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Analysis of Phosphorous Auto-doping in P-Type Silicon using Corona Oxide Silicon Techniques

By Brian Letherer and Greg Horner, KLA-Tencor Corporation

Semiconductor fabrication facilities rely on the integrity of the silicon to manufacture submicron devices. Cross contamination of P-type silicon to N-type carriers or vice versa in the near surface region of the silicon can be detrimental to device performance. Semiconductor processing typically includes numerous diffusion and pre-clean steps, any one of which might auto-dope a P-type silicon substrate with phosphorous. In-line monitoring of these near-surface doping effects enhances the ability to diagnose auto-doping problems.

A non-contact Corona Oxide Silicon (COS) measurement technique has the ability to detect cross-contaminated P-type silicon with phosphorous from wet clean benches and diffusion furnaces. Results show COS flatband (Vfb) and oxide total charge (Qtot) measurements are sensitive to various levels of intentional phosphorous contamination implanted into the silicon at pre-oxidation.

Phosphorous at the silicon/oxide interface can pile up and create an electrically active thin “N” skin. Phosphorous from this thin “N” skin is shown to change the electrical characteristics of near surface region of the silicon. The detection of unwanted phosphorus with the use of COS in-line monitoring can greatly reduce the response time when auto-doping problems occur.

Contamination control has long been an integral part of semiconductor manufacturing.

Yield loss due to small amounts of contamination in silicon can cause catastrophic loss of product due to slight changes in electrical behavior of silicon based devices. The source of contamination can be widespread, as well; there are a variety of potential sources in a silicon manufacturing facility. Isolating and eliminating these sources of contamination 74

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can also be difficult and tedious. Phosphorous, in particular, is used in many areas in a fab as a dopant and can cause significant problems to the near-surface silicon region, the primary region where a device operates.

Detection of contamination problems can be time consuming and timely feedback of detection is also desired to reduce the amount of product at risk. An in-line metrology tool to monitor contamination is essential in a manufacturing environment. The COS, which is commercially available, has the ability to monitor contamination and give timely feedback to ensure the risk of contamination is minimal.

The primary sources of phosphorous are generally POCL3 doping of polysilicon and phosphorous implant. Bare silicon wafers, with high phosphorous content, (test or monitor) processed in wet sinks or high temperature diffusion can readily out-diffuse and will auto-dope wafers with P-type silicon in the same process step or in subsequent process steps. The source of the phosphorous contamination tends to be very localized, as only certain sections of a diffusion furnace or wet-cleaning processes will be contaminated.

Quartzware in diffusion furnaces and wafers that are run continually in the diffusion furnace for thermal mass may retain phosphorous. On subsequent runs phosphorous will out-diffuse at temperatures above 850°C and diffuse into exposed silicon substrates of

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Due to the nature of the low level of doping in the near-surface region after oxidation, there are very few current methods available that can detect unwanted phosphorous in the near-surface region of the silicon.

Many of the technologies employed require that a patterned device must be generated to electrically test for anomalies, such as CV. This requires extra processing and will not allow for timely feedback. COS technology is an inline, non-contact monitoring tool that will provides timely feedback.

COS measurement technique

A KLA-Tencor Quantox COS (Corona Oxide Semiconductor) system was used for all of the electrical characterization work presented here. As in the CV technology, COS analysis requires that an electrical bias be applied to the sample to measure the electrical properties of the near surface silicon and oxide layer. A small amount of charge is precisely deposited on the oxide surface by ionizing moisture and CO2 above the wafer surface5. A sweep is produced in a step-wise fashion by depositing increments of surface charge to bias the underlying silicon from inversion to accumulation.

A vibrating Kelvin probe is then used to measure the surface voltage (Vs) response of the deposited surface charge at each step of the sweep. A high-speed light source photogenerates carriers in the near surface region and the resultant flood of carriers flattens the band bending in the silicon. The resultant surface photo voltage (SPV) is similarly measured during each step of the surface charge sweep and provides a mea-

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Once phosphorous has entered the silicon it will accumulate at the near-surface region of the silicon during subsequent oxidation processes3. It has been shown that phosphorous will induce a positive oxide charge in native oxide, prior to an oxidation. In this mechanism P5+ replaces Si4+ in the oxide4. The data obtained in this study supports the hypothesis that very little phosphorous is incorporated in the oxide during a conventional thermal oxidation.

