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As a result, the capacitor which is connected in series with the RF electrode is charged negatively, and a self-bias potential appears between the RF and grounded electrodes, as shown in Fig. 7.5. The potential of the discharge space is always higher than that of the wall surface and the grounded electrode, and maintai... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
and Miller reported that particles which generated RF sputtering had approximately 104 electrons [14].
In this way, we know that suspension of particles in plasma does not occur if the particles are uncharged, and that particle charging is very important for estimating the particl... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
$$p=-2.57\times10^{-12}d_{p}\,T_{+}^{0.15}\,T_{e}^{0.85}/\epsilon\quad,$$
where dp is in centimeters, T+ and Te are the ion and electron temperatures respectively, and the unit is eV. According to (16), particles of dp = *l/-Lm* have a charge of p = 3000 electrons at T+ = 0.0258 eV and Te = 4 eV.
Also, Sommerer and ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Another phenomenon that needs to be considered is viscous ion drag.
Positive and negative ions in a plasma move toward the electrodes at high velocity accelerated by the electric field. These ions collide with particles, and as a result particles experience a force in the direction of the electrodes.
Sommerer and h... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
From experimental results that show that the position of the particle trap is displaced by the temperature of the electrode, the electrostatic particle trap is thought to be caused by thermophoresis [20J. For the trap, the balance of the Coulomb force due to the bias voltage of the plasma and gravity also cannot be ign... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
It was also observed that particles were trapped inside the ion current to the wafer. These experimental facts suggest that particle deposition in an ECR
plasma can be evaluated by (14) and (18) if the particle charge is obtained.
Particle charging in ECR plasma can generally be expressed by (15).
However, solutio... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
[1] L. Waldmann and K. H. Schmitt: Aerosol Science (ed. C. N. Davies), Academic Press, 137 (1966).
[2] L. Waldmann: Z. Naturf. l4a, 589 (1959).
[3] S. P. Bakanov and B. V. Deryagin: Discuss. Faraday Soc. 30, 130 (1960).
[4] G. S. Selwyn: Jpn. J. Appl. Phys. 32, 3068 (1993).
[5] G. S. Selwyn and E. F. Patterson:... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
[18] M. J. McCaughey and M. J. Kushner: Appl. Phys. Lett. 55, 951 (1989).
[19] M. J. McCaughey and M. J. Kushner: J. Appl. Phys. 69, 6952 (1991).
[20] G. M. Jellum, J. E. Daugherty, and D. B. Garves: J. Appl. Phys. 69, 6923
(1991).
[21] K. T. Whitby and B. Y. H. Liu: Aerosol Science (ed. C. N. Davies), Academic ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
cannot be removed after adhering, regardless of the mounted position of the wafer on the stage.
For this reason, in developing countermeasures for particle reduction, more important than wafer mounting position is to avoid generating particles, and if generated, to prevent their adhesion to the wafer.
## 8.2 The Be... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
where k is the Boltzmann constant and T is the temperature of the gas.
$$({\mathfrak{3}})$$
Both descent velocity v and the average distance moved X caused by Brownian motion are a function of the mean free path .A, so they change according to gas pressure *P (ex* 1/ .A). Figure 8.1 is the result of calculating the ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
There are also data which show that even more swirling occurs during ventilation. Figure 8.3 shows the number of particles adhering to a wafer due to particle swirling for different time constants of pressure changes during pump-down and ventilation as parameters. In fast pressure changes (or for
of particles already existing in the original vacuum chamber can also be achieved, then it is possible to shorten the times of slow pump-down and slow ventilation. To that end, there is a method in which particle swirl caused by rapid pump-down and ventilation can be utilized in a p... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
So, the change in the number of particles adhering to a wafer during ventilation, *i1N* s, is given as
$$\Delta N s=\gamma(1-k)\,a(N+N s)-a N s\quad,$$
$$(4)$$
where --y is the ratio of the space directly over the wafer to the capacity of the whole vacuum chamber. Also, the change in the total number of particles
... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 8.3 Influence Of Static Electricity
As has already been shown in Table 8.1, the contribution of static electricity as a force which adheres particles to the wafer is extremely small. However, there has for a long time been research conducted on the positive use of static electricity to capture particles within the ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Table 8.2 gives the results of a capturing experiment conducted when particles with a diameter of 60 */-lm* (±10 */-lm)* were intentionally placed on substrates (made of SUS, AI, Cu, Si wafer, quartz) at pressures from 1O~4 rv 1O~5 Pa. The ratios of particles captured were measured visually. As a result,
| Table 8.2.... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
over 90% of metallic particles were captured by static electricity alone. When the range of materials shown in Table 8.2 is used in mixed forms, no matter the kind of substrates, particles can be captured by the combined use of electron beams and static electricity provided by a high-voltage power supply whose polarity... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Figure 8.8 shows the results of particle generation measurement from bearings coated with silver for vacuum application. This shows the number of particles of diameter 0.3 */-Lm* and larger, generated when the bearings are subject to rotating in a vacuum at 10 Pa. (Particles generated under this pressure are thought to... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
In the process chamber for plasma CVD or related equipment, measurements have been taken, by laser scanning, of particles floating in the space where the plasma is generated [3-5]. Figure 8.10 gives a schematic of those measurements. Scattered light caused by particles is captured by a video camera, and the space distr... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
(a) Interior structure of equipment as seen from the video camera
(b) Photo of laser scattering in Ar plasma at 27 Pa
(c) Photo of laser scattering in the part near the electric pole 2 mm from (b).
