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2) Wafer Surface Analysis (WSA) Method: The WSA [2] is a method that considerably improves solution preparation for the vapor phase decomposition method. It omits the silicon oxidation step mentioned above, and the native oxide film is decomposed directly by exposing it to HF vapor.
Afterwards, a droplet of *I.d* dil... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
13.2.1.2 Measurement Equipment. The flameless atomic absorption spectrometer has been the most popular metallic contamination analytical tool combined with the above-mentioned sample preparation using the vapor phase decomposition method or the WSA method.
Figure 13.5 shows a sche... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
| Method | Na | Al | Fe | Ni |
|---------------------|------|------|------|------|
| WSA+GFAAS | 0.1 | 0.4 | 0.2 | 0.4 |
| WSA+lCP/MS | 0.04 | 0.02 | 0.04 | 0.07 |
| TRXRF | 100 | 30 | 0.5 | 0.3 |
| [x 1010 atoms/cm2 ] | | | | |
element... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
13.2.2.1 Anions. SOx, NOx , or halogens, such as CI and F, are in general dissolved or extracted in pure water . The most sensitive analysis method is an ion chromatograph, where the anions can be detected up to the ppb level (Fig. 13.7). There remains the problem of extraction of S from the wafer surface because its e... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
SIMS (secondary ion mass spectroscopy), AES (Auger electron spectroscopy)
and XPS (X-ray photoelectron spectroscopy) are commonly used. Physical analysis using beams is a microscopic analysis, and has, even for the most sensitive SIMS method, a detection limit in the ppb region. In general, the physical analysis is la... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
13.3.1.1 Principles. Figure 13.9 gives a schematic of the equipment. When X-rays fall at an extremely small incident angle on a mirror-polished surface, such as the wafer, there is total reflection. Using this phenomenon, an analysis of the fluorescent X-ray due to impurities near the wafer surface is performed.
![19... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
When X-rays fall under total reflection conditions, almost no fluorescent X-rays are induced by the Si wafer, because the X-ray does not penetrate the substrate. Only particles on the wafer surface cause fluorescent X-rays.
Therefore, analysis with an extremely high S /N ratio is possible.
13.3.1.2 Measured Element... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 13.3.1.3 Horizontal Distribution And Depth Distribution.
The TRXRF method is not only a non-destructive and non-contact analysis, but also allows, in contrast to the chemical analysis, local mapping of the distribution of the impurities on the surface, and by varying the incident angle also the distribution in dept... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Recently, non-destructive FT-IR-ATR methods for the detection of organic impurities on the wafer surface have been also employed. At present, to improve of the sensitivity and reliability the tight adhesion of the sample wafer and the prism and the background noise reduction must be improved.
For non-destructive tota... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
In the future, it is expected that due to the growing complexity and increasing level of the process, the requirements of surface impurity assessment techniques will become more and more severe. In the near future, analysis methods for large diameter wafers and impurity analysis on micro-regions of the size of the devi... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
[5] S. S. Ladermann, A. Fischer-Colbrie, A. Shimazaki, K. Miyasaki, S. Brennan, N. Takaura, P. Pianetta, and J. B. Kortight: The 5th Workshop on Total Reflection X-ray Fluorescense Spectroscopy and Related Spectroscopical Methods, Japan Society for Analytical Chemistry, p. 14 (1994).
# 14. Analysis And Evaluation Of ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
The different detection techniques, with their number of detectable atoms depending on the size of the tested area, are shown in Fig. 14.1. The values are standard values, so the uppermost limit of detection is not shown.
For metals, analysis methods are total reflection X-ray fluorescence (TXRF),
secondary ion mass... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
and applies chemical techniques, the sensitivity can be enlarged by two orders of magnitude [2,3]. AES is superior for measurement of minute areas: with the use of a "field emission electron gun" and a multi-channel detector, areas below 20 nm are analyzable. The energy resolution o... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Following Redhead, the desorption rate *N(t)* can be expressed using the following equation [7J:
$$N(t)=-\frac{{\rm d}\sigma}{{\rm d}t}=\nu_{n}\sigma^{n}\exp(-E/RT)\quad.\tag{1}$$
Here, n is the degree of the desorption reaction, a is the surface coverage of the adsorbed component, 1/ is the frequency factor, E is t... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Comparatively large samples can be examined and heated homogeneously.
