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Secondary ion mass spectrometry (SIMS) of silicon
Vacuum/volume
39/numbers
11 /12/pages
1077
to 1087/l
989
0042-207X/89$3.00+.00 (’ 1989 Pergamon Press plc
Printed in Great Britain
Secondary silicon* M Grasserbauer Getreidemarkt
ion mass spectrometry
and G Stingeder.
Institute
for Analytical
Chemistry,
(SIMS) Technical
University
of Vienna,
9, A- 7060 Wien, Austria
This paper describes in a selective manner the potential of high performance SIMS for the development of VLSI silicon devices. Methodological developments are presented for: quantitative analysis of oxygen in silicon in pg/g range, trace element analysis in monolayers on wafers, high accuracy depth profiling analysis of dopant elements for process modelling, characterization of high dopant concentrations, and ultra trace analysis of metallization materials.
1. Introduction Microelectronic devices are the most important high technology products. Most of these are based on silicon technology. VLSI structures have gained a particular significance in this respect. The most advanced VLSI product is the 4 Mbit DRAM (direct random access memory) chip. More than 8,000,OOO transistors are arranged within an area of 1 cm’. The spatial dimensions of the elements in these devices are extremely small-the active areas (source/drain) are about 0.8 pm in diameter, the thickness of the gate oxide is only 15 nm (Ref. 1). VLSI devices are produced in planar technology by a sequence of basic operations which are oxidation, lithography, etching, ion implantation, diffusion, deposition (CVD, sputtering, PVD)2-3. If 4 Mbit DRAMS are made on a 15 cm silicon wafer with a yield of 40% as many as 400 million transistors are produced in one run. Each memory cell has a complex 3-dimensional structure as shown in Figure 1. In total about 400 individual process steps are necessary to produce the 4 Mbit DRAM. New materials and new processes are used to achieve these extremely small structures. The major goals in development of new production techniques are miniaturization and increase of production yield. The yield of device production is determined by physical or chemical defects on a chip. This means that the study of the individual chemical and physical processes occurring in the semiconductor during device production is one of the major tasks in research. The more accurately these processes-like ion implantation, diffusion, oxidation, thin film deposition and removal-an be controlled the higher will be the production
Figure 1. Scanning
electron micrograph of cross section of trench capacitor cell of 4 Mbit DRAM (Courtesy : H Oppolzer, Siemens Munich).
yield. Due to the dramatic miniaturization the chemical and physical processes have to be controlled, calculated and measured within extremely small spatial dimensions-in some cases down to a few tenths of a nanometer (like for the interface Si02/Si in a gate oxide of a MOS Transistor). Chemical and physical processes taking place at small dimensions-e.g. at interfaces, surfaces, crystal defects-are in the center of interest“. In order to develop and produce such highly integrated tures with an acceptable yield an extensive analytical acterization of materials and devices is necessary.
strucchar-
2. Survey of analytical techniques
: AAS, Atomic Absorption Spectrometry ; AES, Auger Electron Spectrometry ; CPAA, Charged Particle Activation Analysis ; DLTS, Deep Level Transient Spectroscopy ; * List
of abbreviations
and acronyms
EPMA, Electron Probe Micro Analysis; GDMS, Glow Discharge Mass Spectrometry; HREELS, High Resolution Electron Energy Loss Spectrometry ; HVTEM, High Voltage Transmission Electron Microscopy ; IR, Infra Red (Spectrometry) ; IRM, Infra Red Microscopy; NAA, Neutron Activation Analysis; OM, Optical Microscopy; RBS, Rutherford Backscattering Spectrometry ; S-AES, Scanning Auger Electron Spectrometry; SCAM, Scanning Acoustic Microscopy; SCOM, Scanr&g Optical Microscopy ; SEM,-Scanning Electron Microscopy ; SIMS, Secondary Ion Mass Spectrometry, TEM, Transmission Electron Microscopy; XPS, X-Ray Photoelectron Spectrometry; XRD, X-Ray Diffraction; XRF, X-Ray Fluorescence (Analysis).
The development and production of microelectronic VLSI devices demands the use of an extensive set of sophisticated analytical techniques. For the determination of chemical features methods for bulk, micro and surface distribution analyiis have to be combined. In addition structural and functional features are of great significance5-‘. Table I contains the major steps of development and production of silicon devices, the analytical information sought and the major analytical techniques applied. Among the techniques listed Secondary Ion Mass Spectrometry [SIMS] provides a unique analytical potential due to the fact that most elements can be analyzed with high sensitivity 1077
M Grasserbauer
and G Stingeder:
Table 1. The IC fabrication Fabrication
process
step, product
Secondary
in relation
ion mass spectrometry
to analytical
(SIMS)
requirements
(according
Information
to Werner’,
Grasserbauer
ef ~l[.~).
Techniques -_______
u;Lfcr production
bulk impurity content physical defects chemical surface structure
NAA. CPAA, IR. DLTS(GDMS) X-ray topography. etching, (HV)TEM, XPS. HREELS, SIMS
R f D wafer processing
thin film composition
EPMA,
1 deposition oxidation implantation etching lithography metallization process integration I processed wafer
dicing. bonding packaging chip
chemical
structure
(chemical) structure topography electrical functions chemical and physical
defects
functions
and isotopic specificity, that elemental and molecular (bonding) information can be gained and that micro, surface and bulk analysis can be peformed8. The major limitations of SIMS are determined by the complexity of the mass spectra, the large variation of the secondary ion yields for different elements, and also for a particular element in different matrices (“chemical matrix effect”), and by the disorder induced into the analytical zone due to the high energy (E,, = l-20 keV) of the primary ions. In order to overcome these limitations in practice the following approach for quantitative trace analysis is most useful :
0 Use of reactive ion sputtering
(oxygen or cesium) to increase the secondary ion yield and to achieve a chemical modification of the surface zone which reduces the chemical matrix effects, l Use of high performance instrumentation which allows the identification (and separation) of interfering species by high mass resolution. lQuantitation with relative sensitivity factors (RSFs) obtained from external or internal calibration. l Systematic study of all sources of analytical errors, particularly of the measurement process. l Development of problem oriented measurement techniques and analytical strategies, combining SIMS with other analytical techniques yielding confirmatory and supplementary information. In the following paragraphs major areas of applications of (high performance) SIMS in the development and production of VLSI silicon devices, focusing on questions associated with the 4 Mbit DRAM will be presented to demonstrate potential and limitations of the method for semiconductor analysis as well as its integration into the arsenal of modern analytical techniques. The figures of merit given refer to a high performance sector field instrument (Camcca IMS 3f). 3. SIMS for the development of VLSI devices 3.1. Wafer production. Wafers as the starting material for device production have to meet stringent requircmcnts in respect to 1078
RBS. XRD, XRF. ellipsometry
depth distribution micro distribution
structure
topography crystallographic
electrical defects
and thickness
analysis: analysis
SIMS. XPS, AES. RBS
: TEM
OM, SCOM, SEM. TEM TEM, RBS, XRD
S-AES, SIMS. TEM OM, SCOM, SEM, TEM non destructive testing OM. SCOM, SEM, SCAM,
IRM. TEM. S-AES, SIMS
non destructive testing (semi-)destructive analysis
purity, base dopant concentration. structural homogeneity. chemical surface structure and topography’. For the highly intcgrated devices usually Czochralski silicon is used which has a rather low specific resistivity (50-l 00 R cm) and is characterized by an oxygen content in the order of 10’7-10’8 cm-‘. This oxygen content is found beneficial because of internal gettering of metallic impurities lo. To utilize this effect outdiffusion of oxygen at I IOO’C to produce an “oxygen free” zone (thickness > IO Alrn) into which the dopant elements are incorporated is combined with growth of SiO,-precipitates (at 100&l IOOC) which act as gettering sites due to the stress field around them. An alternative technique consists in the deposition of “oxygen free” silicon layers by epitaxial growth. For the development of optimized processes a highly accurate determination of oxygen (bulk and distribution analysis) in the concentration range 10’6-10’x cm-‘is necessary. For bulk analysis a combination of techniques is used with IR (for interstitial oxygen only), Inert Gas Fusion Analysis (IGFA) and SIMS exhibiting the best figures of merit for quantitative analysis”. New measurement techniques for SIMS using cesium primary and high energy secondary ions (initial energy = 75- 165 eV) allow to achieve an excellent reproducibility of 24% and an inaccuracy smaller than 10%. This is the basis for highly accurate depth (and lateral) distribution analysis for oxygen with SIMS. Figure 2 shows as an example the oxygen (and boron) depth distribution in the surface layer of a wafer covered with an originally 17 Llrn epitaxial layer (IO pm etched off before analysis) after annealing at 1OOO’C and performing the ASTM2-prccipitation test. As evident from Figure 2 a low oxygen concentration near the surface and the formation of SiO?-prccipitates could be achieved with this process. The increase of the boron concentration is due to a non-reproducible matrix effect caused by the SiO?-precipitates. Compensation could be achieved by analysis with oxygen primary ions and oxygen flooding. Another important area is the determination of impurities on the surface of silicon wafers. High performance Secondary Ion Mass Spectrometry has a great potential for the purity control of the silicon wafer surface due to its inherent high detection power and its high depth resolution. Optimized “quasistatic”
M Grasserbauer
and G Stingeder:
Secondary
ion mass spectrometry
10'9]
(SIMS) 10 ‘1%
I
HYDROPHOBIC
SURFACE
'60 (offset)
lo o~~%::G
I-
10’5 O
10
20
30
60
10
50
Depth (pm)
Figure 2. Depth distribution of oxygen and boron on epi-%/%-structure for 5 min and ASTMZ-test (75O”C, 4 hours and 105O”C, 16 hours) measured with SIMS using Cs+ primary ions. Offset refers to measurement of oxygen ions in the energy range 7% 165 eV (from Stingeder et al. ’ ‘).
