Limitations of Analysis of Metal Impurities Analysis in High-k Film

Ya-Ling Po*, Carol Lin, Shian-Shio Chen, Tings Wang

No.19 Li Hsin Rd., Science-Based Industrial Park, Hsinchu Taiwan, R.O.C. T. 886-3-5663934 / F. 886-3-5663300

emily_po@promos.com.tw

Abstract-This work studies the efficiency of various approaches, using such instruments as Total reflection X-ray Fluorescence (TXRF), Atomic Absorption Spectrometry (AAS) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The target metals are sodium (Na), Aluminum(Al), Calcium(Ca), Chromium(Cr), Ferrum(Fe), Nickel(Ni), Copper(Cu), Zinc(Zn) and Hafnium(Hf). Analytical approaches are employed to elucidate the surface impurities after the wafer has been processed.

I. INTRODUCTION The thickness of conventional silicon gate oxide has been reduced to the point that the sizes of MOS transistors have been shrunk very close to their theoretical limit. A new gate dielectric with a higher dielectric constant (high-κ), allowing larger physical thickness, has emerged as a potential replacement for silicon oxide. Hafnium oxide is a well-known candidate material for high-k gate insulators. Hence, this experiment involves multi-metal analysis of the surface of hafnium oxide to guarantee gate quality, and performs hafnium microanalysis to prevent residual surface cross-contamination by: hafnium species in the fabrication areas.

II. EXPERIMENTS AND RESULTS Total reflection X-ray Fluorescence (TXRF), Atomic Absorption Spectrometry (AAS) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) are the traditional means of detecting trace metals on the surface of silicon wafers in ultra-low quantities. They have been adopted successfully to identify impurities at the level of 1010 atoms/cm2. Residual metal on a silicon dioxide surface and metal- impurities on the hafnium oxide surface can be detected under suitable analytical conditions following appropriate sample treatment. TXRF is superior to ICP-MS or AAS, because TXRF is a method of non-destructive analysis. ICP-MS and AAS are destructive analytical methods. Processed wafers can be examined directly by TXRF without any pre-treatment. When ICP-MS or AAS is used to check metal impurities on the wafer surface, the wafers must be treated with acid; the acid solution is collected, and then the ICP-MS or AAS analysis is performed. The ICP-MS or AAS procedures are complex, but components of the film cause TXRF spectral interference. A. TXRF Unlike the analysis of a silicon dioxide film or native oxide on a silicon surface, metallic contamination on the

surface is difficult to analyze by a conventional W-Lβ1 source, because large Hf-L emissions give rise to high background around 6 - 8 keV (Fig. 1) and interfere in the detection of K lines of cobalt, nickel and copper from surface contamination. Therefore, the analysis of a hafnium oxide using TXRF overestimates amounts of cobalt, nickel and copper. As expected, and increase in the signal of cobalt, nickel and copper is observed with increasing Hf concentration (Fig 2). Figure 3 indicate that the intensity of metal vs. hafnium create a good linear relationship. The Ir source is suggested to replace W source for Hf film.4 Since the energy of Ir-Lα1, 9.175 keV, is slightly lower than conventional W-Lβ1 (9.769 keV) radiation, it does not cause Hf-L absorption and the corresponding emissions.

Fig. 1. TXRF spectrum of Hf. Fig. 2. Various Hf concentrations cause interference in TXRF analysis.

Co & Hf

Cu & Hf

0.1

1.0

10.0

100.0

1000.0

10000.0

Co Ni Cu Hf

cps

1ppm Hf

2ppm Hf

0.5ppm Hf

2681-4244-0653-6/07/$20.00 ©2007 IEEE 2007 IEEE/SEMI Advanced Semiconductor Manufacturing Conference

Fig. 3. Hafnium concentration is correlated with Co, Ni and Cu concentrations in TXRF analysis. B. ICP-MS The sensitivity of ICP-MS analysis and AAS analysis exceeds that of direct TXRF, because the pre-treatments are similar to extraction and concentrate. The SRM (Standard Reference Material) is applied to confirm the results of detection by ICP-MS (Table 1). The recovery rate indicates the efficiency of the technique. For the spike recovery test, the 2%HNO3 matrix solution divided into two same volume samples (C0), and one of samples spike metal standards into one of the 2%HNO3 matrix (C1). Intentional spiking of 10 pg/ml, 20 pg/ml and 50 pg/ml hafnium standards (C1) is used to perform the recovery trial. ICP-MS analyzed all of them. The recovery rate is calculated as below: Recovery rate(%) = (C1-C0) · 100/C1 (1) Table 2 summarizes the recovery rates obtained using various amounts of hafnium. The ideal recovery is 100%. However, a higher spiking concentration corresponds to greater recovery of hafnium in the 2%HNO3 matrix. We suspect that multi-dilution of hafnium causes the lower recovery. Figure 4 presents the various Hf contents used in the trials to evaluate the matrix interference. The analysis is problematic due to the very complex matrix by ICP-MS. The matrix consists of Hf, HF, H2O2, H2O and Si-base species in this trial. Clearly, the 2ng/ml concentrations of numerous metals are underestimated, because the 50 ug/ml (ppm) and 100 ug/ml (ppm) hafnium could be formed oxide refractory to inhibit the signal of ICP-MS. The calibration with a working curve would require standards that closely match the composition of the sample in standard addition method. Hence, interference associated with the high-content hafnium matrix was eliminated by the method of standard addition (Fig. 5). C. AAS AAS technique is based on the absorption spectrum that is atomized at liquid sample due to electrothermal atomizer, and determines metallic concentration on the liquid material. The general construction of an AAS, which contains radiation source, atomizer, monochromator,