The adhered molecules desorb into the gas phase and can be collected efficiently in the mass spectrometer. Therefore the sensitivity is high. System (c)
uses atmospheric pressure ionisation mass spectrometry (APIMS) or a gas chromatograph, which m... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
This method allows accurate measurements of the water molecules adhered to the Si surface to be made. Figure 14.4 shows a comparison with a spectrum obtained with an unmoved sample. The dotted line is the background, and the solid line is the TDS spectrum obtained when the sample is... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
The sample used was cut from mirror-polished (both sides) n-type Si(100).
It was cleaned with the RCA method and dilute HF solution. Figure 14.5b shows the data obtained from the background only, while (a) is the spectrum obtained when the wafer is present. The different background mass peaks
 and (3) at about 400 °C and about *500°C,* respectively. The (100) surface changes from 1 x 1 structure to the mono hydride 2 x 1 structure at about *400°C,* and at about 500 °C also to another 2 x 1 structure without hydrogen [10].
$$\mathrm{SiH_{2}\longr... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
In the native oxide film that is formed in the chemical, as well as in the interface of the surface or the Si substrate, hydrogen is also present.
Figure 14.7 shows the TDS spectrum of H2 and D2 for wafers terminated by D after growth of the native oxide film for three different cases:
1) The surface is terminated w... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 14.3.2 Water
When water is on the Si surface, it is not only the reason for silanol and the creation of a native oxide film, but it also dissolves the different ions present and becomes the source of new contamination. Therefore, water adsorbed in the surface should be removed. A water desorption spectrum with a pe... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
When the wafer is exposed to atmospheric air or gas containing water this is not the whole story. Independent of the surface structure, in less than one second several atomic layers of water are adhered. In 10 layers the amount of water is 0.1%, in 100 layers about 3% [11 J. When TDS is performed in Ar gas with less th... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Since in many experiments organic molecules with mass number 28 at most have been observed, attempts have been made to identify this molecule.
With the TOF-SIMS having a very high mass resolution, as shown in Fig. 14.10, it became clear that the peak is not due to CO or N2 but to C2H4 . The background observed with T... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
14.3.3.3 Effects of Teflon Zig. When the wafer is in contact with Teflon, as shown in Fig. 14.13, at over 500 DC, peaks at *M/e* = 31,50,69 corresponding to CF, CF2 , CF3 occur in the spectrum. In order to make sure that Teflon is the reason for the contamination, three sample wafers were examined with TDS (Fig. 14.14)... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
The distinction between HF, H2 180 and D2 0 having the same mass number can be done by comparing the resemblance to the desorption spectrum of H2 0 with *M/e* = 18. Compared with hydrogen, ammonia desorbs at a rate
The detection of contaminants adsorbed onto the wafer surface afte... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
N. Yabumoto: Ultra Clean Technology 4, 304 (1992) [in Japanese].
Y. Matsushita, N. Tsuchiya, and M. Kageyama: Proc. 13th Ultra Clean Techno!. Symp. Tokyo, 127 (1991) [in Japanese].
H. Mizokami and Y. Ajioka: Trans. Inst. Electron. Inform. and Communication. Eng. J 76-C-II, 67 (1993) [in Japanese].
T. Hattori: Ult... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
N. Hirashita, M. Kobayakawa, A. Arimatsu, F. Yokoyama, and T. Ajioka:
J. Electrochem. Soc. 139, 794 (1992).
K. Murase, N. Yabumoto, and Y. Komine: J. Electrochem. Soc. 140, 1722
(1993).
K. Machida, N. Shimoyama, J. Takahashi, Y. Takahashi, N. Yabumoto, and E. Arai: IEEE Trans. Electron Devices 41, 709 (1994).
H... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Furthermore, methods, principles, and evaluation examples are described for microwave photoconductive decay *(ft-PCD)*, SPY, the reverse bias leakage current, MOS C-t, and the evaluation method of gate oxide integrity. The degradation mechanisms of the oxide film caused by metal contamination have been covered in Chap.... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
An example *S-p* characteristic is shown in Fig. 15.2. In this example, wafers with resistivity below 1 Dcm or over 200 Dcm have small LS and therefore the microwave sensitivity is not sufficient to determine *T1.* The distribution of carriers after light exposure is theoretically calculated under the assumptions that ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
$$F=\frac{P}{(h c/\lambda)\pi\phi^{2}}\quad.$$
$$\left(2\right)$$
The number of injected carriers (L'::.n = *L'::.p)* is given by
$$\triangle n=\triangle p=\alpha F T p\quad.$$
$$\left({\boldsymbol{3}}\right)$$
$$\left(4\right)$$
Tb was completely analyzed by Shockley and Read. The recombination velocity U is give... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
$$\tau_{b}=\triangle n/U\quad.$$
Tb = *6njU* (9)
From (4) it follows that
$$\tau_{b}=\frac{n_{0}+n_{l}+\triangle n}{(n_{0}+p_{0}+\triangle n)C_{p}}+\frac{p_{0}+p_{l}+\triangle n}{(n_{0}+p_{0}+\triangle n)C_{n}}\quad.\tag{10}$$
In this equation, Po and no denote the density of holes and electrons at thermal equilib... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
metals from the surface into the bulk with a heat treatment. Furthermore, in order to investigate only the influence of heavy metals, it is necessary to use wafers without gettering sites, because the gettering sites may shorten lifetimes and interact with heavy metals.
15.2.1.2 I... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
To examine the reasons for the lowering of *Tl,* deep-level transient spectroscopy (DLTS) is used for the trap evaluation, and Spy is used for the Fe contamination evaluation and the examination of the bulk lifetime. Furthermore, analysis by total reflection X-ray fluorescence spect... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Interstitial Fe forms a deep level with Ev + 0.39 eV, which lowers Tl [9,10].
Fig. 15.7. Optical micrograph of the wafer contaminated with Ni and subjected to oxidation at 1000 ° C for 10 minutes in dry 02 after bevel polishing followed by selective etching
Ni: At high temperatur... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
W: Figure 15.8 shows the relation between the surface W-concentration analyzed with VPD-AAS and the trap density due to W measured by DLTS.
The deep level of W in n-Si is *E c* - 0.17 e V, and in p-Si Ev + 0.42 e V [6].
Since their concentrations differ by one order of magnitude, W in n-Si is electrically almost in... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 15.2.2 Spy Method
15.2.2.1 Principles. For the Spy method it is necessary to form a depletion layer or an inversion depletion in the bulk near the wafer surface.
Therefore, charges of the same type as the majority carriers are attached to the wafer surface. n-Si wafers are boi... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Space charge density *(Qsc)* + Surface state charge density *(Qss)* = o. (13)
Under the assumption that L:.V «: *kTlq* it follows from (11) and the relation of L:. V und L:.p
$$\Phi_{e f f}/\triangle V=A(S+D/L)(1+1/L\alpha)\quad.$$
$$(14)$$
For the case that an depletion layer is formed
$$A=(q n_{0}/k T)\exp(q V/... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
On the other hand, for the linear Spy method a light intensity of 1012_
1014 photons per cm2 is used; furthermore, it is not necessary to adjust the light intensity. However, due to the restriction of the measurable diffusion length by the wafer width (L < *dI2),* it is not possible to measure long diffusion lengths. ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
The diffusion lengths of wafers without controlled contamination are restricted by the wafer thickness, and the measured Tr is smaller than the real value. In the following the influences of the different metals on the lowering of T1 in Si are mentioned.
Fe: 7 r became shorter whe... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Cu: Cu contamination led to a lowering of 7 r in n-Si, but not in p-Si. With the *{L-PCD* method, the same tendency was detected for 71. As described before, Cu diffusing in n-Si creates deep levels leading to a lowering of 7 r ;
however, Cu is in an electrically inactive state in p-Si.