From th... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 9. Particle Adhesion In Liquids
Akio Saito and Katsuhiro Dta, Hitachi Particle adhesion in liquids can be thought of as resulting from the van der Waals and electrostatic repulsive forces which exist between the substrate and the particle. In contrast with that existing in air, the static electric repulsive force i... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
The total potential energy is expressed by Va + *Vr,* and depends on the ionic strength of the solution, the surface potential (zeta potential) of the wafer and particle, and the particle diameter.
Furthermore, the diffusion equation for the stationary state is expressed as the following (here considered in one dimen... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
The solution obtained is the following theoretical formula (7) which describes particle adhesion.
## 9.2 Test Results Using Standard Polystyrene Particles
$$\left(7\right)$$
j: number of adhered particles nd: particle concentration D: particle diffusion coefficient d: thickness of diffusion layer w: potential of va... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
formula (7) are easily calculable from the experimental conditions. The experiment which employed the above-mentioned method 1 was conducted in order to clarify the diffusion layer thickness d. With a rotating disk, the laminar flow boundary layer becomes a uniform thickness, so the diffusion layer thickness d value ca... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
per unit time (the slopes in Fig. 9.4) are in proportion to particle density.
These are in fundamental agreement with the results obtained from formula
(7), and it is thought that this formula can additionally be adapted to elucidate adhesion in baths.
Furthermore, even if the ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 9.3.2 Zeta Potential
The zeta potential is the surface charge of a particle in a liquid. A particle's zeta potential can be measured by electrophoresis [4]. Furthermore, it is clear that the zeta potential is unaffected by the diameter of the particle [5], so it can be extrapola... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Next, in order to consider the conditions under which particle adhesion to the wafer occurs, the effects of D and d are omitted. Figure 9.8 shows the relationship between diameter and the deposition coefficient (in (7), the
number of deposited ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
As shown in Figs. 9.3 and 9.4, even a line extrapolated to an immersion time of 0 minutes will not pass through the origin. The reasons for this can be thought of as the following:
1) Particles adhere while the wafer is being immersed or pulled up out of the bath.
2) Rapid deposition in a short interval (within a mi... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
$$(10)$$
2 Fig. 9.9. Non-stationary state adhesion
The solution was obtained from [7] and calculations using the experimental conditions of Fig. 9.3 produce the results shown in Fig. 9.9.
We know that rapid deposition occurs within 1 minute, and after 1 minute stationary adhesion is experimentally viewable. Further... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
[1] R. Hogg *et al.:* Transactions of the Faraday Society 62, 1638 (1966).
[2] A. Prock *et al.:* Topics in Chemical Physics, Elsevier (1962).
[3] M. Noguchi *et al.:* Jpn. J. Appl. Phys. 32, 352 (1993). [4] A. Delgado *et al.:* Acta Polymerica 37, 361 (1986).
[5] K. Ota *et al.:* Proceedings of the 23rd Autumn M... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
The fundamental interactions occurring between particles and the wafer surface in solutions are the van der Waals force (a molecular reciprocal action)
and the electrostatic force (the reciprocal action of an electrical double layer).
Research related to the particle adhesion mechanism on wafer surfaces in solutions... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Figure 10.2 shows a schematic diagram of a particle and a wafer surface in a solution. Both surfaces are conjectured to be, for example, positively charged due to the absorption of H+ ions. In the vicinity of the charged surface, negative ions are influenced by static electricity attraction and irregular Brownian motio... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
a... 10-18 Fig. 10.3. An example of a calculation of the reciprocal inter10-17 action between a particle and the wafer surface
potential. It is speculated that the zeta potentials of both the particle and the wafer surface are the same, so the electrostatic potential which accompan... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Next, some experimental results related to particle deposition will be introduced. Figure 10.4 shows the relationship between the density of 0.5 pm PSL (polystyrene latex) particles floating in Dr water and the number of particles adhering to a 5 inch wafer surface.
