam Dark Field Wide PMT

Brightfield Channels SNT Tangential Nomaski DIC

Dark Field Narrow PMT

Ellipsoidal Collector

Lens Collector

Wafer

Figure 1. SP1 TBI optics layout.

Oblique Incidence Beam

SNT Radial Deflection

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SP1TBI recipe: - Oblique C-UU Full - LSE-diam: min 0.15 µm max 1.0 µm Narrow-channel

Figure 2. Using an oblique incident beam and “C” polarization, the wide-channel exhibited a much higher LPD count than the narrow channel.

detected only in the wide channel were primarily microscratches (Figure 3). To further verify our hypothesis, we deposited approximately 5000 PSL spheres to the wafer containing microscratches. The LPD count increased about 5000 counts in both channels. To be sure that the effect witnessed is not just limited to the properties of PSL spheres, we also measured standard CMP polished wafers that were dipped into a solution of slurry to add typical residual slurry particles. Similar amounts of LPDs were counted in both channels in this case as well. In summary, the microscratch counts were detected predominately in the wide collection channel, whereas surface particles were counted in both the wide collection channel and the narrow collection channel.

In the next phase of the experiment, “snake” and “fork” patterns were deposited on a polished oxide surface to test for intra-level defects represented by (a) shorts between the snake and fork lines and (b) discontinuity of the snake lines (Figures 4 and 5). Inter-level defects characterized as electrical breakdown of the polished

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oxide between metal capacitor plates were also tested. Two sets of wafers were polished using a standard slurry and two sets were polished using a slurry contaminated or “spiked” with large silica particles greater than 1 µm to induce a larger percentage of microscratches. One set of the standard slurry polished wafers received a scrubber clean with dilute ammonia and one set was cleaned with ammonia plus an HF wet etch.

No significant difference in the density of intra-level electrical shorts or discontinuities in the standard slurry was detected as compared to the density of defects in the contaminated slurry for either cleaning condition.

CMP condition: - HDP oxide - slurry: fumed SiO2 (ILD-1300) with 1.5 µm alumina particles - cleaning: scrubber only

Wide-channel

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particle microscratch

microscratch particle Al

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oxide Si

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b Figure 4. Inter-metal defects on

Figure 5. Electrical breakdown

metal meander-fork structures

characteristics of polished oxide

deposited on a polished oxide

(inter-level defects between metal

surface (0.4 µm line/0.4 µm

capacitor plates).

spacing). (a) shorts between meander-forks (b) discontinuity of meander lines.

The effect of cleaning chemistry on interlevel defects was more significant. For the standard slurry polish and the contaminated slurry polish with just a dilute ammonia scrub, the results are acceptable.

But, when applying an HF wet etch to a contaminated slurry polish (more scratches present), the results are catastrophic. The HF etches the oxide surface scratch defects and causes early electrical breakdown between the capacitor plates.

Figure 3. Microscope review confirms the spiked slurr y (right) exhibits a higher number

The study has shown that the Surfscan SP1TBI is an effective tool for monitoring CMP slurry residue and microscratch defects that can lead to device yield problems.

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of microscratches.

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