15.2.2.4 Methods for Evaluati... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
15.2.2.5 Influence of Surface Copper Contaminants on Surface Re-**combination Velocity.** An Spy analysis on a wafer after Cu contamination was performed [21]. After contaminating 1/4 of the p-Si surface with BHF
solution containing 0.1 ppm Cu the SPY analysis was performed. The measured diffusion length showed almost... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Here, Dn is the diffusion coefficient of the electrons, f is the Fe-B pair dissociation ratio, and *Lo, L1* are the diffusion lengths before and after the FeB pair dissociation, respectively. Cn (Fei), Cn (Fe-B) are the electron capture rates of Fe+ donors and Fe-B pairs, respectively. With the DLTS measurement of [Fe]... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
15.2.2.7 Measuring Fe Concentration in Bulk. Figure 15.11 shows the relation between the [Fe] obtained with the SPY and DLTS methods. For a wafer with a thickness of 650 *j.Lm,* the relation becomes worse when Fe concentration is below 2 x lQllcm-3 . Therefore it is considered that the detection limit of this Spy syste... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
$$U=-n_{i}/\tau_{g}$$
$$N_{g e n}=-(n_{i}/2\tau_{g})W_{d}A\quad.$$
The number of carriers generated in the depletion layer, *Ngen* , is given by
$$(20)$$
$$(21)$$
$$(222)$$
Here, A is the area of the pn junction and Wd the depletion layer thickness.
Therefore, the generation current Ig is given by
$$(23)$$
$$I_... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Figure 15.12 shows the relation between surface metal concentration measured by VPD-AAS and IT [26]. In the following the influence of the metals on IT and the cause of the phenomenon will be discussed.
Fe: *IT"* slightly increased as the surfa... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Cu: Only at a surface Cu concentration of 2 x 1013cm-2 was a distinct increase of Ir observed. From a Cu depth profile by SIMS in the n+ region of the sample with surface Cu concentration of 2 x 1013cm- 2 , Cu in the n+
region is almost totally gettered also, as shown in Fig. 15.16. However, as shown in Fig. 15.14, ma... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
This corresponds to the density of holes, which are minority carriers already injected in the n-region at t = 0; the density decreases from the distribution at t = 0 shown in Fig. 15.20b to that at t = tl when the reverse bias is applied to the p... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
In the following the influences of Fe, Ni, or Cr on Trr are described.
Fe: Compared with the influence on Ir in Fig. 15.12 the influence of Fe on Trr was larger. As follows from (26), Trr corresponds to the minority carrier recombination lifetime. The recombination lifetime is lowered sufficiently at energy levels wh... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
15.4.1.1 Measurement Technique. After measuring the dopant concentration by a high-frequency C-V method, the C-t characteristic is measured.
The C-V and C-t measurements are performed by the author in the apparatus described in Fig. 15.22.
The LCR meter has the function of a cap... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
C denotes the total capacitance, *Cox* the capacity of the oxide layer, Cf the balancing capacitance, ni the intrinsic concentration of carriers, *Nsub* the concentration of dopant, and *KsfO* the permittivity of Si.
Equation (21) can be derived because Tg is due to the deep level... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
increased by doping during crystal growth [29]. As shown in Fig. 15.4, as the deep levels formed by Fe deviate from Ei , they do not contribute to T 9 . On the other side, the surface Fe contamination shortens T9 due to the formation of micro-defects induced by Fe in the bulk near S... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
15.4.2.1 Gate Oxide Integrity (GOI). In general, two different methods of analysis of the breakdown strength of the oxide film are in use. As shown in Fig. 15.26, one method is the measurement of current during the imposition of a step-like voltage in the accumulation direction between the MOS capacitor electrode and t... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
A schematic circuit using the second method is shown in Fig. 15.28. A
constant current is applied to the MOS capacitor and the voltage is measured.
Good quality is defined for the case that a certain time at a constant field strength (for example 8 MV *jcm)* is sustained. This method is called the con-
![242_image_... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Figure 15.31 shows the TDDB measurement results for MOS capacitors depending on the surface Fe concentration [30]. The measurement was performed at 24 DC in a 11 MV /cm electric field. At a surface Fe concentration of as little as 1O-llcm2 TDDB degradation was detected.
The actual... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
[4J M. Hourai, T. Naridomi, Y. Oka, K. Murakami, S. Sumita, N. Fujino, and T. Shiraiwa: Jpn. J. Appl Phys. 27, L2361 (1988).
[5J M. Miyazaki, T. Yanai, M. Sano, and N. Fujino: Ext. Abstr. 38th Spring Meet.
Japan Society of Appl. Phys., 30p-ZL-6 (1991) [in JapaneseJ.