’ 1,
10 6
3.2. R+D wafer processing. In the production of integrated circuits the following major processes are used : deposition of thin films, oxidation, ion implantation (combined with annealing to heal crystal defects and to activate dopant elements), etching, lithography, metallization 2*3*5 . In the research phase these processes have to be studied to gain understanding about basic mechanisms and quantitative relationships between the parameters of a process and the product. In the development phase these processes are optimized for specific IC structures’. Implantation and annealing. Type, concentration and distribution of the dopant elements (8 for p-doping, As, P and Sb for n-doping) determine the electrical properties of a device2v3. Dopant elements are introduced into the active regions of the device usually by ion implantation and diffusion. The thickness
1000
HYDROPHILIC
TIMEIs]
1500
SURFACE
3Os1+ I, 101
I-7,
\
afterannealing at 1000°C
measurement techniques had to be developed for the different elements to achieve the optimum between (high) detection power and (low) sample consumption per data point’*. An elaborate analytical procedure based on the combination of high and low mass resolution depth profiles with the evaluation of isotope ratios and the shape of the profiles allows an extensive characterization of the trace elements which are concentrated within the native oxide layer (thickness l-2 nm). It was found that the impurity content in the oxide depends on the surface treatment of the wafer (Figure 3). The detection limit for trace analysis with a sample consumption of less than one tenth of one monolayer per data point (ca. 10” atoms) is about 1 pg/g for aluminium (which serves as a tracer element for contamination) measured with high mass resolution. This corresponds to an absolute detection power of 5 x lo- I9 g, or cu. 10“ aluminium atoms.
500
?
ul
10 =
I’---. i.. ‘.\‘.
\
:::j
1:::::::::
+ 0
500
1000
1500
2000
SPUTTERING
TIMEIs
Figure 3. Typical distributions of contaminants after wafer cleaning. SIMS depth profiles obtained with 0: primary ions applying a quasistatic sputtering mode. For each data point on the profiles about 0.1 atomic layers are tonsumed (from Stingeder Edal. 12).
of the doping zone is in the range between 50 nm and 1 pm ; its lateral dimensions are about 1 pm or less. Ion implantation (E, = 30-200 keV) causes amorphization of the single crystal. Consequently a thermal annealing step is necessary to repair crystal damage and activate the dopant atoms. This high temperature process-typically carried out at temperatures around 1000”C--causes diffusion of the dopant elements yielding a different distribution as generated by ion implantation. Therefore the processes of ion implantation and diffusion have to be studied thoroughly with the goal to establish an accurate relationship between the process parameters applied (ion energy, temperature, time, atmosphere of annealing) and the distribution of the dopant elements. Due to the small size of the active regions in highly integrated devices analysis of the dopant elements in the devices is possible only to a limited extent. For fundamental studies of the mechanism of diffusion and for establishing quan1079
M Grasserbauer and G Stingeder:
Secondary ion mass spectrometry
Table 2. Analytical ligures of merit for depth profiling ofdopant in silicon with SIMS (data of Technical
University
elements
Vienna)
Primary
Dopant
ion species
B P AS Sb
0; cs+ CS’ Cs’
Detection limit Secondary [cm--‘] [ppba] ions .~._. .._~. .._ “8+ IxlO” 7 “pm 1x10’) ‘0 ,‘As~“Si 1x10” -2 “‘Sb’YSi 4x IO” IJ2S8’XSi 3xlo’J :
Inaccuracy +5-lO’% 7 20% & 10% *20”/, *5- 10%
titative models for diffusion highly accurate measurements of the distribution of the dopant elements are necessary. For this purpose large scale samples are usually prepared and investigated. The individual steps of device production are carried out on these samples and the physical and chemical behaviour of the dopant elements is investigated. The accurate distribution analysis of the (total and electrically active) dopant elements serves as a basis for establishing mathematical-physical models which describe the behaviour of these trace elements as a function of (production) process parameters (“process modelling”) I3~Ii, Such models can then be used to calculate the properties determining distribution in the small scale devices (reduction of lateral dimensions from millimeters to nanometers, depth scale remains constant). This “transfer of information” enables the optimization ofdevice production, the study of physical processes in devices and the prediction of electrical device properties (“device modelling”) “. Methodological aspects. The major requirements for surface (depth) distribution analysis in semiconductor materials arc : Large dynamic range of analysis and high detection power: centration range C(I. IO”-5 X IO” cm- ‘. Large spatial (depth) resolution : several nm. High accuracy of analytical information (concentration depth).
con-
vs.
Methods of surface analysis of dopant elements. Electrical measurements (like spreading resistance, differential sheet resistance or C/V-measurements) are used for distribution analysis of the electrically active fraction of the dopnnt elements. For elemental surface distribution analysis the major techniques are Neutron Activation Analysis (NAA), SIMS and Rutherford Backscattering Spectrometry (RBS). Extensive comparative work has shown that for B in silicon SIMS and NAA offer similar figures of merit. For P, As and Sb SIMS is superior to the other techniques. At the present state of development SIMS is the most important technique for the distribution analysis of dopant elements ’ 9. With optimized analytical conditions detection limits for distribution analysis are obtained which are in the range between 4 x 10’ ’ cm- 3 and I x 10 ’ 5 cm 3 (Table 2). The analytical accuracy can be estimated and derived from evaluation of different calibration techniques: it is found to be in the order of 5% (B) to 20% (P) for the concentration scale (data point on a profile) and between I- 10 nm for the depth scale (using profilometry to determine the depth of craters). Elaborate techniques had to bc developed for the measurement of the distribution of phosphorus (necessary mass resolution = 4500) in silicon and SiO,/Si-layer 1080
(SIMS)
systems”‘. These new techniques allow not only precision depth profiling. but also the compensation of charging and chemical matrix effects. For the study of the mutual intluence of dopant clemcnts on diffusion the simultaneous measurement of several dopants is necessary. Marker exeriments are used to determine the influence of one dopant element (e.g. P) on the diffusion of the others (e.g. B. As, Sb). In the optimized measurement technique ccsium primary ions arc used, the signals of Si-. P , BSi~~, AsSi~-. and SbSi are detected. and a mass resolution of approximately 4500 is applied to separate interferences”. The main problem of simultaneous measurement with high mass resolution (HMR) is the precise and reproducible setting of the magnetic field. Hysteresis effects are eliminated by cycling sequence. many times (- 20-100) through the measurement The adjustment of the mass scale is performed by computer control using the centroid algorithm’“. The settling time to obtain a stable signal depends on the mass difference and on the mass resolution and is between 1.5 and IO s. These long waiting times, in addition to integration times of 3-10 s for each clement, limit the maximum usable erosion rate and thus the depth resolution. Drifts in the magnetic field are compensated for by the transfer of the deviations of the magnetic field value of a reference mass (e.g. “‘Si-) to the analytical ions. The absolute shift AB and the relative shift AS/B were applied (B denotes the magnetic field value of the reference mass). Best results were obtained using AB. Figure 4 shows as an example the depth profiles of B. P and Sb measured simultaneously in high mass resolution. The depth scale is accurate to 3% for all elements. For process modelling the ratios of the concentrations of the different dopants versus depth are important. To avoid errors due to different craters, the distribution of all elements should be determined simultaneously with HMR. This is not always possible with sufficient quality. Under HMR conditions intensity is lost (in this case by a factor of - 50), which causes poor signal to noise ratios at low concentrations. For shallow distributions are also a limitthe long settling times (especially for “‘Sb’*Si-) ing factor. Thus for low concentrations and shallow profile measurements low mass resolution (LMR, M/AM - 300) and HMR (M/AM - 4500) are combined. At least one element is detected in both modes The fine tuning of the combined depth scale is performed by matching the profiles of the element that was followed under both conditions. An example for B and P is shown in Figure 5, With low mass resolution the fine details in the profile are represented much more accurately since the number of data points per depth unit is increased by a factor of 3. Matching of the P-profiles leads to an accuracy of the depth scale for the combined quantitative depth profile of 4%. With these elaborate measurement techniques the influence of phosphorus on the diffusion of B and Sb could be determined. At high concentrations of phosphorus a negative electrical field is induced. Thus the diffusion of P and Sb is enhanced, and for B it is retarded in the zone where a high phosphorus concentration exists (Figure 4). The enhancement for boron at low concentrations of phosphorus indicates the presence of a supcrsaturation of interstitial silicon atoms, because boron is mainly diffusing via the interstitialcy mechanism. These results provided the basis for a new model to describe the diffusion of phosphorus and the coupled diffusion with other dopants”.