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lesser doping concentration1. Wet cleaning processes can also accumulate impurities, such as phosphorous, from wafers that came from a heavily doped high temperature process, like diffusion POCl3 step. Phosphorous from the contaminated clean step will deposit on the surface of the silicon and create an N-skin2. The phosphorous is then activated in the near surface region of the silicon during successive high temperature process steps.

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Deposited Charge x1E-8 (C/cm2) Figure 1. A diagram of a surface voltage vs. deposited charge cur ve and a surface photo voltage vs. deposited charge cur ve as measured

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surement of the silicon band-bending. From these two methods (Vs and SPV), charge versus surface voltage (QV) and charge versus SPV curves can be generated as displayed in Figure 1. From these curves measurements such as flatband voltage (Vfb), total charge in the oxide (Qtot) and density of interface traps (Dit) can be extracted.

Vfb is calculated when the SPV is 0 on the SPV verses surface voltage curve. Qtot is the amount of charge in the oxide at flatband. Dit is derived from the QV curve using the Burglund method.

COS also has the ability to measure near surface doping in the silicon. This is accomplished by biasing the silicon into strong inversion using onto a site. A guard ring is then placed around the site with opposite signed charge to place the silicon into accumulation. A known charge pulse is then applied to the central region into deep depletion. The deep depletion transient response is analyzed to extract both the doping level and the generation lifetime of the near-surface silicon.

Experimental

In this study, phosphorous was intentionally introduced into the silicon to mimic a contamination problem.

Bare P-type silicon eight inch wafers were used with a doping level of 1.5E15 #/cm3. The samples were implanted with various amounts of phosphorous ranging from a control wafer of no implant, to a wafer that received 3E12 #/cm2 dose of phosphorous implantation.

Oxidation was then done in a vertical furnace utilizing a standard CV oxidation process at 900°C, which did not include the use of chlorine, to produce 950 Å of Summer 2000

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oxide. Wafers were then etched back to varying oxide thicknesses, using an HF wet stripping process. Each wafer was stripped back three separate times to obtain oxide thickness levels of 850, 750 and 300 Å. COS measurements were performed after the etch step, at each thickness level.

The wafers were stripped back to determine the sensitivity levels of measurements to detect phosphorous on thin oxides. In general, higher measurement precision is required to monitor thin oxide layers for potential contamination in the near-surface substrate. We show here that the COS measurements retained excellent sensitivity as the oxide thickness decreased.

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Figure 3. Simplified band diagrams for a) phosphorous N-skin, and b) phosphorous N-skin pulled to the flatband condition by negative surface charge. To first order, the surface charge density at the flatband condition equals the phosphorous dose.

Results

CV versus COS measurements have been well-documented (5). Changes in flatband due to changes in the charge in the oxide and underlying substrate show better correlation with COS compared to conventional CV methods in previous studies.

ative charge density equal to Dphos must be applied to the oxide surface to pull the underlying silicon to the flatband condition (Figure 3b). The resultant voltage scales linearly with Tox and Dphos (the probe-to-silicon workfunction difference and 2nd order band bending effects are neglected in the following equations): Qsurface flatband = DPhos

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Note that while Vfb is an excellent indicator of N-skin contamination on relatively thick oxides (Tox>200 Å), the sensitivity drops steadily as the oxide thickness is decreased (see, for instance, the 300 Å oxide in Figure 2).

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Phosphorous (#/cm2) Figure 2. Phosphorous implant dose vs. Vfb as measured using COS techniques.

Figure 2 shows the dependence of Vfb on implant dose.

The 950 Å thermal oxides were repeatedly wet-etched to provide measurements at several oxide thicknesses.

The Vfb behavior is similar to the dependence of threshold voltage on implant dose, as outlined here.

Consider a thermal oxide with initial total oxide charge which is much lower than the contamination level that must be detected. We assume that the phosphorous contamination is incorporated in the near-surface region of the silicon, but not in the oxide (as in Figure 3a; this assumption was verified experimentally with SIMS and COS analysis). In CV or COS testing, a neg76

DPhos DPhos • Tox = Cox εox

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The Quantox is also capable of measuring the total oxide charge, Qtot, a parameter that contacting techniques such as conventional CV are not able to measure.