M. Miyazaki, T. Yanai, K. Ide, S. Mizouchi, K. ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Lett. 63, 2902 (1993).
J. Lagowski, M. Dexter, and P. Edelman: The National Renewable Energy Laboratory Workshop, Colorado (1992).
G. Zoth and W. Bergholz: J. Appl. Phys. 67, 6764 (1990).
D. K. Shroder: IEEE Trans. on Electron. Dev., ED-29(8), 1336 (1982).
M. Miyazaki, M. Sano, S. Sadamitsu, S. Sumita, N. Fujin... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Furthermore, the electronic and physical properties can be analyzed simultaneously. There are almost no restrictions on the observation environment; the analysis is possible in air or in gas, in vacuum and in liquids [3]. These features are superior to electron or optical microscopes.
STM is of interest for the analy... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Furthermore, by changing the bias voltage it is possible to detect the density of states of the sample. Analysis of the density of states by measuring the change in the tunneling current when altering the voltage is possible using scanning tunneling spectroscopy (STS) [4,5] and, esp... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
There are two methods of force detection: (1) measurement of the force in the repulsive region and (2) measurement of the force gradient in the attractive region. The first method uses the bending of the cantilever directly.
(a) (b)
Here, the ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
An image of adsorbed molecules on the Si surface can be obtained in ultrahigh vacuum after thermal cleaning of the Si surface at high temperature.
Figure 16.3a shows the image of a 7 x 7 reconstruction Si(l11) surface. As can
be seen, STM has a resolution up to atomic order. Rec... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Recently, dynamic observation of changing surfaces during MBE or annealing has become popular. In the following, some examples of the research are described.
## 16.3.2 In Situ Observation During Crystal Growth
Hasegawa *et al.* used *in situ* STM for the observation of Si crystal... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Avouris et *al.* [23] and Hasegawa et *al.* [24] use UHV-STM for the analysis of the initial silicon oxidation process. When the Si(lll) 7 x 7 reconstruction surface is covered with only a 0.2 oxygen atomic monolayer, dark spots and bright spots appear at places where oxygen is absorbed at locations of displaced Si ato... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Furthermore, boiling this in ultrapure water leads to the creation of a surface terminated with a monohydride (SiH) layer. They showed that this surface treatment prevents growth of the native oxide layer on the sample.
## 16.3.6 Observation With Atomic Resolution Using Afm
Many layer-by-Iayer materials can be obser... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Observation of the distribution of impurities and the pn junction, as well as of the defect distribution, are also studied.
## 16.4.1 Evaluation Of Insulator Surface Morphology
Many scientists have investigated this theme. In the following, only representative examples are described.
16.4.1.1 Degradation in Oxide ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 16.4.2 Dependence Of Native Oxide Breakdown Voltage On Si Facet Surface [38]
Recently, it has become clear that the breakdown voltage of the oxide film depends on the oxidation temperature. Degradation of the breakdown voltage is especially severe at temperatures above 900°C. The oxidation temperature dependence of... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
16.4.2.1 Si Facet and Native Oxide Film Growth. For samples, many facets were formed on Si(111) surface by electric heating in ultrahigh vacuum, and those that formed a native oxide film after leaving them in air for a long time were used in the experiment. As samples, n-type Si(111) with conductance of 0.5 *Dcm* was u... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
The above analysis shows that surfaces with low coefficients, such as (111),
(110), and (001), have high resistance against -10 V, but the resistance of surfaces near the (111) surface with specific high coefficients gives rise to native oxide with weak breakdown voltage. This result can be obtained by measurement of ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
In order to understand why the dispersion becomes large, AFM and SEM
(scanning electron microscope) analysis of the poly-Si/Si02 was performed.
After removal of the top layer of poly-Si, small mountains like meshes have been observed.
Figure 16.9 shows AFM images for annealing ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
growth rate of oxidized material influences the dispersion. The TEM analysis
(Nanoprobe-TEM-EDX, transmission electron microscope-energy dispersive X-ray spectroscopy) of the granular surface of poly-Si shows that the phosphorous atoms are segregated.