3IP _________-_ IIB --.lZISb
3oSi-
l40-60eV)
0 SIMULATION
(a)
TIME [nin .I
IQ6
IO5 I1
0.0
3.0
2.0
1.0
4.0
3oSi-
-g 1.04
(60-80eV)
31P*% i s-, li5 (S i 1-)
0 DEPTH
hJm1
(a)
1021 31P _________--
102O
IO0
~-.-
0
0 SIMULATION
10’9
15
lb)
30
45
60
75
TIME [min.1
1022
F: 10*a
31P -.-.-
102'
z
l*B
:
E IO"
5 1020
4
F 5 1ol6 g
z p
1019
0
z z y 1018
1015
8
lOI
10’7
1013
1016 0.0
0.0
2.0
1.0
DEPTH
3.0
4.0
fun1
(b) Figure 4. Redistribution of B, Sb and P after pre-annealing at 800°C for 15 min and annealing at 1170°C for 20 min. Implantation: boron and antimony (see Figure 5), phosphorus, 140 keV, 1 x 10” cm-r (a), phosphorus, 140 keV, 1.5 x lOI cm-’ (b). The SiO, layer was etched before measurement. The profiles were measured simultaneously in the HMR mode. (M/AM _ 4500) (from Stingeder et al. * ‘)
(c)
4
0.2
0.4 DEPTH
0.6
0.8
1.0
[pm1
Figure 5. Comparison between measurements with high mass resolution (HMR, M/AM - 4500) and low mass resolution (LMR, M/AM - 300). Technological process steps: implantation of boron, 200 keV, 2.5 x IO” atom cm-*; 60 keV, 9 x 10” atom cmm2; 20 keV, 5 x 10” atom cm-*; were perphosphorus, 50 keV, 7 x lOI atom cm- *. The implantations formed through 20 nm SiOz. The SiOr layer was etched before measurement. Annealing was performed at 800°C for 15 min under Nr (a) HMR. (b) LMR. (c) Quantified measurements. Distribution ofphosphorus from part c1 (HMR) and distribution of boron from part b (LMR) (from Stingeder ef al.*‘). 1081
M Grasserbauer
Secondary
and G Stingeder:
Combination
of analytical
ion mass
spectrometry
techniques for obtaining maximum
to gain maximum information about the distribution of the dopant elements SIMS being the major technique at the present state is frequently combined with electrical measurements, RBS and electron microscopy. The combination of SIMS with electrical measurements allows a separate determination of the distribution of the total dopants (elemental analysis) and the electrically active fraction which may be considerably lower at high dopant concentrations. Such investigations are extremely important in the study of high concentration effects”. Two examples should be presented here to demonstrate the underlying philosophy and the information content of such an analytical system. Antimony. SIMS and RBS allow to combine elemental distribution with structural information for Sb (and As). RBS in random alignment of the He+-beam yields spectra which are produced by collisions with all target particles within the accessible surface zone irrespective of their structural positions. In channelling alignment only atoms at non-substitutional positions contribute to backscattering. Effects occurring during annealing of high dose Sb implants render themselves suitable for a study by a combination of SIMS and RBS’J. Figure 6 shows the random and aligned Rutherford backscattering spectra of an antimony-implanted and annealed sample. They show that a part of Sb is non-substitutional. This non-substitutional part can be correlated with the SIMS measurement in Figure 7. Below a concentration of -6 x 10” cm-‘, the annealed profile broadens because antimony diffusion occurs. Above this concentration, the antimony atoms are immobile. Therefore, in this concentration range diffusion does not take place. Curve 2 indicates that the immobile Sb makes up -7O-80% of the implantation dose (area of profile above 6 x lOI cm-3). RBS shows that these atoms are not found on substitutional sites. It can be concluded that these immobile antimony atoms are precipitated, because the concentration of 6 x lOI cmm3 at which Sb diffusion takes place is higher than the Sb solubility limit of -4 x lOI cme3 (Ref. 24). Combination with electrical measurements (spreading resistance, 4-point probe in conjunction with anodic etching, C/V-measurements) allow us to distinguish further between substitutional (electrically active) information.
In order
I
,
9,
,
I
(SIMS)
r
I
0.1
0.2 DEPTH
0.3
0.4
1pm)
Figure7. SIMS measurement ofan implanted (squares) (120 keV, 3 x 10” cm-‘) and then annealed (solid circles) (IOOtPC, 60 min. NZ) Sb-doped sample. The annealed sample is the same ‘IS in Figure 6 (from Guerrero f’i ol.‘J).
and non-substitutional antimony. It was found that diffusion takes place before precipitation has reached the thermodynamical equilibrium. In this case a supersaturation of clectrically active antimony is obtained. The extension of this analytical system by electron microscopy allows the study of defects and precipitates occurring in high concentration implants”. Figure 8 shows the TEM cross section micrograph of a specimen annealed at 900’ exhibiting a supersaturation of substitutional Sb by more than a factor of 2 (7 x lOI cm3 chemical solutility, 3 x 10” cm3 substitutional concentration) and an immobile fraction of Sb of ccI. 70%. Precipitates are concentrated near the center of damage at a depth of 25 nm. Tangles of dislocations, extending from the surface to a depth of 60 nm, are formed from small faulted loops.
Sb I
00
ALIGNED
100
150
200 CHANNEL
Figure 6. Random (squares) and (100) aligned (circles) RBS spectrum (1.7 MeV, He+) ofa Sb implanted (120 keV, 3 x 10’5cm~ ‘) and annealed (1OOOC for 60 min, NJ sample. For comparison (100) aligned signal (dotted line) of an unimplanted specimen is given (from Guerrero et Ul.Z4). 1082
Figure 8. TEM cross-section micrograph after implantation of 50 keV Sb 3 x lOI cm-I and annealing at T = 900°C for 60 min. The damage zone (precipitates, tangles of dislocations and a few faulted loops) extends to a depth of 75 nm below the surface (from Stingeder et n/.2’).
M Grasserbauer
and G Stingeder:
Secondary
ion mass spectrometry
(SIMS)
P, PONGRATZ, APPLIED PHYSICS TU VIENNA
Figure 10. High resolution
silicon TEM [OI I] lattice image with a pa$ially ccherent antimony precipitate. Orientation relationship: (11 l)Si// (1012)Sb. Process parameters: 50 keV Sb, 3 x 10” cm-*; T= 900°C 60 min (from Stingeder et ~1.‘~).
lOOO”C/ ___________ 1000°C/ -----.-
Figure 9. Plan view bright-field TEM micrograph near [OOI] pole after implantation of 50 keV, 3 x 1015cm-2 and annealing at T = 900°C for 60 min. (a) Precipitates shown with structure factor contrast. (b) Precipitates and dislocations imaged in bright field multibeam diffraction contrast near the [OOl] orientation to see both dislocations with Burgers vectors a [I lo]/2 and a [[TO]/;? (parallel to the surface) (from Stingeder et ~1.~‘).
On a bright field micrograph in plane view mode a large number of precipitates with a size between 5 and 10 nm can be observed using structure factor contrast (Figure 9a). In bright field dislocations and precipitates can be seen (Figure 9b). The morphology of the dislocations indicates climb processes and pipe diffusion to precipitates which are found along their lines and nodes. This can be concluded from the fact that precipitates on dislocation lines and nodes are larger in size than others and that most dislocations do not lie on { 111) glide planes. A high resolution (TEM) Si[ 1 lo] lattice image is shown in Figure 10. Partially coherent (7012) Sb lattice planes parallel to (1il)Si planes are visible. The nature of the precipitates is thus directly explained as Sb (Ref. 23). Image processing of plan view micrographs allows to determine the size distribution of precipitates, which is the experimental basis of simulation of particle growth*‘. Figure 11 shows the SIMS profiles of specimens annealed at IOOO’C for 30. 90 and 180 min indicating that a large fraction of the antimony is precipitated. For annealing at 1000°C and 30 min this fraction is 75%. Figure 12 represents the TEM micrographs and the corresponding size distributions. The total amount of Sb pre-
30min 9Omin
1000°C/180min
10'6
0.0
0.1
0.2
0.3
0: 4
DEPTH [urn1
Figure 11. SIMS profiles of three specimens of Sb implants annealed for 30, 90 and 180 min at 1000°C. (Oxygen primary ions, surface saturation with oxygen Po, = 6 x lo-’ mbar). Process parameters : implantation : 100 keV Sb, 2.5x 10” cm-* through 28 nm SiO,; pre-annealing at 1150°C for 5 s. The interface SiO,/Si is indicated by the verfical dashed line (from Stingeder et al. *I).
cipitated after 30 min was found to be 60%. It is smaller than the corresponding SIMS value, which can be attributed to the fact that very small precipitates cannot be resolved in the TEM. Phosphorus. In marker experiments the influence of high concentrations of P on the diffusion of other dopants is studied as described before. TEM is used to examine the influence of lattice defects on diffusion. In Figure 13 the distributions of Sb, B and P of an annealed specimen are shown. At a depth of 230 nm the boron distribution exhibits a slight enhancement. It corresponds to a kink in the phosphorus profile. Figure 14 shows a cross section TEM micrograph of this specimen. Up to a depth of -50 nm a defect free zone is visible. This corresponds to the region of high Sb concentration. Due to the different covalent atomic radii of P and Sb part of the elastic strain may be compensated for in this region and dislocations from deeper zones are repelled. Up to a 1083
M Grasserbauer
and G Stingeder:
Secondary
ion mass spectrometry
(SIMS)
200
t=30min
140
I I
0
Figure 12, Plan view TEM micrograph
and area distributions
of the precipitates
depth of approximately 250 nm large perfect dislocation loops are present. A distinct band of defects is observed at a depth of - 250 nm. Therefore it can be concluded that boron is segregated at the dislocation band. The edge in the phosphorus profile (Figure 13) at a depth of 230 nm also indicates phosphorus redistribution near the dislocation band. It is also possible that P-B-complexes are formed. Rod like defects with a size of 500 nm or more are visible below the defect band. The rods are narrow dislocation dipoles of extrinsic type, containing Si-interstitials and presumably boron. Investigation of high concentrations of phosphorus after annealing at 900°C respectively 1000°C showed enrichment of phosphorus at the SiOJSi interface and formation of SiP precipitates2’. Oxidation. Oxide films fulfill the functions of dielectric layers, doping masks and protective coatings. Chemical and physical vapor deposition processes and thermal oxidation are used to form the oxide films. The latter is a high temperature process (- IOOOC) which strongly influences the dopant elements through diffusion and segregation. A major reason for this influi 084
30
t = 90 min
60
of the three specimens
90
120
150
shown in Figure
I1 (from Stingeder
el 01.“‘).
ence is that thermal oxidation of silicon causes a perturbation of the equilibrium concentration of silicon point defects26. Incomplete oxidation at the Si/SiO, interface leads to a supersaturation of interstitial silicon at the surface. This supersaturation spreads into the bulk and leads to recombination with the vacancies in the bulk. The result is a decreased vacancy concentration associated with the increased interstitial concentration. Since Group III and V elements diffuse via point defects, the deviations of point defect concentrations from their equilibrium values will strongly affect the diffusivities of these elements. The diffusion of Group III or V elements can be split into two contributions. One results from the diffusion of impurities linked to silicon selfinterstitials (interstitialcy mechanisms). The other is originated by the diffusion of impurities via silicon vacancies (vacancy mechanism). The diffusion coefficient of a dopant can therefore be expressed as
(1)
M Grasserbauer
-
1020
:
I!