The Qtot parameter is distinctly different from the Vfb measurement, since it’s sensitivity does not fall as Tox is reduced. Again assuming that the oxide total charge is much less than the dose of contaminant, we find (neglecting 2nd order band bending effects): Qsurface flatband = –Qtot = DPhos The Qtot parameter is often used on thin gate oxides (down to 20 Å), where conventional Vfb measurements provide a poor signal-to-noise ratio. Figure 4 shows Qtot acquired on the same set of phosphorous contaminated samples. As before, the samples were repeatedly wet-etched to demonstrate the sensitivity of the technique as a function of Tox.

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Phosphorous Implant (#/cm2) Figure 4. Phosphorous implant dose vs. Qtot using COS measurement techniques.

Dit does not show any significant increases upon differing levels of phosphorous implantation, except in the case at high dose of phosphorous in the 3E12 #/cm2 range, shown in Figure 5. This shows that the silicon/silicon dioxide interface is not a factor in influencing Vfb and Qtot measurements. The oxidation at post implant was able to pacify any damage that may have incurred at the silicon/silicon dioxide interface during the implant. Upon higher doses at and above 3E12 #/cm2, however, implantation damages to the silicon/silicon dioxide interface creates a significant jump in Dit for all oxide thicknesses. Dit is effected by implant damage but not effected by phosphorous pile up at the silicon/silicon dioxide interface.

Secondary Ion Mass Spectroscopy (SIMS) was used on sister wafers with implant levels similar to the ones used in the study to show the doping profile. The depth of dopant for phosphorous after oxidation on these wafers ranged from 600 to 1500 Å into the silicon.

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depth was too shallow to be measured by conventional doping techniques, including COS and CV. The pulsed doping measurements, a technique used by COS, were not able to monitor the effects of the shallow phosphorous contamination in dose levels explored in this study.

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The changes in Vfb and Qtot due to phosphorous implants in the doses used in this study were significant.

These changes will severely alter the device electrical characteristics and parameters. This has been independently confirmed by extended flow experiments that were tested at metal 1 using a conventional parametric test.

Summary

COS measurement technology is an in-line non-contact method that provides timely feedback for monitoring possible phosphorous contamination from various sources.

Vfb maintains good sensitivity on samples with oxide thicknesses down to 300 Å, although the sensitivity is expected to scale with Tox. Qtot measurements retain high sensitivity even on oxides with thicknesses of less then 300 Å. This is in contrast to the Vfb measurements in Figure 2, where the measurement sensitivity decreased as Tox was reduced. The Qtot , therefore, is the preferred monitoring measurement for oxide thicknesses of less then 300 Å.

References 1. “Silicon Processing, Vol. 1” S Wolf and R. N. Tauber, Lattice Press, Sunset Beach CA (1986).

2. In line Charge-trapping Characterization of dielectrics for sub-0.5 um CMOS Technologies, P. K. Roy, C. Chacon, Y. Ma, G.S. Horner Mat. Res. Soc. Symp. Proc. Vol. 473 (1997).

3. “Oxynitridation-Enhanced Diffusion of Phosphorus in <100> Silicon” N. K. Chen and C. Lee, Elect. Chem.

Soc. Vol. 142, No. 6 (1995).

4. Phosphorous-induced positive charge in native oxide of silicon wafers, H. Shimizu, C. Munakta, Appl. Phys. Lett., 64 (26), pp. 3598-3599, 27 June 1994.

5. M.S. Fung and R.L. Verkuil, (Spring Electrochemical Meeting, abstract no. 169, (1988)).

6. Replacing C-V Monitoring with NON Contact COS Charge Analysis, K. Catmull, R. Cosway, B. Letherer and G. Horner, Mat. Res. Soc. Symp. Proc. Vol. 473 (1997).

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Phosphorous Implant (#/cm2) Figure 5. Phosphorous implant dose vs. Dit using COS measurement

Reprinted with permission from SPIE. Presented at SPIE ‘99 Microlithography.

Vol. 3884-14.

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