The reason is that when the ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 16.4.4 Distribution Of Impurities On Poly-Si Surfaces (Stm-Tbi Measurement) [9]
TBI (tunneling barrier-height imaging) detects the change of the tunneling barrier height, dependent on the density of impurities, leading to an alternation of the band structure. In the following, the analysis of the distribution of im... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 16.4.5 Observation Of Silicon Pn Junctions (Stm-Cits Measurement) [7]
A pn junction treated with annealing and As ion implantation on a p-Si substrate is observed with the STM-CITS system. The STM observation is performed in vacuum immediately after chemical etching of the surface oxide film. Figure 16.11a is the S... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
At 2 V, no current flowed in the p-region, only in the n-region. This property is inverse to the ideal property. This could be caused by an inversion layer for the generation of trap levels in the surface, leading to growth of the depletion layer on the surface when electrons are incident on the surface from
![258_im... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 16.4.6 Other Measurement Techniques
There exist many detection techniques related to STM and AFM. In the following, we present only some of them.
16.4.6.1 BEEM (Ballistic Electron Emission Microscopy) [39]. This method uses the dependence of the electron emitted from the STM tip on the height of the Schottky barr... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 16.5 Advice On Stm And Afm Operation
1) Important for the observation of atomic images are the questions of if the sample surface is clean and whether the tip terminates with just one atom. The images obtained are influenced by these questions. In particular, multiple tips lead to different images of the atomic con... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
2) For analysis or observation at nm order, AFM is used more than STM, | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
because the observed samples contain almost insulating materials. The shape of the end of the tip is a key point for the AFM observation. The tip is less sharp than in the STM system; therefore care must be taken when high aspect ratio samples are observed. At the edge of the surface structure, the shape of the tip man... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
system, the STM system can be used for observation of high-aspect ratio structures, but not for sample surfaces containing insulating material and native oxide film covered surfaces. When the native oxidation of a Si surface is suppressed by HF treatment, STM observation remains possible under atmospheric pressure for ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
We have presented the STM and AFM systems and their use in measuring techniques and showed that it is possible to analyze microscopic areas of the Si surface. In contrast to standard techniques, these techniques have nm or atomic orders imaging resolution. However, to interpret the nm order analysis described in Sect. ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
[8] G. Binnig and H. Rohrer: IBM J. Res. Develop. 30, 355 (1989).
[9] S. Hosaka, K. Sagara, S. Hosoki, T. Hasegawa, and T. Takata: J. Vac. Sci.
Technol. A8, 270 (1990).
[10] G. Meyer and N. M. Amer: Appl. Phys. Lett. 53, 1045 (1998). [11] Y. Martin and H. K. Wickramasinghe: Appl. Phys. Lett. 50, 1455 (1987). [12]... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
213, 157 (1989).
Ph. Avouris, In-Whan Lyo, and F. Bozso: J. Vac. Sci. TechnoI. B9, 424 (1991).
T. Hasegawa, M. Kohno, S. Hosaka, and S. HosoH: Surf. Sci. 312, L753 (1994).
J. J. Boland: Surf. Sci. 244, 1 (1991).
Y. Morita, K. Miki, and H. Tokumoto: AppI. Phys. Lett. 59, 1347 (1991).
Y. Morita, Y. Miki, and H.... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Ito *et al.:* Ext. Abstract '94 Spring Meeting, Jpn. Inst. Ele. Inform. Com.
Eng. (1994) 25 [in Japanese].
S. Hosaka, H. Koyanagi, T. Hasegawa, S. Hosoki, and A. Hiraiwa: J. AppI.
Phys. 72, 688 (1992).
W. J. Kaiser and L. D. Bell: Phys. Rev. Lett. 60, 1406 (1988) and Phys. Rev.
Lett. 61, 2368 (1988).
P. Mur... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Furthermore, up to now particle assessment has for the most part entailed measuring the number of particles and their diameters, but in order to determine the origin of contamination and devise an effective countermeasure for its prevention, it is also essential to analyze the particle composition and chemical states. ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
For a more detailed explanation of these principles, see Chap. 12.