.-__
I? 2
and G Stingeder:
1019
Secondary
ion mass spectrometry
also used for the diffusion coefficient of Sb in the oxidized domain of the wafer in the modified form
-.-.-
-
31P
(HMR)
-----_
31P
(LMR)
-
iif3
ILMR)
121Sb
--__
--__
(LMR)
fv =
..,.q
1.0 DEPTH
where
---___
0.5
I 1.5
[urn1
Figure 13. Redistribution of P, B and Sb in silicon. Implantation : P : 140 keV, 1.5x lOI cm-*; B: 200 keV, 1.5x IO“’ cm-*; 60 keV, 9x IO” cmm2; 20 keV, 5 x IO” cm-‘; Sb: 120 keV, 2.5 x lOI cm-*; annealing at 800°C. 15 min under Nz. Phosphorus was measured with low mass resolution (LMR, M/AM - 300), analytical ion: P- and SiH-) and high
mass resolution (HMR, M/AM - 4500, P-) (from Stingeder et aLZ3).
Figure 14. Cross section TEM micrograph 13 (from Stingeder
of specimen
shown in Figure
et ~1.‘~).
C, is the actual and C;q is the equilibrium concentration of selfinterstitials while Cv and Cy denote the analogous quantities for vacancies. Dv and D, are the interstitialcy and vacancy diffusion coefficients. D is the resulting diffusivity, which depends on the actual point defect concentrations. This should be illustrated by the influence of oxidation on the diffusion of Sb in silicon”. In these experiments the diffusion of Sb under an Si,N,/SiO, mask occurs at thermal equilibrium of the point defects and under intrinsic conditions. In this case one obtains from equation (1)
D* = Dv+D,
(SIMS)
(2)
D* in equation (2) is the time independent intrinsic diffusion coefficient of Sb in the domain under the mask. Equation (1) is
$
and
fl = $
=
1-fv.
Equation (3) is obtained by combination of equations (1) and (2). The coefficient fi is the relative contribution by the interstitialcy mechanism and fv is the relative contribution by the vacancy mechanism to the total Sb diffusion. High accuracy SIMS measurements and numerical simulations complement each other and permit a quantitative determination of the retardation factors. An experimental SIMS result is shown in Figure 15. In this figure the oxidation-retarded diffusion of Sb is clearly visible. It is more pronounced at the upper part of the profile because of the steeper initial concentration gradient. The values of _fv and fi are calculated from the above experiments. Under the assumption of local equilibrium between I and V during thermal oxidation, it was found that fi = 0.01 and fv = 0.99, which means that antimony diffuses by 99% via vacancies”. Metallization. The metallization process serves to produce interconnections and electrodes between the active areas of a chip. Highly doped poly-silicon, metals and metal silicides are used. Due to the increasing miniaturization leading to line widths of less than 1/cm current densities of about 1 million A/cm* are encountered in the metallic interconnections. This poses the stringent requirement for an extreme purity of the metals (sputter targets) used to produce these lines, because any impurity might be transported into the sensitive region of the transistor by electromigration or diffusion at elevated temperatures. The maximum tolerable impurity levels for sputter target materials are now : 10 rig/g for alkaline elements (high mobility, danger of break-through at gate oxide), 100 rig/g for transition metals (which can form nano-precipitates at the interfaces isolation oxide-silicon thus reducing the insulating properties) and 10 rig/g for U and Th (being u-emitters these can cause soft errors in a memory cell). The necessary detection power is provided by high performance SIMS and GDMS. SIMS needs an elaborate analytical scheme, which includes the preparation and certification of suitable standards (powders doped with the elements of interest in the pg/g range) to establish relative sensitivity factors but provides bulk a&distribution information**. In addition the method can be used for analysis of thin films on a device, enabling the study of incorporation of impurities during thin film deposition.
4. Concluding remarks
It is evident that secondary
ion mass spectrometry can provide important information for the development of microelectronic materials and devices. The future will still increase the challenges for the analytical chemist since miniaturization of silicon devices is continuing to the 16,64, 128.. Mbit DRAM. New techniques will have to be developed and applied (e.g. SNMS for extremely shallow doping profiles29, nanoanalysis of devices with liquid metal ion sources30.3’, three-dimensional characterization of 1085
M Grasserbauer
and G Stingeder:
Secondary
ion mass spectrometry
(SIMS)
I _ 1020
102(
Sb 1OOOT. inert, 500 mm
-0
----_-v ---II
_-_-__.
--m
10'5
-
0
1
2
3
DEPTH [pm]
(a)
Sb,lK0'C. Inert. 1500 min
-0
Sb. !OOOT. 02, 500mm Sb Initial profile
(b)
Sb 1OOVC. 02, 1500mm !3 lrllhl profde
1
2
Figure 15. Oxidation-retarded diffusion (ORD) of Sb: measured (0, 0, 0) and calculated (curves) antimony profiles. profile before the final annealing step. The final annealing step at 1000°C lasted for 500 min (a), resp. 1500 min (b). The (under the Si,N,/SiO, mask) are shown by circles. Triangles indicate the results for the oxidized domain of the sample. keV ; dose : 4.3 x 10IJ cm-‘; annealing at 1150°C for 30 min in N, to heal implantation defects ; 1.2Llrn epi-silicon layer 1000°C with half of wafer protected by SiSN, (inert annealing profiles). Oxide and nitride removed before analysis (from
using image processing will have to be adopted.
structures
strategies
techniques3’),
new analytical
Acknowledgements Support of the research activities on which this paper is based by the Austrian Scientific Research Council (Proj. No. S 43/10), the Austrian National Bank, the Federal Ministry for Science and Research, the Central Research Laboratories of Siemens, Munich, Wacker Chemitronic, Burghausen, and the Metallwerk. Plansee, Reutte, is gratefully acknowledged. The authors want to thank Dr A Virag, Dr G Friedbacher, Mr K Piphts and Mrs A Nikiforov for performance of SIMS measurements and support in the preparation of this paper. Furthermore the cooperation with the following colleagues is gratefully acknowledged : Prof H Piitzl, D I M Budil, Prof P Skalicky, Dr P Pongratz, D I W Kuhnert (Technical University Vienna), Prof H Ortner, Dr P Wilhartitz (Metallwerk Plansee), Dr M Grundner, Mr S Pahlke, Dr E Guerrero (Wacker Chemitronic), Dr L Mader (Siemens
Munich).
References
’ B 0 Kolbesen and W Pamler, Fres Z Analyf Chem, 333, 561 (1988). ‘I Brodie and J J Muray, The Physics ofMicrofabrication. Plenum Press, New York (1982). ‘S M Sze (Editor), VLSI Technology. McGraw-Hill, New York (1983). “A L Ruoff and K S Chan in Microstructure Science, Vol. 5, p. 329. Academic Press, New York (1982). 5 H W Werner, in Microelectronic Materials and Processes (Edited by R A Levi). M Niijhoff, Amsterdam (1987). ‘M Grasserbauer, G Stingeder, H P&l and E Guerrero, Fres Z A&y/ Chem, 323,421 (1986). ‘M Grasserbauer, in Review in Analytical Chemistr_r-VI (Edited by E Roth), Les Editions Physique, Paris (1988). 1086
3
DEPTH rtml1 Squares indicate the “initial” results for the inert diffusion Implantation of “‘Sb at 80 on wafer; annealing in 0: at Guerrero et a(.“).
*A Benninghoven, F Riidenauer and H W Werner, Secondary Ion Mass Spectrometry, Wiley Interscience, New York (1987). 9B 0 Kolbesen, Characterization of bulk properties of silicon, Parts I and II; 2nd Brasilian Summer School on Semiconductor Physics, Sao Paulo (1981) [available from the author at Siemens Research Laboratories, Otto-Hahn-Ring 6, D-8000 Munich 83.1 I0 J L Regolini, J P Stoquert, C Ganter and P Siffert J Eiectrochem Sot, 133, 2160 (1986). “G Stingeder, S Gara, S Pahlke, H Schwenk, E Guerrero and M Grasserbauer, Fres Z Analyt Chem, 333,576 (1989). ‘*G Stingeder, M Grundner and M Grasserbauer, Surface Interface Anal, 11,407 (1988). “J D Plummer, Rep. No. TR DXG 501-84, Stanford University, July (1984). i4D A Antoniadis, S E Hansen and R W Dutton (1978) Integrated Circuits Laboratory, Stanford Electronics Laboratories, Rep. No. 50192. “R Tielert, in Process and Device Simrrlalion for MOS--VLSI Circuits (Edited by A Antognetti). NATO AS1 Series E, Vol. 62. p. 343. Martinus Niijhof (1983). I6D Chin, M R Kump, H G Lee and R W Dutton, IEEE Truns Elcclrun Der, ED 29, 336 (1982). “H Ryssel, K Haberger, K Hoffmann, G Prinke, R Diimke and A Sachs, IEEE Trans Electron Dev, ED 27, 1484 (1980). ” S Selberherr Analysis and Simularion of Semiconducror Deciccs, Springer, Wien’(1984). 19M Grasserbauer, Pure Appl Chem, 60, (1988). ‘“G Stingeder, Anal Chem 60, 1524 (1988). ” G Stingeder, K Piplits, S Gara, M Grasserbauer, M Budil and H Pijtzl, Anal Chem, 61,412 (1989). “M Budil, H Potzl, G Stingeder, M Grasserbauer and K Goser, hfer Sci Forum, 38-41, 719 (1989). 12G Stingeder, P Pongratz, W Kuhnert and T Brabec, Fres Z AnczlyI Chem, 333, 191 (1989). “E Guerrero, H P&z], G Stingeder, M Grasserbauer, K Piplits, W K Chu, J Elecfrochem Sot, 132, 3048 (1985). *5T Brabec, M Schrems, M Bud& H Piitzl, W Kuhncrt, P Pongratz, G Stingeder and M Grasserbauer, J Electrochem Sot, 136, 1542 (1989). ‘“T Y Yan and U G(iscle, Phys Letr, 40, 616 (1982).
M Grasserbauer and G Stingeder:
Secondary
ion mass spectrometry
“E Guerrero, W Jiingling, H P&l, U GBsele, L Mader, M Grasserbauer and G Stingeder, J Ekcfrochem Sot, 133, 2181 (1986). *“A Virag, G Friedbacher. M Grasserbauer, H M Ortner and P Wilhartitz, J Mater Res, 3, 694 (1988). 29H Oechsner, in Thin Film and Depth Profile Analysis (Edited by H Oechsner), Topics in Current Physics. p. 63 Springer, Berlin (1984).
