Autumn99 diaprocess

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Diagnosing Processing Problems through Electrical Charge Characterization

by Greg Horner, Senior Scientist; Brian Letherer, Senior Applications Engineer

Process engineers commonly require front-end electrical testers that are both sensitive to process deviations and powerful enough to diagnose any problems that arise in production. For day-to-day monitoring, these engineers usually prefer to monitor just a few electrical test parameters. However, when the process limits on these ‘front-line’ parameters are exceed ed, the electrical test equipment must be able to analyze the problem thoroughly in the shortest possible time. For test data to be meaningful, the results of the various parameters must complement each other in order to present a complete picture of the process deviation.

The KLA-Tencor Quantox system is a noncontact metrology tool capable of characterizing the electrical properties of both dielectric and silicon. It provides process engineers with conventional electrical test data without the need for metallization or other processing. It also offers the ability to improve processing capabilities by reducing the time needed to gather information for monitoring critical processes and process tools.

This system provides highly detailed information that separates the electrical charge contamination into individual categories. It provides an electrical measurement of oxide thicknesses as small as 2 nm, and produces “maps’ or “fingerprints” that provide a quick overview of wafer uniformity. It also monitors heavy metal contamination, both on the surface and in the bulk of the silicon.

Quantox combines three non-contacting techniques to perform the measurement functions. A corona discharge is used to bias the surface and emulate the function of the MOS (metal oxide semiconductor) electrical contact. A vibrating Kelvin probe monitors the wafer surface potential as a function of surface charge. A pulsed light source linked to the Kelvin probe enables the stimulus and detection of surface photovoltage, which, in turn, provides additional information on the silicon bandbending.

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These three techniques provide SPV-Q-V curves that are used to calculate and extract the system’s electrical test parameters.

Heavy metal contamination

Recently, the experience of one user illustrated the complementary nature of Quantox system’s parameters and led to the source of a processing problem.

Engineers at a logic manufacturer monitor their diffusion furnaces on a weekly basis for mobile ionic contamination with an Upper Control Limit (UCL) of 7E10 q/cm2. On one occasion (figure 1), their Qm data showed two wafers (T16 and T3) were contaminated at 1E11 q/cm2 and 7.3E10 q/cm 2 respectively. A third wafer (T8) was within specification at 5.8E10 q/cm2.

Note that the ‘first-line’ Qm measurement may have been affected due to alkali contamination (Na, K, Li) or increased oxide leakage incurred by metal contamination. To isolate the cause of the process variation in the first two wafers, they first tested all three wafers for

F i g u re 1. Mobi le i onic contamination /bulk iro n .

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F i g u re 2. Oxide re s i s t i v i t y.

F i g u re 4. Bul k recombinati on lifetime .

oxide resistivity and E tunnel. These tests are designed to measure the low-field (~0.5MV/cm) and high-field (~12MV/cm) leakage characteristics of the oxides.

Figures 2 and 3 indicate that both oxide resistivity and Etunnel data for T16 and T3 were low, suggesting metal contamination of the oxide, while wafer T8 was within the fab’s SPC limits. To clarify the type of contaminant in wafers T16 and T3 further, the engineers measured the bulk minority carrier lifetime of the underlying silicon. While heavy metal contamination strongly affects bulk lifetime, alkali contamination typically has no impact on bulk lifetime. Figure 4 clearly indicates the presence of heavy metal contamination in the bulk of the silicon; the values for wafers T16 and T3 were far lower than the typical BτR Lower Control Limit of approximately 500 µsec.

the high iron samples revealed a metal washer that was beginning to corrode. They fixed the problem, then ran another set of monitor wafers to verify that the tool and the process were back in specification.

