Advances in Wafer Analysis by VPD ICP-MS

Advances in Wafer Analysis by VPD ICP-MS

Advances in Wafer Analysis by VPD ICP-MS

Inductively Coupled Plasma Mass Spectrometry

Vapor Phase Decomposition (VPD)

Samantha Tan, “Application of Vapor Phase Decomposition Technique (VPD ASS and ICP-MS for Trace Element Analysis in Oxide Coatings on Silicon” in Nuclear Instruments and Methods in Physics Research, B99, p. 458-461, 1995

Introduction

Metal contamination affects IC device

operation in various ways. Alkali metals, such as Na, K and Li, can cause MOSFET

threshold voltage shifts, Al and Zn affect the oxidation rates of silicon, and Fe, Cr, and Cu can cause junction leakage currents and oxide integrity degradation. For example, an increase in Fe concentration from 5 x 10⁹ at/

cm² to 5x10¹¹ at/cm² reduces the breakdown field from 11.6 MV/cm to 6.8 MV/cm for a 10 nm thick oxide.

Vapor phase decomposition (VPD) inductively coupled plasma-mass spectrometry (ICP-MS) is one of the most utilized analytical

techniques to monitor trace metals on wafer surfaces. VPD ICP-MS is referenced in SEMI E45-95, SEMI E45-0301, and ASTM F 1617- 95. ChemTrace Corporation has refined the technique to analyze a variety of process films (e.g. low- and high-k dielectric films, BPSG, and polysilicon films) and to perform localized analysis (e.g. radial scans). The detection limit of VPD ICP-MS for bare silicon wafers is in the range of 10⁷ - 10¹⁰ at/cm² and meets the detection limit criteria for future IC technologies.

ChemTrace Corporation offers:

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VPD ICP-MS for

≤

300 mm bare wafers

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DFS ICP-MS of films on

≤

300 mm wafers

ƒ

BSE ICP-MS of poly-Si and Si

ƒ

Localized VPD ICP-MS

ƒ

Radial Scan VPD ICP-MS

Application Note 10

44050 Fremont Boulevard, Fremont CA 94538 USA Tel: 510.687.8000 Fax: 510.687.9054

Detection Limits of Analytical Techniques

* Cleanroom compatible





 













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Vapor Phase Decomposition ICP-MS

Vapor Phase Decomposition and Scanning Droplet Process

During VPD, the native or thermal oxide on < 300 mm silicon wafers are dissolved in the presence of HF vapor. The metals in the oxide layer are released and soluble fluorides formed are collected by the scanning process.

Fe₂O₃ + 6HF ---> 2FeF₃ +3H₂O SiO₂ + 6HF ---> H₂SiF₆ + H₂O

The scanning solutions are optimized to collect all metal contaminants on the wafer surface.

Advanced VPD Techniques

The accuracy of the measurement is related to the recovery of the trace metals, wafer preparation steps, and to the calibration of the ICP-MS instrument. ICP-MS instrument is calibrated with NIST traceable standards that are used to verify secondary standards. All samples, solutions and standards, are spiked with an internal standard to monitor and correct for matrix interference and instrument drift. The overall accuracy and precision of the analysis is + 15% for concentration

Accuracy and Precision

values 10x above the DL. Results close to the DL have large variations. Excellent trace metals recoveries are observed using ChemTrace wafer processing protocols. Table 1 shows the recovery of duplicate wafers that were spiked with multi-element standard solutions by two different analysts.

VPD Minienvironment


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Vapor Phase Decomposition (VPD) ICP-MS

In Depth Information

Direct Film Stripping (DFS) ICP-MS

Thin Film: Thermal/

TEOS/CVD oxide, SiOC, SiON, BPSG, Si3N4, Ta2O5, Cu,Ti, TiN, TiW

Wafer Surface Scan (WSS) ICP-MS

Bulk silicon Etch (BSE) ICP-MS

ChemTrace Corporation Wafer Analysis

To achieve ultra-low detection limits, the VPD or other wafer processing experiments must be performed in a highly controlled environment.

Routine monitoring of wafers exposed in our mini- environment shows negligible metal contamination.

0 0.1 0.2 0.3

Al Ca Cr Cu Fe Mg Ni K Na Zn Concentration (x1010 atoms/cm2 )

Slot 6 Slot 20 t = 0 min t = 15 min Table 1: Recoveries of Critical Metals

Figure 1: VPD ICP-MS results of control wafer and a wafer exposed in a minienvironment for 15 minutes.