(SIMS) ‘OR Levi-Setti, J Chabala and Y L Wang, Scanning Microsc Suppl. 1, 13 (1987).
” R von Criegern H Zeininger and S RBhl, Proc. SIMS VI Conference, Versailles 1987, p: 419. Wiley, Chichester (1988). ” F G Riidenauer and W Steiger, Proc SIMS VI Conference, Versailles 1987, p. 361. Wiley, Chichester (1988).
1087
39/numbers
11 /12/pages
1077
to 1087/l
989
0042-207X/89$3.00+.00 (’ 1989 Pergamon Press plc
Printed in Great Britain
Secondary silicon* M Grasserbauer Getreidemarkt
ion mass spectrometry
and G Stingeder.
Institute
for Analytical
Chemistry,
(SIMS) Technical
University
of Vienna,
9, A- 7060 Wien, Austria
This paper describes in a selective manner the potential of high performance SIMS for the development of VLSI silicon devices. Methodological developments are presented for: quantitative analysis of oxygen in silicon in pg/g range, trace element analysis in monolayers on wafers, high accuracy depth profiling analysis of dopant elements for process modelling, characterization of high dopant concentrations, and ultra trace analysis of metallization materials.
1. Introduction Microelectronic devices are the most important high technology products. Most of these are based on silicon technology. VLSI structures have gained a particular significance in this respect. The most advanced VLSI product is the 4 Mbit DRAM (direct random access memory) chip. More than 8,000,OOO transistors are arranged within an area of 1 cm’. The spatial dimensions of the elements in these devices are extremely small-the active areas (source/drain) are about 0.8 pm in diameter, the thickness of the gate oxide is only 15 nm (Ref. 1). VLSI devices are produced in planar technology by a sequence of basic operations which are oxidation, lithography, etching, ion implantation, diffusion, deposition (CVD, sputtering, PVD)2-3. If 4 Mbit DRAMS are made on a 15 cm silicon wafer with a yield of 40% as many as 400 million transistors are produced in one run. Each memory cell has a complex 3-dimensional structure as shown in Figure 1. In total about 400 individual process steps are necessary to produce the 4 Mbit DRAM. New materials and new processes are used to achieve these extremely small structures. The major goals in development of new production techniques are miniaturization and increase of production yield. The yield of device production is determined by physical or chemical defects on a chip. This means that the study of the individual chemical and physical processes occurring in the semiconductor during device production is one of the major tasks in research. The more accurately these processes-like ion implantation, diffusion, oxidation, thin film deposition and removal-an be controlled the higher will be the production
Figure 1. Scanning
electron micrograph of cross section of trench capacitor cell of 4 Mbit DRAM (Courtesy : H Oppolzer, Siemens Munich).
yield. Due to the dramatic miniaturization the chemical and physical processes have to be controlled, calculated and measured within extremely small spatial dimensions-in some cases down to a few tenths of a nanometer (like for the interface Si02/Si in a gate oxide of a MOS Transistor). Chemical and physical processes taking place at small dimensions-e.g. at interfaces, surfaces, crystal defects-are in the center of interest“. In order to develop and produce such highly integrated tures with an acceptable yield an extensive analytical acterization of materials and devices is necessary.
strucchar-
2. Survey of analytical techniques
: AAS, Atomic Absorption Spectrometry ; AES, Auger Electron Spectrometry ; CPAA, Charged Particle Activation Analysis ; DLTS, Deep Level Transient Spectroscopy ; * List
of abbreviations
and acronyms
EPMA, Electron Probe Micro Analysis; GDMS, Glow Discharge Mass Spectrometry; HREELS, High Resolution Electron Energy Loss Spectrometry ; HVTEM, High Voltage Transmission Electron Microscopy ; IR, Infra Red (Spectrometry) ; IRM, Infra Red Microscopy; NAA, Neutron Activation Analysis; OM, Optical Microscopy; RBS, Rutherford Backscattering Spectrometry ; S-AES, Scanning Auger Electron Spectrometry; SCAM, Scanning Acoustic Microscopy; SCOM, Scanr&g Optical Microscopy ; SEM,-Scanning Electron Microscopy ; SIMS, Secondary Ion Mass Spectrometry, TEM, Transmission Electron Microscopy; XPS, X-Ray Photoelectron Spectrometry; XRD, X-Ray Diffraction; XRF, X-Ray Fluorescence (Analysis).
The development and production of microelectronic VLSI devices demands the use of an extensive set of sophisticated analytical techniques. For the determination of chemical features methods for bulk, micro and surface distribution analyiis have to be combined. In addition structural and functional features are of great significance5-‘. Table I contains the major steps of development and production of silicon devices, the analytical information sought and the major analytical techniques applied. Among the techniques listed Secondary Ion Mass Spectrometry [SIMS] provides a unique analytical potential due to the fact that most elements can be analyzed with high sensitivity 1077
M Grasserbauer
and G Stingeder:
Table 1. The IC fabrication Fabrication
process
step, product
Secondary
in relation
ion mass spectrometry
to analytical
(SIMS)
requirements
(according
Information
to Werner’,
Grasserbauer
ef ~l[.~).
Techniques -_______
u;Lfcr production
bulk impurity content physical defects chemical surface structure
NAA. CPAA, IR. DLTS(GDMS) X-ray topography. etching, (HV)TEM, XPS. HREELS, SIMS
R f D wafer processing
thin film composition
EPMA,
1 deposition oxidation implantation etching lithography metallization process integration I processed wafer
dicing. bonding packaging chip
chemical
structure
(chemical) structure topography electrical functions chemical and physical
defects
functions
and isotopic specificity, that elemental and molecular (bonding) information can be gained and that micro, surface and bulk analysis can be peformed8. The major limitations of SIMS are determined by the complexity of the mass spectra, the large variation of the secondary ion yields for different elements, and also for a particular element in different matrices (“chemical matrix effect”), and by the disorder induced into the analytical zone due to the high energy (E,, = l-20 keV) of the primary ions. In order to overcome these limitations in practice the following approach for quantitative trace analysis is most useful :
0 Use of reactive ion sputtering
(oxygen or cesium) to increase the secondary ion yield and to achieve a chemical modification of the surface zone which reduces the chemical matrix effects, l Use of high performance instrumentation which allows the identification (and separation) of interfering species by high mass resolution. lQuantitation with relative sensitivity factors (RSFs) obtained from external or internal calibration. l Systematic study of all sources of analytical errors, particularly of the measurement process. l Development of problem oriented measurement techniques and analytical strategies, combining SIMS with other analytical techniques yielding confirmatory and supplementary information. In the following paragraphs major areas of applications of (high performance) SIMS in the development and production of VLSI silicon devices, focusing on questions associated with the 4 Mbit DRAM will be presented to demonstrate potential and limitations of the method for semiconductor analysis as well as its integration into the arsenal of modern analytical techniques. The figures of merit given refer to a high performance sector field instrument (Camcca IMS 3f). 3. SIMS for the development of VLSI devices 3.1. Wafer production. Wafers as the starting material for device production have to meet stringent requircmcnts in respect to 1078
RBS. XRD, XRF. ellipsometry
depth distribution micro distribution
structure
topography crystallographic
electrical defects
and thickness
analysis: analysis
SIMS. XPS, AES. RBS
: TEM
OM, SCOM, SEM. TEM TEM, RBS, XRD
S-AES, SIMS. TEM OM, SCOM, SEM, TEM non destructive testing OM. SCOM, SEM, SCAM,
IRM. TEM. S-AES, SIMS
non destructive testing (semi-)destructive analysis
purity, base dopant concentration. structural homogeneity. chemical surface structure and topography’. For the highly intcgrated devices usually Czochralski silicon is used which has a rather low specific resistivity (50-l 00 R cm) and is characterized by an oxygen content in the order of 10’7-10’8 cm-‘. This oxygen content is found beneficial because of internal gettering of metallic impurities lo. To utilize this effect outdiffusion of oxygen at I IOO’C to produce an “oxygen free” zone (thickness > IO Alrn) into which the dopant elements are incorporated is combined with growth of SiO,-precipitates (at 100&l IOOC) which act as gettering sites due to the stress field around them. An alternative technique consists in the deposition of “oxygen free” silicon layers by epitaxial growth. For the development of optimized processes a highly accurate determination of oxygen (bulk and distribution analysis) in the concentration range 10’6-10’x cm-‘is necessary. For bulk analysis a combination of techniques is used with IR (for interstitial oxygen only), Inert Gas Fusion Analysis (IGFA) and SIMS exhibiting the best figures of merit for quantitative analysis”. New measurement techniques for SIMS using cesium primary and high energy secondary ions (initial energy = 75- 165 eV) allow to achieve an excellent reproducibility of 24% and an inaccuracy smaller than 10%. This is the basis for highly accurate depth (and lateral) distribution analysis for oxygen with SIMS. Figure 2 shows as an example the oxygen (and boron) depth distribution in the surface layer of a wafer covered with an originally 17 Llrn epitaxial layer (IO pm etched off before analysis) after annealing at 1OOO’C and performing the ASTM2-prccipitation test. As evident from Figure 2 a low oxygen concentration near the surface and the formation of SiO?-prccipitates could be achieved with this process. The increase of the boron concentration is due to a non-reproducible matrix effect caused by the SiO?-precipitates. Compensation could be achieved by analysis with oxygen primary ions and oxygen flooding. Another important area is the determination of impurities on the surface of silicon wafers. High performance Secondary Ion Mass Spectrometry has a great potential for the purity control of the silicon wafer surface due to its inherent high detection power and its high depth resolution. Optimized “quasistatic”
M Grasserbauer
and G Stingeder:
Secondary
ion mass spectrometry
10'9]
(SIMS) 10 ‘1%
I
HYDROPHOBIC
SURFACE
'60 (offset)
lo o~~%::G
I-
10’5 O
10
20
30
60
10
50
Depth (pm)
Figure 2. Depth distribution of oxygen and boron on epi-%/%-structure for 5 min and ASTMZ-test (75O”C, 4 hours and 105O”C, 16 hours) measured with SIMS using Cs+ primary ions. Offset refers to measurement of oxygen ions in the energy range 7% 165 eV (from Stingeder et al. ’ ‘).