Two wafers were immersed for 10 minutes in Dr water... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Fig. 10.4. Relationship between the number of particles adhering to a 5 inch wafer and PSL density floating in DI water
condition the electrical double layer repulsive force was superior to the van der Waals force, so there was no PSL particle a... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 10.2 Relationship Between Ph In Solution And Particle Adhesion On The Wafer
In the reciprocal interaction between a particle and a wafer in a solution, the electrical double layer reciprocal action is dependent upon the relationship of the zeta potentials of the particle and wafer; if they are of the same signs rep... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
According to this formula, the degree of movement U is related to the zeta potential ( through the following formula:
$$(1)$$
$$U=\frac{\epsilon\zeta}{4\pi\eta}\quad,$$
where E is the permittivity of the solution, and rJ is the viscosity of the solution. In other words, with this equipment, when voltage is applied t... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Not only the oxidized surface and Si02 , but also Si3N4 and PSL particles, are influenced by the pH of the solutions. Their zeta potentials show positive charges with acidity, and negative with alkalinity. Furthermore, the Si surface shows a negative charge regardless of changes in pH. Next we will discuss the results ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Figure 10.9 shows the relationship between particle deposition number on a 4 inch wafer surface and 0.6 *11m* diameter PSL particle concentration in
0.5 wt% HF. The wafer immersion period was 10 minutes. The zeta potentials of the PSL particles... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
value of the deposition number was different from that for PSL particles, the trend was the same.
In this way, with particles and wafer surfaces in solutions, differently signed zeta potentials facilitate the action of both the electrical double layer static electricity attraction and the van der Waals attraction, an... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 10.3 Controlling Particle Adhesion With Surfactants
Surfactants are made up of hydrophobic groups and hydrophilic groups, and can be divided into three categories according to the ionization of the latter.
Surfactants containing negative hydrophilic ions such as R-S03 are called anionic surfactants; those contain... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
In Table 10.1 the anionic and cationic surfactants were each added in amounts of 200 ppm.
## 10.3.1 Controlling Particle Adhesion On The Si Surface
First, control of particle deposition On the Si surface by the addition of surfactants will be discussed. Figure 10.11 shows the particle deposition number of a 4 inch w... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
| PSL zeta | Si zeta | Particle deposition | |
|-----------------------------------|-----------------|-----------------------|--------|
| potential (m V) | potential (m V) | (particles/4" wafer) | |
| without | +39 ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
However, in this experiment nearly 10000 adhered PSL particles were seen. Particle adhesion at the time of the addition of the cationic surfactant is thought to be due to the following:
1) In 0.5 wt% HF supplemented by a cationic surfactant, PSL particles are controlled with a positive zeta potential.
2) In 0.5 wt% ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Figure 10.13 shows the typical particle deposition control results when the various surfactants have been added to HF. When no surfactant is added, because the Si surface has a negative zeta charge and the PSL has a positive zeta
potential, particles deposit due to the attractive ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 10.3.2 Controlling Particle Adhesion On An Si3N4 Surface
In Table 10.1, zeta potentials have been shown. However, the Si3N4 surface differs from the Si surface in that in pH 3.3 Hel, without the addition of a surfactant, it carries a positive zeta potential. Therefore, if an anionic surfactant is applied to Si3N4, ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
I wi1l>Oul.url_ ..-ic A cationic A I ..... -.-...... Fig. 10.15. Relationship between particle deposition number and immersion period in 0.5% HF
Figure 10.15 shows the relationship between PSL particle deposition and immersion period of a 5 inch Si3N4 wafer when 0.6 */-lm* PSL part... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
positive zeta charges; the addition of cationic surfactant produces the same result: a positive zeta potential. However, with the Si3N4 surface, although no increases were noticed due to the immersion period in surfactant-augmented dilute HF or in cationic surfactants, PSL particles did adhere and deposit.
The Si3N4 ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
3) With surfaces that have opposite signs before and after the orientation of either an anionic surfactant or cationic surfactant, particle adhesion at the time of wafer immersion cannot be controlled. Therefore, even if both surfactants are employed, cleaning solutions containing the absolute least possible number of ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
| Si surface ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
| Anionic A ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 10.3.4 Controlling Particle Adhesion With Surfactants
Figure 10.16 summarizes the methods by which particles adhere to the inprocess wafer surface when surfactants are used. In acidic solutions such as HF, the Si surface shows a negative zeta potential, and the Si3N4 surface shows a positive potential. Therefore, i... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
2) In alkali solutions, particles and wafer surfaces show a negative zeta potential, so between the surface and the particles which are separated from the surface, the electrical double layer electrostatic repulsive force acts to remove particles.