Surface Only

A l 8 8 % 9 6 % 1 0 0% 1 0 0%

C a 9 0 % 8 9 % 9 5 % 9 5 %

C r 9 0 % 9 1 % 1 0 1% 9 9 %

C u 8 8 % 9 2 % 9 6 % 9 3 %

F e 9 1 % 9 5 % 1 0 8% 1 0 6%

M g 9 7 % 9 8 % 1 0 3% 1 0 1%

N i 9 9 % 9 8 % 1 0 3% 1 0 5%

K 1 0 0% 9 8 % 1 0 5% 1 0 3%

N a 9 8 % 1 0 2% 1 0 5% 1 0 5%

Z n 1 0 4% 1 0 4% 1 0 4% 1 0 4%

1 A 1 B 2 A 2 B R E C O V E R Y E X P E R IM E N T

A n a lys t I A n a lys t II

1.0E+14 1.0E+15 1.0E+16 1.0E+17 1.0E+18 1.0E+19 1.0E+20 1.0E+21

0 100 200 300 400 500 600

Depth (Angstroms)

Silicon Substrate Film

Concentration (atoms/cm3)

Al (Areal Density = 6.4E12 at/cm2) SIMS DEPTH PROFILE

IC Processing Applications

Process Films (CVD, PVD and Electroplating)

Direct Film Stripping (DFS) ICP-MS is well suited for monitoring trace metals in films, to ensure film specifica- tions are met.

„ Dielectric (silicon dioxide, BPSG, silicon nitride, low-k dielectric, high-k dielectric)

„ Metal (tungsten silicide, titanium nitride)

After determining the metal contaminants, SIMS may be used to

Polysilicon Film and Bulk Silicon

Bulk Silicon Etch (BSE) ICP-MS may be used to determine 30 or more metal impurities in a polysilicon film. Film thickness of 50 Å to 20 µm may be measured. BSE ICP-MS complements SIMS in providing information of the film’s purity and uniformity. BSE provides in-depth information of a 100-150 cm² surface area whereas SIMS only analyzes a small micro-area. BSE may also be performed in the silicon substrate to determine impurity diffusion at the interface (Figure 3).

ChemTrace Corporation Wafer Analysis

Figure 2: In-film trace metal survey using DFS ICP-MS

profile targeted metals for their in-film distribution.

The cause of contamination can usually be traced Element Film Aluminum 480 Calcium <10 Chromium <2 Copper 29 Iron 19 Magnesium <10 Nickel 36 Potassium <10 Sodium 12 Zinc 10

Units: x10¹⁰ atoms/cm²

DFS ICP-MS

Figure 4: SIMS depth profile of the same film analyzed by DFS ICP-MS shown in Figure 2

Figure 3: Two consecutive analyses of BSE in the Si substrate reveals a highly contaminated layer below the wafer surface

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to the tool components or to the starting materials of the film, such as the gas or liquid source materials and the chemical precursors.

Prime, Reclaim, and SOI Wafers

The selection of starting materials is important to IC fabrication and for equipment marathon experiments requiring hundreds of wafers. To qualify the wafers, a whole-wafer surface analysis method is required.

VPD ICP-MS is the only cost effective analysis technique capable of monitoring the whole-wafer surface. Using witness wafers, VPD ICP-MS maybe used to evaluate:

ƒ

Cleaning chemicals purity

ƒ

Efficacy of cleaning protocols

ƒ

Quality of cleanroom air

ƒ

Process tools for acceptance

ƒ

Tool components cleanliness

ƒ

Cleanliness of electrostatic chuck (ESC)

2

2

IC Processing Applications

44050 Fremont Boulevard, Fremont CA 94538 USA Tel: 510.687.8000 Fax: 510.687.9054

© 2002 ChemTrace Corporation • Printed in USA • 6/02

Element 200 Å Aluminum 12,300 Antimony 7.3 Boron 230 Chromium <1 Copper <1 Iron <3 Magnesium <5 Nickel <0.5 Phosphorus 287 Zinc 1

Units: x10¹⁰ atoms/cm²

BSE ICP-MS

Figure 7: BSE ICP-MS characterization of an arsenic implant

Chemical Mechanical Polishing

CMP is a critical process in the fabrication of cop- per-based semiconductors. Contamination control of the bevel area is important to avoid causing defects in subsequent processing steps. Radial scanning of the VPD droplet is a specialized protocol unique to ChemTrace Corporation. For this application, the wafer is scanned along the bevel edge of the wafer to inspect for residual wet chemical slurry.

Figure 5: Comparison of metal contamination on the wafer surface and at the bevel edge

Ion Implantation

Metal contamination on a wafer may arise from ion beam sputtering of exposed surfaces, such as wafer clamps and apertures. The surface metals may be monitored using VPD ICP-MS, a survey technique capable of monitoring 30 elements or more on the wafer surface.

Figure 6: A schematic representation of knock-on implantion of surface metals

Knock-on implantation of surface metals may occur during the implantation process, driving them below the wafer surface. The surface metals may also diffuse into the wafer bulk during subsequent thermal processing. These contaminants are often referred to as energetic contamination. BSE ICP-MS can provide a survey of 30 or more elements of the Si substrate at an analysis depth >10 Å. A typical bulk etch depth for this application is 200 Å, as shown in Figure 7. Another application of BSE ICP- MS is to determine the implant dose of ultra-shallow implants.

0 1000 2000 3000

Al Ca Cr Cu Fe Mg Ni K Na Zn

0 10 0 20 0 30 0 40 0 50 0

A l C a C r C u F e M g N i

Concentration (x1010 atoms/cm2)

w ith 2 m m ed ge e xclusio n w ithou t ed ge e xclusio n Surface Bevel


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