’ 1,
10 6
3.2. R+D wafer processing. In the production of integrated circuits the following major processes are used : deposition of thin films, oxidation, ion implantation (combined with annealing to heal crystal defects and to activate dopant elements), etching, lithography, metallization 2*3*5 . In the research phase these processes have to be studied to gain understanding about basic mechanisms and quantitative relationships between the parameters of a process and the product. In the development phase these processes are optimized for specific IC structures’. Implantation and annealing. Type, concentration and distribution of the dopant elements (8 for p-doping, As, P and Sb for n-doping) determine the electrical properties of a device2v3. Dopant elements are introduced into the active regions of the device usually by ion implantation and diffusion. The thickness
1000
HYDROPHILIC
TIMEIs]
1500
SURFACE
3Os1+ I, 101
I-7,
\
afterannealing at 1000°C
measurement techniques had to be developed for the different elements to achieve the optimum between (high) detection power and (low) sample consumption per data point’*. An elaborate analytical procedure based on the combination of high and low mass resolution depth profiles with the evaluation of isotope ratios and the shape of the profiles allows an extensive characterization of the trace elements which are concentrated within the native oxide layer (thickness l-2 nm). It was found that the impurity content in the oxide depends on the surface treatment of the wafer (Figure 3). The detection limit for trace analysis with a sample consumption of less than one tenth of one monolayer per data point (ca. 10” atoms) is about 1 pg/g for aluminium (which serves as a tracer element for contamination) measured with high mass resolution. This corresponds to an absolute detection power of 5 x lo- I9 g, or cu. 10“ aluminium atoms.
500
?
ul
10 =
I’---. i.. ‘.\‘.
\
:::j
1:::::::::
+ 0
500
1000
1500
2000
SPUTTERING
TIMEIs
Figure 3. Typical distributions of contaminants after wafer cleaning. SIMS depth profiles obtained with 0: primary ions applying a quasistatic sputtering mode. For each data point on the profiles about 0.1 atomic layers are tonsumed (from Stingeder Edal. 12).
of the doping zone is in the range between 50 nm and 1 pm ; its lateral dimensions are about 1 pm or less. Ion implantation (E, = 30-200 keV) causes amorphization of the single crystal. Consequently a thermal annealing step is necessary to repair crystal damage and activate the dopant atoms. This high temperature process-typically carried out at temperatures around 1000”C--causes diffusion of the dopant elements yielding a different distribution as generated by ion implantation. Therefore the processes of ion implantation and diffusion have to be studied thoroughly with the goal to establish an accurate relationship between the process parameters applied (ion energy, temperature, time, atmosphere of annealing) and the distribution of the dopant elements. Due to the small size of the active regions in highly integrated devices analysis of the dopant elements in the devices is possible only to a limited extent. For fundamental studies of the mechanism of diffusion and for establishing quan1079
M Grasserbauer and G Stingeder:
Secondary ion mass spectrometry
Table 2. Analytical ligures of merit for depth profiling ofdopant in silicon with SIMS (data of Technical
University
elements
Vienna)
Primary
Dopant
ion species
B P AS Sb
0; cs+ CS’ Cs’
Detection limit Secondary [cm--‘] [ppba] ions .~._. .._~. .._ “8+ IxlO” 7 “pm 1x10’) ‘0 ,‘As~“Si 1x10” -2 “‘Sb’YSi 4x IO” IJ2S8’XSi 3xlo’J :
Inaccuracy +5-lO’% 7 20% & 10% *20”/, *5- 10%
titative models for diffusion highly accurate measurements of the distribution of the dopant elements are necessary. For this purpose large scale samples are usually prepared and investigated. The individual steps of device production are carried out on these samples and the physical and chemical behaviour of the dopant elements is investigated. The accurate distribution analysis of the (total and electrically active) dopant elements serves as a basis for establishing mathematical-physical models which describe the behaviour of these trace elements as a function of (production) process parameters (“process modelling”) I3~Ii, Such models can then be used to calculate the properties determining distribution in the small scale devices (reduction of lateral dimensions from millimeters to nanometers, depth scale remains constant). This “transfer of information” enables the optimization ofdevice production, the study of physical processes in devices and the prediction of electrical device properties (“device modelling”) “. Methodological aspects. The major requirements for surface (depth) distribution analysis in semiconductor materials arc : Large dynamic range of analysis and high detection power: centration range C(I. IO”-5 X IO” cm- ‘. Large spatial (depth) resolution : several nm. High accuracy of analytical information (concentration depth).
con-
vs.
Methods of surface analysis of dopant elements. Electrical measurements (like spreading resistance, differential sheet resistance or C/V-measurements) are used for distribution analysis of the electrically active fraction of the dopnnt elements. For elemental surface distribution analysis the major techniques are Neutron Activation Analysis (NAA), SIMS and Rutherford Backscattering Spectrometry (RBS). Extensive comparative work has shown that for B in silicon SIMS and NAA offer similar figures of merit. For P, As and Sb SIMS is superior to the other techniques. At the present state of development SIMS is the most important technique for the distribution analysis of dopant elements ’ 9. With optimized analytical conditions detection limits for distribution analysis are obtained which are in the range between 4 x 10’ ’ cm- 3 and I x 10 ’ 5 cm 3 (Table 2). The analytical accuracy can be estimated and derived from evaluation of different calibration techniques: it is found to be in the order of 5% (B) to 20% (P) for the concentration scale (data point on a profile) and between I- 10 nm for the depth scale (using profilometry to determine the depth of craters). Elaborate techniques had to bc developed for the measurement of the distribution of phosphorus (necessary mass resolution = 4500) in silicon and SiO,/Si-layer 1080
(SIMS)
systems”‘. These new techniques allow not only precision depth profiling. but also the compensation of charging and chemical matrix effects. For the study of the mutual intluence of dopant clemcnts on diffusion the simultaneous measurement of several dopants is necessary. Marker exeriments are used to determine the influence of one dopant element (e.g. P) on the diffusion of the others (e.g. B. As, Sb). In the optimized measurement technique ccsium primary ions arc used, the signals of Si-. P , BSi~~, AsSi~-. and SbSi are detected. and a mass resolution of approximately 4500 is applied to separate interferences”. The main problem of simultaneous measurement with high mass resolution (HMR) is the precise and reproducible setting of the magnetic field. Hysteresis effects are eliminated by cycling sequence. many times (- 20-100) through the measurement The adjustment of the mass scale is performed by computer control using the centroid algorithm’“. The settling time to obtain a stable signal depends on the mass difference and on the mass resolution and is between 1.5 and IO s. These long waiting times, in addition to integration times of 3-10 s for each clement, limit the maximum usable erosion rate and thus the depth resolution. Drifts in the magnetic field are compensated for by the transfer of the deviations of the magnetic field value of a reference mass (e.g. “‘Si-) to the analytical ions. The absolute shift AB and the relative shift AS/B were applied (B denotes the magnetic field value of the reference mass). Best results were obtained using AB. Figure 4 shows as an example the depth profiles of B. P and Sb measured simultaneously in high mass resolution. The depth scale is accurate to 3% for all elements. For process modelling the ratios of the concentrations of the different dopants versus depth are important. To avoid errors due to different craters, the distribution of all elements should be determined simultaneously with HMR. This is not always possible with sufficient quality. Under HMR conditions intensity is lost (in this case by a factor of - 50), which causes poor signal to noise ratios at low concentrations. For shallow distributions are also a limitthe long settling times (especially for “‘Sb’*Si-) ing factor. Thus for low concentrations and shallow profile measurements low mass resolution (LMR, M/AM - 300) and HMR (M/AM - 4500) are combined. At least one element is detected in both modes The fine tuning of the combined depth scale is performed by matching the profiles of the element that was followed under both conditions. An example for B and P is shown in Figure 5, With low mass resolution the fine details in the profile are represented much more accurately since the number of data points per depth unit is increased by a factor of 3. Matching of the P-profiles leads to an accuracy of the depth scale for the combined quantitative depth profile of 4%. With these elaborate measurement techniques the influence of phosphorus on the diffusion of B and Sb could be determined. At high concentrations of phosphorus a negative electrical field is induced. Thus the diffusion of P and Sb is enhanced, and for B it is retarded in the zone where a high phosphorus concentration exists (Figure 4). The enhancement for boron at low concentrations of phosphorus indicates the presence of a supcrsaturation of interstitial silicon atoms, because boron is mainly diffusing via the interstitialcy mechanism. These results provided the basis for a new model to describe the diffusion of phosphorus and the coupled diffusion with other dopants”.