The data obtained to that point implied the presence of heavy metals in the oxide as well as in the silicon. To narrow down the type of contaminant further, they ran the Quantox system’s iron detection algorithm. Figure 1 shows that wafers T16 and T3 were contaminated with iron (Fe), while T8 was not. Since Fe in the oxide is known to increase the oxide leakage, they concluded that the contamination must have occurred during oxidation. A close inspection of the furnace that generated

PETEOS optimization for better step coverage development

At a R&D facility in the US, the Quantox system was used in the development of a new process for interlevel dielectric plasma enhanced TEOS films. Earlier publications have indicated that PETEOS grown in a low oxygen and low pressure environment exhibits improved step coverage. To test the effects of this environment on the electrical characteristics of the oxide, the R&D team grew an oxide film with lower oxygen and tested it on the Quantox system for surface voltage (VS). The data obtained (table 1) indicated the low oxygen films had lower surface charge than the baseline process. They also found that by lowering the total pressure during deposition, the Si/SiO2 interface quality also improved, as shown by the flatband voltage (Vfb) measurements detailed in table 1.

Process Baseline Low O 2 Low pressure

DVS 6.2 1.0 7.0

VFB -39 -31 -28

Stress -135 - 66 -119

Tabl e 1. Electrical chara cteristics of low oxygen or low pre s s u re films.

F i g u re 3. Tunnel ing field.

Initially, these results led them to conclude that this new process provided lower surface charging, as well as the improved step coverage that was originally sought.

The data encouraged them to explore this new process further by combining low oxygen and low total pressure in the same run. The new film surface voltage results, shown in table 2, again indicated low charging.

However, they were unable to obtain any measurement Autumn 1999

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of the flatband voltage. Since this is usually the case with leaky oxides, they next used the Quantox system to measure the oxide resistivity and found it to be 2000 times lower than that of the baseline process.

Leaky oxides were causing the corona applied on the surface of the oxide to leak away too fast for the system to make measurements. Further experiments showed that the new film was grown in a tensile condition, which explained its high leakage property.

Process Baseline Low O 2 & low pressure

VS 6.2 0.5

VFB -39 NA

Rox(Ω-cm) 5E17 2E14

Stress -135 +113

Table 2. Electrical char acteris tics of low ox ygen and low pre s s u re fil m.

As a result of these tests, the R&D team was able to get quick feedback for their process parameter development.

They concluded that an environment that combined low oxygen and low pressure produces films with significantly poorer dielectric properties than their baseline process. Relying on VS maps alone would have led them to interpret the data improperly; the leakier films showed low surface voltage because the surface charge was dissipating quickly. By characterizing the oxide fully with Vfb and resistivity measurements, and with VS maps, they were able to develop a far more complete picture of the process.

Phosphorous contamination

exposed to back-end POCL or BPSG processes, then subsequently either cleaned in the wrong tools or misrouted for use as front-end monitors. Phosphorous out-gasses readily, so when these wafers found their way into front-end furnaces, the contamination spread rapidly.

These Quantox system users were alerted to the problem when routine process monitors triggered SPC alarms.

F i g u re 6. Surface voltage map.

In one case, a process engineer was monitoring surface voltage and surface generation lifetime, and was able to confirm and trace the phosphorus problem by using the Quantox system’s Qtot parameter. Figure 5 shows results from daily monitoring of surface generation lifetime; the lot in question experienced a severe drop in lifetime to less than 100 µsec, indicating contamination on the wafer’s surface. The source of the problem was traced back to a wafer boat contaminated with phosphorus.

Figure 6 depicts a surface voltage map; the crescent shape that appears on the wafer’s upper left edge is characteristic of boat contamination, which occurs where the wafer touches the boat.

Figure 7 represents the total oxide charge (Qtot) on two locations on the same wafer, indicating non-uniformity across the wafer and confirming the presence of local charging problems. Fab managers estimated their savings due to early detection at $2.5 million.

Recently, six fabs around the world have reported a severe problem that was ultimately identified as phosphorous contamination. The source of the problem was usually traced back to misrouted wafers. In each case, either monitor wafers or dummy wafers had been

In all of these instances, without the Quantox system, the fabs were unlikely to have detected the problem until end-of-line parametric tests were performed, leading to a loss of several thousand wafers per fab. ❈

F i g u re 5. Surface generat ion lif eti me.

F i g u re 7. Total oxi de charg e .

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cir cle RS#035