Vapor Phase Decomposition - Inductively Coupled Plasma Mass Spectrometry (VPD-ICP-MS) Analysis
Introduction
To help maximize wafer yields, it is critical that implanters, etchers and furnaces as well as cleaning baths are free of low-level contaminants. Vapor Phase Decomposition (VPD) is a method by which trace element contamination on the surface of a process silicon wafer is collected into a liquid sample and subsequently measured by inductively coupled plasma-mass spectrometry (ICP-MS). The need for lower detection limits, lower instrument background, higher sensitivity, and well-controlled analytical blanks is met by using high resolution ICP-MS (HR-ICP-MS).
Sample Preparation
Bare silicon wafers or wafers with thin films of SiO2, Si3N4 or SiON are prepared using a GeMeTec wafer surface perparation system (WSPS). This is an automated VPD preparation tool. The GeMeTec places the wafers in a sealed Teflon chamber and exposes them to HF vapor. The HF vapor forms a condensate on top of the wafer that decomposes the thin film layer on the wafer surface, and reacts with any trace metals that might be present. The condensate and trace metals are easily collected by rolling a droplet of 0.1ml acid solution across the entire wafer surface. The droplet is then transferred from the surface of the wafer into a clean vial and analyzed directly by HR-ICP-MS.
Molecular Ion Interferences
A typical VPD acid solution consists of a mixture of HF, HNO3, H2O2 and water. Samples can also contain as much as 0.1% silicon depending on the thickness of the thin film layer on the wafer being analyzed. This sample matrix gives rise to many molecular ion interferences that can adversely affect detection limits (Table 1).
Table 1
|
Element
|
Molecular Ion Interferences
|
|
31P
|
30SiH / 19F12C
|
|
42Ca
|
40ArH2
|
|
44Ca
|
28Si16O
|
|
46Ti
|
30Si16O / 28Si18O / 28Si16OH2
|
|
47Ti
|
36Ar12C / 28Si19F
|
|
48Ti
|
30Si18O / 28Si19FH / 14N16O2H2
|
|
51V
|
40Ar11B
|
|
52Cr
|
40Ar12C
|
|
53Cr
|
40Ar12CH
|
|
55Mn
|
40Ar14NH
|
|
Co
|
40Ar19F
|
|
58Ni
|
28Si2H2
|
|
60Ni
|
28Si16O2
|
|
63Cu
|
28Si16O19F
|
|
64Zn
|
36Ar28Si / 28Si19F16OH
|
|
66Zn
|
40Ar12C2H2 / 28Si12C2N / 38Ar28Si
|
|
69Ga
|
40Ar28SiH /28Si2H12C
|
|
71Ga
|
40Ar1H30Si
|
|
89Y
|
29Si30Si
|
|
98Mo
|
40Ar28Si30Si
|
|
39K
|
38ArH
|
|
56Fe
|
40Ar16O
|
|
57Fe
|
40Ar16OH / 28Si2H
|
For example, 30SiH, 19F12C, and 28SiH3 at mass 31 interfere with 31P (Figure 1), and 30Si18O at mass 48 interferes with 48Ti (Figure 2). Other molecular ion interferences arise from Ar, which is the support gas for ICP/MS instruments. For example, 40Ar16O at mass 56 interferes with 56Fe (Figure 3), and 40Ar12C at mass 52 interferes with 52Cr (Figure 4). Analysis of samples with such interferences results in elevated background signals and high detection limits. These interferences can be eliminated by performing the analysis of VPD samples using a high resolution ICP-MS.
Figure 1
Finnigan Element 2 HR-ICP-MS
Signal for 100 ppt 31P - 366 ppm Si in Matrix
Figure 2
Finnigan Element 2 HR-ICP-MS
Signal for 100 ppt 48Ti - 366 ppm Si in Matrix
Figure 3
56 Fe Peak, Finnigan Element 2 HR-ICP-MS
Figure 4
Finnigan Element 2 HR-ICP-MS
Signal for 100 ppt 52Cr - 366 ppm Si in Matrix
Cerium Laboratories performs VPD sample analysis using a Finnigan Element 2 high resolution ICP-MS. At a resolution setting of 4000, all of the molecular ion interferences listed in Table 1 can be resolved from the analytes of interest. Non-interfered elements can be analyzed at the resolution setting of 300 for greater sensitivity and improved detection limits.
Method Capabilities
Recovery data and average detection limits for the VPD method are presented in Table 2. Elements have recovery results between 75 and 125%. Many of the elements present on the wafer surface are converted into fluoride compounds by exposure to HF vapor. Compounds such as AsF5, AsF3 and BF3 are highly volatile at room temperature. Other compounds such as YF3, TeF3, CeF3, and BiF3 are either insoluble or only slightly soluble in VPD acid solution. Gold and silver are insoluble in VPD acid solution, and compounds like AuF3 and AgF3 are not readily formed. This explains lower recovery rates for the elements As, B, Bi, Ce, Au, Ag, Te, and Y.
The average detection limits for VPD-ICP-MS analysis of bare silicon wafers using the Finnigan Element 2 High Resolution ICP-MS are on the order of 108 atoms cm-2 or less for 39 of 42 elements tested (Table 2).
Table 2
|
|
Average
|
Example
|
|
|
% Recovery
|
Detection Limits
|
|
Element
|
Rate
|
(E10atoms/cm2)
|
|
Li
|
101
|
0.0221
|
|
Be
|
100
|
0.0547
|
|
B
|
64
|
1.0846
|
|
Na
|
101
|
0.0035
|
|
Mg
|
102
|
0.0036
|
|
Al
|
101
|
0.0155
|
|
P
|
104
|
2.7160
|
|
K
|
98
|
0.0080
|
|
Ca
|
100
|
0.0362
|
|
Ti
|
100
|
0.0031
|
|
V
|
100
|
0.0013
|
|
Cr
|
99
|
0.0017
|
|
Mn
|
101
|
0.0013
|
|
Fe
|
103
|
0.0089
|
|
Co
|
102
|
0.0006
|
|
Ni
|
102
|
0.0035
|
|
Cu
|
94
|
0.0011
|
|
Zn
|
103
|
0.0089
|
|
Ga
|
101
|
0.0012
|
|
Ge
|
98
|
0.0266
|
|
Se
|
100
|
0.0061
|
|
As
|
25
|
0.0156
|
|
Sr
|
99
|
0.0003
|
|
Y
|
77
|
0.0009
|
|
Zr
|
100
|
0.0051
|
|
Nb
|
98
|
0.0001
|
|
Mo
|
96
|
0.0003
|
|
Ag
|
7
|
0.0031
|
|
Cd
|
101
|
0.0015
|
|
In
|
100
|
0.0006
|
|
Sn
|
102
|
0.0009
|
|
Sb
|
100
|
0.0004
|
|
Te
|
40
|
0.0917
|
|
Ba
|
97
|
0.0001
|
|
Ce
|
77
|
0.0003
|
|
Hf
|
102
|
0.0001
|
|
Ta
|
94
|
0.0002
|
|
W
|
99
|
0.0001
|
|
Au
|
55
|
0.0384
|
|
Tl
|
99
|
0.0006
|
|
Pb
|
99
|
0.0006
|
|
Bi
|
76
|
0.0007
|
|