3IP _________-_ IIB --.lZISb
3oSi-
l40-60eV)
0 SIMULATION
(a)
TIME [nin .I
IQ6
IO5 I1
0.0
3.0
2.0
1.0
4.0
3oSi-
-g 1.04
(60-80eV)
31P*% i s-, li5 (S i 1-)
0 DEPTH
hJm1
(a)
1021 31P _________--
102O
IO0
~-.-
0
0 SIMULATION
10’9
15
lb)
30
45
60
75
TIME [min.1
1022
F: 10*a
31P -.-.-
102'
z
l*B
:
E IO"
5 1020
4
F 5 1ol6 g
z p
1019
0
z z y 1018
1015
8
lOI
10’7
1013
1016 0.0
0.0
2.0
1.0
DEPTH
3.0
4.0
fun1
(b) Figure 4. Redistribution of B, Sb and P after pre-annealing at 800°C for 15 min and annealing at 1170°C for 20 min. Implantation: boron and antimony (see Figure 5), phosphorus, 140 keV, 1 x 10” cm-r (a), phosphorus, 140 keV, 1.5 x lOI cm-’ (b). The SiO, layer was etched before measurement. The profiles were measured simultaneously in the HMR mode. (M/AM _ 4500) (from Stingeder et al. * ‘)
(c)
4
0.2
0.4 DEPTH
0.6
0.8
1.0
[pm1
Figure 5. Comparison between measurements with high mass resolution (HMR, M/AM - 4500) and low mass resolution (LMR, M/AM - 300). Technological process steps: implantation of boron, 200 keV, 2.5 x IO” atom cm-*; 60 keV, 9 x 10” atom cmm2; 20 keV, 5 x 10” atom cm-*; were perphosphorus, 50 keV, 7 x lOI atom cm- *. The implantations formed through 20 nm SiOz. The SiOr layer was etched before measurement. Annealing was performed at 800°C for 15 min under Nr (a) HMR. (b) LMR. (c) Quantified measurements. Distribution ofphosphorus from part c1 (HMR) and distribution of boron from part b (LMR) (from Stingeder ef al.*‘). 1081
M Grasserbauer
Secondary
and G Stingeder:
Combination
of analytical
ion mass
spectrometry
techniques for obtaining maximum
to gain maximum information about the distribution of the dopant elements SIMS being the major technique at the present state is frequently combined with electrical measurements, RBS and electron microscopy. The combination of SIMS with electrical measurements allows a separate determination of the distribution of the total dopants (elemental analysis) and the electrically active fraction which may be considerably lower at high dopant concentrations. Such investigations are extremely important in the study of high concentration effects”. Two examples should be presented here to demonstrate the underlying philosophy and the information content of such an analytical system. Antimony. SIMS and RBS allow to combine elemental distribution with structural information for Sb (and As). RBS in random alignment of the He+-beam yields spectra which are produced by collisions with all target particles within the accessible surface zone irrespective of their structural positions. In channelling alignment only atoms at non-substitutional positions contribute to backscattering. Effects occurring during annealing of high dose Sb implants render themselves suitable for a study by a combination of SIMS and RBS’J. Figure 6 shows the random and aligned Rutherford backscattering spectra of an antimony-implanted and annealed sample. They show that a part of Sb is non-substitutional. This non-substitutional part can be correlated with the SIMS measurement in Figure 7. Below a concentration of -6 x 10” cm-‘, the annealed profile broadens because antimony diffusion occurs. Above this concentration, the antimony atoms are immobile. Therefore, in this concentration range diffusion does not take place. Curve 2 indicates that the immobile Sb makes up -7O-80% of the implantation dose (area of profile above 6 x lOI cm-3). RBS shows that these atoms are not found on substitutional sites. It can be concluded that these immobile antimony atoms are precipitated, because the concentration of 6 x lOI cmm3 at which Sb diffusion takes place is higher than the Sb solubility limit of -4 x lOI cme3 (Ref. 24). Combination with electrical measurements (spreading resistance, 4-point probe in conjunction with anodic etching, C/V-measurements) allow us to distinguish further between substitutional (electrically active) information.
In order
I
,
9,
,
I
(SIMS)
r
I
0.1
0.2 DEPTH
0.3
0.4
1pm)
Figure7. SIMS measurement ofan implanted (squares) (120 keV, 3 x 10” cm-‘) and then annealed (solid circles) (IOOtPC, 60 min. NZ) Sb-doped sample. The annealed sample is the same ‘IS in Figure 6 (from Guerrero f’i ol.‘J).
and non-substitutional antimony. It was found that diffusion takes place before precipitation has reached the thermodynamical equilibrium. In this case a supersaturation of clectrically active antimony is obtained. The extension of this analytical system by electron microscopy allows the study of defects and precipitates occurring in high concentration implants”. Figure 8 shows the TEM cross section micrograph of a specimen annealed at 900’ exhibiting a supersaturation of substitutional Sb by more than a factor of 2 (7 x lOI cm3 chemical solutility, 3 x 10” cm3 substitutional concentration) and an immobile fraction of Sb of ccI. 70%. Precipitates are concentrated near the center of damage at a depth of 25 nm. Tangles of dislocations, extending from the surface to a depth of 60 nm, are formed from small faulted loops.
Sb I
00
100
150
200 CHANNEL
Figure 6. Random (squares) and (100) aligned (circles) RBS spectrum (1.7 MeV, He+) ofa Sb implanted (120 keV, 3 x 10’5cm~ ‘) and annealed (1OOOC for 60 min, NJ sample. For comparison (100) aligned signal (dotted line) of an unimplanted specimen is given (from Guerrero et Ul.Z4). 1082
Figure 8. TEM cross-section micrograph after implantation of 50 keV Sb 3 x lOI cm-I and annealing at T = 900°C for 60 min. The damage zone (precipitates, tangles of dislocations and a few faulted loops) extends to a depth of 75 nm below the surface (from Stingeder et n/.2’).
M Grasserbauer
and G Stingeder:
Secondary
ion mass spectrometry
(SIMS)
P, PONGRATZ, APPLIED PHYSICS TU VIENNA
Figure 10. High resolution
silicon TEM [OI I] lattice image with a pa$ially ccherent antimony precipitate. Orientation relationship: (11 l)Si// (1012)Sb. Process parameters: 50 keV Sb, 3 x 10” cm-*; T= 900°C 60 min (from Stingeder et ~1.‘~).
lOOO”C/ ___________ 1000°C/ -----.-
Figure 9. Plan view bright-field TEM micrograph near [OOI] pole after implantation of 50 keV, 3 x 1015cm-2 and annealing at T = 900°C for 60 min. (a) Precipitates shown with structure factor contrast. (b) Precipitates and dislocations imaged in bright field multibeam diffraction contrast near the [OOl] orientation to see both dislocations with Burgers vectors a [I lo]/2 and a [[TO]/;? (parallel to the surface) (from Stingeder et ~1.~‘).
On a bright field micrograph in plane view mode a large number of precipitates with a size between 5 and 10 nm can be observed using structure factor contrast (Figure 9a). In bright field dislocations and precipitates can be seen (Figure 9b). The morphology of the dislocations indicates climb processes and pipe diffusion to precipitates which are found along their lines and nodes. This can be concluded from the fact that precipitates on dislocation lines and nodes are larger in size than others and that most dislocations do not lie on { 111) glide planes. A high resolution (TEM) Si[ 1 lo] lattice image is shown in Figure 10. Partially coherent (7012) Sb lattice planes parallel to (1il)Si planes are visible. The nature of the precipitates is thus directly explained as Sb (Ref. 23). Image processing of plan view micrographs allows to determine the size distribution of precipitates, which is the experimental basis of simulation of particle growth*‘. Figure 11 shows the SIMS profiles of specimens annealed at IOOO’C for 30. 90 and 180 min indicating that a large fraction of the antimony is precipitated. For annealing at 1000°C and 30 min this fraction is 75%. Figure 12 represents the TEM micrographs and the corresponding size distributions. The total amount of Sb pre-
30min 9Omin
1000°C/180min
10'6
0.0
0.1
0.2
0.3
0: 4
DEPTH [urn1
Figure 11. SIMS profiles of three specimens of Sb implants annealed for 30, 90 and 180 min at 1000°C. (Oxygen primary ions, surface saturation with oxygen Po, = 6 x lo-’ mbar). Process parameters : implantation : 100 keV Sb, 2.5x 10” cm-* through 28 nm SiO,; pre-annealing at 1150°C for 5 s. The interface SiO,/Si is indicated by the verfical dashed line (from Stingeder et al. *I).
cipitated after 30 min was found to be 60%. It is smaller than the corresponding SIMS value, which can be attributed to the fact that very small precipitates cannot be resolved in the TEM. Phosphorus. In marker experiments the influence of high concentrations of P on the diffusion of other dopants is studied as described before. TEM is used to examine the influence of lattice defects on diffusion. In Figure 13 the distributions of Sb, B and P of an annealed specimen are shown. At a depth of 230 nm the boron distribution exhibits a slight enhancement. It corresponds to a kink in the phosphorus profile. Figure 14 shows a cross section TEM micrograph of this specimen. Up to a depth of -50 nm a defect free zone is visible. This corresponds to the region of high Sb concentration. Due to the different covalent atomic radii of P and Sb part of the elastic strain may be compensated for in this region and dislocations from deeper zones are repelled. Up to a 1083
M Grasserbauer
and G Stingeder:
Secondary
ion mass spectrometry
(SIMS)
200
t=30min
140
I I
0
Figure 12, Plan view TEM micrograph
and area distributions
of the precipitates
depth of approximately 250 nm large perfect dislocation loops are present. A distinct band of defects is observed at a depth of - 250 nm. Therefore it can be concluded that boron is segregated at the dislocation band. The edge in the phosphorus profile (Figure 13) at a depth of 230 nm also indicates phosphorus redistribution near the dislocation band. It is also possible that P-B-complexes are formed. Rod like defects with a size of 500 nm or more are visible below the defect band. The rods are narrow dislocation dipoles of extrinsic type, containing Si-interstitials and presumably boron. Investigation of high concentrations of phosphorus after annealing at 900°C respectively 1000°C showed enrichment of phosphorus at the SiOJSi interface and formation of SiP precipitates2’. Oxidation. Oxide films fulfill the functions of dielectric layers, doping masks and protective coatings. Chemical and physical vapor deposition processes and thermal oxidation are used to form the oxide films. The latter is a high temperature process (- IOOOC) which strongly influences the dopant elements through diffusion and segregation. A major reason for this influi 084
30
t = 90 min
60
of the three specimens
90
120
150
shown in Figure
I1 (from Stingeder
el 01.“‘).
ence is that thermal oxidation of silicon causes a perturbation of the equilibrium concentration of silicon point defects26. Incomplete oxidation at the Si/SiO, interface leads to a supersaturation of interstitial silicon at the surface. This supersaturation spreads into the bulk and leads to recombination with the vacancies in the bulk. The result is a decreased vacancy concentration associated with the increased interstitial concentration. Since Group III and V elements diffuse via point defects, the deviations of point defect concentrations from their equilibrium values will strongly affect the diffusivities of these elements. The diffusion of Group III or V elements can be split into two contributions. One results from the diffusion of impurities linked to silicon selfinterstitials (interstitialcy mechanisms). The other is originated by the diffusion of impurities via silicon vacancies (vacancy mechanism). The diffusion coefficient of a dopant can therefore be expressed as
(1)
M Grasserbauer
-
1020
:
I!