## 17.2.1 Measuring Particles On Polished Wafers
When a laser beam irradiates particles adhered to the wafer, the scattered light generated is comprised of (1) reflection of forward scattering light off the surface; (2) back and side scattering light... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
| inspection equipment Com- Model Detection | Light | Scan | Through- | Wafer | Cell size | | |
|----------------------------------------------|-------------|-----------|------------|---------|-------------|------|---------------|
| pany | s... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
In general, in order to increase high sensitivity, besides the above-mentioned approaches to widening the solid angle of light gathering, there have also been efforts to reduce the laser's wavelength, increase the laser's output power, increase the application energy density by beam focusing, and reduce the various for... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
As a result, there are many occasions on which the particle-size classification cannot be accomplished with full certainty. Though fundamental solutions for this problem are difficult, a combination of two kinds of polarized light is being used in an attempt to improve the accuracy of the classification [5].
Because ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Recently, equipment specialized for wafers with films deposited on them has been produced (with functions so that even a wafer with a film can be detected with relatively high sensitivity - see Table 17.2). The method adopted here diminishes the influence of reflection from the film surface by making the laser light's ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Depending on the wafer, most of the things counted as particles of about 0.10 *J.Lm* are not particles at all, but rather surface micro-roughness and etch pits. One of the most urgent tasks, then, is to develop a technology which can distinguish particles from pits. Without this, competition for high sensitivity is poi... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 17.2.2 Measuring Particles On Patterned Wafers
In order to detect particles adhered to the surface of patterned product wafers, equipment must be modified so that it can distinguish scattered light from a particle and scattered light from a pattern edge. In fact, there is wafer surface inspection equipment currentl... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Methods which incorporate things other than light scattering include automatic pattern defect inspection equipment, which conducts die-to-die (or cell-to-cell) pattern image comparison by computer, but to inspect the entire wafer takes some time. Recently, with the progress of technology, because the speed of image pro... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
At the site of semiconductor manufacturing, the use of this patterned wafer inspection equipment enables in-process particle management. Part i-
| Table 17.3. Representative patterned wafer surface detection equipment Company Model Throughput Light source Detection method Hitachi IS-3270 20 " Semicond. Scattering H... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Table 17.3. Representative patterned wafer surface detection equipment cle emission during each of the process steps, including that from transport and reactions, can be assessed. Also, it is becoming possible to grasp quantitatively the way that particles adhered to wafer products in intermediate process steps exert a... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 17.2.3 Measuring Particles In The Lsi Manufacturing Lines
On mass production lines, particle generation has always been checked with mirror wafer surface inspection equipment, but recently particle generation in the entire process has been monitored with patterned wafer inspection equipment, and both are being used... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
when it is treated together with dirtier product wafers. This is also true with ion implementation of resist-coated product wafers. Therefore, depending on the process step to be tested, it is essential to choose appropriately single treatment or treatment simultaneous with that of ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
particle generation by application of the particle analysis method described later.
(III) Stable Type. In the case that stability of particle generation can be confirmed based on the data of a particular ter... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
A method which displays great power in the morphology observation of particles adhered to the wafer is scanning electron microscopy (SEM). Even ultrasmall particles impossible to distinguish with a spectroscopic microscope can be observed morphologically by using SEM, and can, in some cases, be identified. Furthermore,... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
(B) in a way similar to WDX. With this equipment it is possible to identify particles with diameters of hundreds of nm. For analysis in a domain smaller than this, in general micro-Auger electron spectroscopic analysis (J1-AES, or scan-type Auger, SAM) are effective. However, until recently, with J1-AES,
if the wafer ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
In contrast to organic compounds, most of which emit fluorescent light, inorganic substrate materials such as the silicon (Si) substrate, conventional silicon oxide film (Si02 ), and aluminum do not, so organic particles themselves look conspicuous in fluorescent light. The resolution of micro-fluorescent spectroscopy ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Microscopic Raman spectroscopy, which employs a laser as light source, is at present generally not suitable for the analysis of organic particles. It is easy for particles to melt or even burn due to the application of the laser light, and the fluorescent light emitted by organic compounds in this case registers as a n... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
In the case of mirror surfaces, focusing on the surface is difficult. As a result, getting the analyzed particle into the field of observation requires patience
and consumes a great deal of time; it is inefficient and becomes a large obstacle in terms of conducting analysis. Incid... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Because this analysis system is not just one body of equipment, improvements in performance by renewal of parts that have shown conspicuous development are easy; also, it has another advantage in that each piece of
## 254 Takeshi Hattori
equipment can be used in place at separate sites (for example, at company headq... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers |
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