Table 1. Hafnium background (BEC) and detection limit (D.L.) in different matrices, determined by ICP-MS analysis

Matrix DIW 2%HNO3 S Solution

BEC(pg/ml) 0.66 0.44 1.57

D.L.(pg/ml) 1.13 0.47 1.03

Table 2 Comparison of recovery rates of hafnium by ICP-MS analysis

Element Hf Hf Hf Hf

Mass 177 178 179 180

10 pg/ml Hf Recovery rate

88.80% 90.70% 83.70% 96.50%

20 pg/ml Hf Recovery rate

99.00% 100.50% 103.00% 98.10%

50 pg/ml Hf Recovery rate

100.50% 100.30% 100.20% 100.40%

Fig. 4. Recovery of 2ng/ml metals in various Hf matrices by ICP-MS using external calibration. Fig. 5. Comparison between recovery achieved by the method of external calibration and that achieved by standard addition in various Hf matrices

50

75

100

125

150

Na Al Ti Cr Fe Co Ni Cu Zn W

Rec

over

y,%

External Caribration+100 ppm Hf

External Caribration+200 ppm Hf

Standard Addition+100 ppm Hf

Standard Addition+200 ppm Hf

UCL=125%

LCL=75%

y = 0.0094x + 0.6714Co R2 = 0.9941

y = 0.0057x - 2.6211Cu R2 = 0.9457

y = 0.0017x - 0.1616Ni R2 = 0.9808

-5

0

5

10

15

20

25

30

35

40

0 500 1000 1500 2000 2500 3000 3500 4000

Hf Intens ity, cps

Me

tal I

nte

nsi

ty, c

ps

Co

Ni

Cu

45

55

65

75

85

95

105

115

125

135

Na Al Ca Cr Fe Ni Cu Zn Hf

Rec

ove

ry,%

0.1ppm Hf

0.5 ppm Hf

1ppm Hf50 ppm Hf

100 ppm Hf

UCL=125%

LCL=75%

269

Fig.6 Recovery of 1ng/ml metals in100ng/ml Hf matrix by AAS analysis detector and electrical measuring system with readout device, is simple. The success or failure of a determination is dependent on the atomization step. The atomization step, i.e. the transfer of the sample into free atoms in the gaseous state, is the most important process in an analysis by AAS. That Hafnium oxide refractory was formed and remained in atomizer induced unstable thermal progress at atomization, which resulted in the fluctuant recovery of 1 ng/ml metals (Fig. 6). Consequently, GFAAS is incompatible with any hafnium analysis.

III. CONCLUSIONS The matrices of the solvent-like components of the film both affect the accuracy. As new high-k materials become more diverse, the use of analytical instruments by the semiconductor involves new challenges. Selecting the most appropriate tool for monitoring contamination is critical. This work clearly reveals the potential risk of interference in quantitative analysis. However few hafnium-content is, spectral interference is occurred between cobalt, nickel, copper and hafnium by TXRF analysis; hafnium refractory induces poor recovery by GFAAS analysis. And, high hafnium matrix effects on ICP-MS analysis. Nevertheless, matrix interference could be reduced by standard addition method on ICP-MS. Additionally, software will be modified and hardware improved in the future to detect contamination accurately and help in selecting the most effective tool for treating samples.

REFERENCES [1] Y.-S. Lin, R. Puthenkovilakam, and J. P. Changa, “Dielectric property

and thermal stability of HfO2 on silicon”, Applied Physics Letters, vol.81, No.11, pp. 2041-2043, 2002

[2] V. Lowalekar, V. Pandit, S. Raghavan, H. G. Parks and J. Jeon, “Contamination of SiO2 by Aqueous Zirconium and Hafnium Species”,2003 The Electrochemical Society

[3] Hye-Young Chung, Sang-Hak Lee, Young-Hun Kim, Ki-Sang Lee, Dae-Hong Kim, “Determination of Metallic Impurity in a Silicon Wafer by Local Etching and Electrothermal Atomic Absorption Spectrometry”, Analytical Sciences, vol.19, pp. 1051-1054,2003

[4] C. Sparks, J. Barnett, D. K. Michelson, C. Gondran, S. Song, A. Martinez, H. Takahara, H. Murakami, T. Kinashi, Abstract of UCPSS2006 (Antwerp, Belgium).

50

75

100

125

150

Na Al Ca Cr Fe Ni Cu Zn

Rec

ove

ry,% Recovery,%

LCL,75%UCL,125%

270

271