.-__
I? 2
and G Stingeder:
1019
Secondary
ion mass spectrometry
also used for the diffusion coefficient of Sb in the oxidized domain of the wafer in the modified form
-.-.-
-
31P
(HMR)
-----_
31P
(LMR)
-
iif3
ILMR)
121Sb
--__
--__
(LMR)
fv =
..,.q
1.0 DEPTH
where
---___
0.5
I 1.5
[urn1
Figure 13. Redistribution of P, B and Sb in silicon. Implantation : P : 140 keV, 1.5x lOI cm-*; B: 200 keV, 1.5x IO“’ cm-*; 60 keV, 9x IO” cmm2; 20 keV, 5 x IO” cm-‘; Sb: 120 keV, 2.5 x lOI cm-*; annealing at 800°C. 15 min under Nz. Phosphorus was measured with low mass resolution (LMR, M/AM - 300), analytical ion: P- and SiH-) and high
mass resolution (HMR, M/AM - 4500, P-) (from Stingeder et aLZ3).
Figure 14. Cross section TEM micrograph 13 (from Stingeder
of specimen
shown in Figure
et ~1.‘~).
C, is the actual and C;q is the equilibrium concentration of selfinterstitials while Cv and Cy denote the analogous quantities for vacancies. Dv and D, are the interstitialcy and vacancy diffusion coefficients. D is the resulting diffusivity, which depends on the actual point defect concentrations. This should be illustrated by the influence of oxidation on the diffusion of Sb in silicon”. In these experiments the diffusion of Sb under an Si,N,/SiO, mask occurs at thermal equilibrium of the point defects and under intrinsic conditions. In this case one obtains from equation (1)
D* = Dv+D,
(SIMS)
(2)
D* in equation (2) is the time independent intrinsic diffusion coefficient of Sb in the domain under the mask. Equation (1) is
$
and
fl = $
=
1-fv.
Equation (3) is obtained by combination of equations (1) and (2). The coefficient fi is the relative contribution by the interstitialcy mechanism and fv is the relative contribution by the vacancy mechanism to the total Sb diffusion. High accuracy SIMS measurements and numerical simulations complement each other and permit a quantitative determination of the retardation factors. An experimental SIMS result is shown in Figure 15. In this figure the oxidation-retarded diffusion of Sb is clearly visible. It is more pronounced at the upper part of the profile because of the steeper initial concentration gradient. The values of _fv and fi are calculated from the above experiments. Under the assumption of local equilibrium between I and V during thermal oxidation, it was found that fi = 0.01 and fv = 0.99, which means that antimony diffuses by 99% via vacancies”. Metallization. The metallization process serves to produce interconnections and electrodes between the active areas of a chip. Highly doped poly-silicon, metals and metal silicides are used. Due to the increasing miniaturization leading to line widths of less than 1/cm current densities of about 1 million A/cm* are encountered in the metallic interconnections. This poses the stringent requirement for an extreme purity of the metals (sputter targets) used to produce these lines, because any impurity might be transported into the sensitive region of the transistor by electromigration or diffusion at elevated temperatures. The maximum tolerable impurity levels for sputter target materials are now : 10 rig/g for alkaline elements (high mobility, danger of break-through at gate oxide), 100 rig/g for transition metals (which can form nano-precipitates at the interfaces isolation oxide-silicon thus reducing the insulating properties) and 10 rig/g for U and Th (being u-emitters these can cause soft errors in a memory cell). The necessary detection power is provided by high performance SIMS and GDMS. SIMS needs an elaborate analytical scheme, which includes the preparation and certification of suitable standards (powders doped with the elements of interest in the pg/g range) to establish relative sensitivity factors but provides bulk a&distribution information**. In addition the method can be used for analysis of thin films on a device, enabling the study of incorporation of impurities during thin film deposition.
4. Concluding remarks
It is evident that secondary
ion mass spectrometry can provide important information for the development of microelectronic materials and devices. The future will still increase the challenges for the analytical chemist since miniaturization of silicon devices is continuing to the 16,64, 128.. Mbit DRAM. New techniques will have to be developed and applied (e.g. SNMS for extremely shallow doping profiles29, nanoanalysis of devices with liquid metal ion sources30.3’, three-dimensional characterization of 1085
M Grasserbauer
and G Stingeder:
Secondary
ion mass spectrometry
(SIMS)
I _ 1020
102(
Sb 1OOOT. inert, 500 mm
-0
----_-v ---II
_-_-__.
--m
10'5
-
0
1
2
3
DEPTH [pm]
(a)
Sb,lK0'C. Inert. 1500 min
-0
Sb. !OOOT. 02, 500mm Sb Initial profile
(b)
Sb 1OOVC. 02, 1500mm !3 lrllhl profde
1
2
Figure 15. Oxidation-retarded diffusion (ORD) of Sb: measured (0, 0, 0) and calculated (curves) antimony profiles. profile before the final annealing step. The final annealing step at 1000°C lasted for 500 min (a), resp. 1500 min (b). The (under the Si,N,/SiO, mask) are shown by circles. Triangles indicate the results for the oxidized domain of the sample. keV ; dose : 4.3 x 10IJ cm-‘; annealing at 1150°C for 30 min in N, to heal implantation defects ; 1.2Llrn epi-silicon layer 1000°C with half of wafer protected by SiSN, (inert annealing profiles). Oxide and nitride removed before analysis (from
using image processing will have to be adopted.
structures
strategies
techniques3’),
new analytical
Acknowledgements Support of the research activities on which this paper is based by the Austrian Scientific Research Council (Proj. No. S 43/10), the Austrian National Bank, the Federal Ministry for Science and Research, the Central Research Laboratories of Siemens, Munich, Wacker Chemitronic, Burghausen, and the Metallwerk. Plansee, Reutte, is gratefully acknowledged. The authors want to thank Dr A Virag, Dr G Friedbacher, Mr K Piphts and Mrs A Nikiforov for performance of SIMS measurements and support in the preparation of this paper. Furthermore the cooperation with the following colleagues is gratefully acknowledged : Prof H Piitzl, D I M Budil, Prof P Skalicky, Dr P Pongratz, D I W Kuhnert (Technical University Vienna), Prof H Ortner, Dr P Wilhartitz (Metallwerk Plansee), Dr M Grundner, Mr S Pahlke, Dr E Guerrero (Wacker Chemitronic), Dr L Mader (Siemens
Munich).
References
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3
DEPTH rtml1 Squares indicate the “initial” results for the inert diffusion Implantation of “‘Sb at 80 on wafer; annealing in 0: at Guerrero et a(.“).
*A Benninghoven, F Riidenauer and H W Werner, Secondary Ion Mass Spectrometry, Wiley Interscience, New York (1987). 9B 0 Kolbesen, Characterization of bulk properties of silicon, Parts I and II; 2nd Brasilian Summer School on Semiconductor Physics, Sao Paulo (1981) [available from the author at Siemens Research Laboratories, Otto-Hahn-Ring 6, D-8000 Munich 83.1 I0 J L Regolini, J P Stoquert, C Ganter and P Siffert J Eiectrochem Sot, 133, 2160 (1986). “G Stingeder, S Gara, S Pahlke, H Schwenk, E Guerrero and M Grasserbauer, Fres Z Analyt Chem, 333,576 (1989). ‘*G Stingeder, M Grundner and M Grasserbauer, Surface Interface Anal, 11,407 (1988). “J D Plummer, Rep. No. TR DXG 501-84, Stanford University, July (1984). i4D A Antoniadis, S E Hansen and R W Dutton (1978) Integrated Circuits Laboratory, Stanford Electronics Laboratories, Rep. No. 50192. “R Tielert, in Process and Device Simrrlalion for MOS--VLSI Circuits (Edited by A Antognetti). NATO AS1 Series E, Vol. 62. p. 343. Martinus Niijhof (1983). I6D Chin, M R Kump, H G Lee and R W Dutton, IEEE Truns Elcclrun Der, ED 29, 336 (1982). “H Ryssel, K Haberger, K Hoffmann, G Prinke, R Diimke and A Sachs, IEEE Trans Electron Dev, ED 27, 1484 (1980). ” S Selberherr Analysis and Simularion of Semiconducror Deciccs, Springer, Wien’(1984). 19M Grasserbauer, Pure Appl Chem, 60, (1988). ‘“G Stingeder, Anal Chem 60, 1524 (1988). ” G Stingeder, K Piplits, S Gara, M Grasserbauer, M Budil and H Pijtzl, Anal Chem, 61,412 (1989). “M Budil, H Potzl, G Stingeder, M Grasserbauer and K Goser, hfer Sci Forum, 38-41, 719 (1989). 12G Stingeder, P Pongratz, W Kuhnert and T Brabec, Fres Z AnczlyI Chem, 333, 191 (1989). “E Guerrero, H P&z], G Stingeder, M Grasserbauer, K Piplits, W K Chu, J Elecfrochem Sot, 132, 3048 (1985). *5T Brabec, M Schrems, M Bud& H Piitzl, W Kuhncrt, P Pongratz, G Stingeder and M Grasserbauer, J Electrochem Sot, 136, 1542 (1989). ‘“T Y Yan and U G(iscle, Phys Letr, 40, 616 (1982).
M Grasserbauer and G Stingeder:
Secondary
ion mass spectrometry
“E Guerrero, W Jiingling, H P&l, U GBsele, L Mader, M Grasserbauer and G Stingeder, J Ekcfrochem Sot, 133, 2181 (1986). *“A Virag, G Friedbacher. M Grasserbauer, H M Ortner and P Wilhartitz, J Mater Res, 3, 694 (1988). 29H Oechsner, in Thin Film and Depth Profile Analysis (Edited by H Oechsner), Topics in Current Physics. p. 63 Springer, Berlin (1984).
(SIMS) ‘OR Levi-Setti, J Chabala and Y L Wang, Scanning Microsc Suppl. 1, 13 (1987).
” R von Criegern H Zeininger and S RBhl, Proc. SIMS VI Conference, Versailles 1987, p: 419. Wiley, Chichester (1988). ” F G Riidenauer and W Steiger, Proc SIMS VI Conference, Versailles 1987, p. 361. Wiley, Chichester (1988).
1087