In the section that follows, the particle removal mechanism will be ex... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
4 : 1 : 5 -> pH 9.8, 0.26 nm/min.
9 : 1 : 5 -> pH 10.5, 0.28 nm/min.
(when 20 wt% TMAH was used)
2 : 1 : 5 -> pH 11.2, 0.32 nm/min.
4 : 1 : 5 -> pH 12.0, 0.35 nm/min.
With NH4 0H/H2 0 2 /H2 0 and TMAH/H2 0 2 /H2 0 solutions it is possible to control the Si etching rate by modifying the NH4 0H or TMAH mixture r... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 10.4.2 Comparison With Acidic Solutions
Alkali/hydrogen peroxide solutions have a high efficiency of particle removal when the Si etching rate is 0.25 nm/min and above. The results of a comparison between these particle removal efficiencies and those of typical acidic solutions are shown in Table 10.6. Among the ac... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
| Table 10.6. Comparison of particle removal efficiencies Solution Contaminant Hot UPW NH4 0H· H2 0 2 NH4 0H· H2 0 2 | TMAH· H2 0 2 | | |
|----------------------------------------------------------------------------------------------------------------... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
| 80 8.8 | 98.7 84.0 97.9 82.3 9: 1 80 10.5 | | |
| 80 - ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
| 97.1 | 9.7 | | |
| 55.2 ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
the zeta potentials of the wafer surface and particles are all negative, particles that separate from the surface due to slight etching are removed by the action of the repulsive force of the electric double layer. Additionally, for wafers that were contaminated with city water or outside the cleanroom environment the ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
It has been pointed out that as the pH of alkali/hydrogen peroxide solution becomes higher, its oxidation ability increases [14]. Figure 10.19 shows the forms obtained after the addition of 1 *J-Lm* diameter PSL particles to each of the cleaning solutions, followed by a treatment for 10 minutes at 80 oe.
![159_image_... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
[1] M. Itano, F. W. Kern, Jr., I. Kawanabe, M. Miyashita, R. W. Rosenberg, and T. Ohmi: Particle deposition and removal in RCA cleaning process, in Proceedings of 8th Workshop on Ultraclean Technology, Tokyo, 35-55
(Dec. 1990).
[2] M. Itano, F. W. Kern, Jr., I. Kawanabe, M. Miyashita, R. W. Rosenberg, and T. Ohmi: P... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
[8] A. Saito et *al.:* Proceedings of 8th Workshop on Ultraclean Technology, Tokyo, 19-31, Dec. 1990.
[9] M. !tano *et al.:* Proceedings of 27th Workshop on Ultraclean Technology, Tokyo, 31-40, June 1995.
[10] T. Kezuka, M. Ishii, H. Hosomi, M. Suyama, M. Itano, and M. Kubo: The behavior of particles and their depo... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 11.1 Introduction
Charges on the Si wafer and liquid crystal substrate have great and harmful effects on product yield and reliability. Most of these problems, such as the deposition of airborne particles due to static electricity, device defects due to static discharge, electron beam exposure time, etc., constitut... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 11.2 Particle Adhesion Caused By Static Electricity [2,3]
11.2.1 Results of Measuring Particle Adhesion Speed on a Charged Wafer Surface To determine the effects of surface charges on the adhesion of airborne particles to the cleaned surface, we conducted the following measurements and comparisons of particle depos... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 11.2.2 Calculated Comparison Of Floating Range For Particles To Adhere To A Charged Wafer Surface
We have seen from experimental results that when a particle adheres to a wafer surface the effect of static electricity is great. Using the charge of the surface where the particle adheres and the size of the particles... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
been assigned using wafer radius as scale units which proceed outward from the center. For a wafer with a 50 V potential, the floating range of adhered particles is quite narrow, and the deposition amount is very small. However, when the wafer potential climbs to 300 V or 1800 V, th... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 11.3 Actual Wafer Charging Situations In Production Processes [3,4]
Using an electrostatic voltmeter (Monroe Electronics Model 244), we estimated the charge conditions of the semiconductor manufacturing process and the surrounding environment. What follows is a summary of the results of the investigation.
Even af... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
| Table 11.1. Wafer charging in the photolithographic process Process Wafer potential [V] Process Wafer potential [V] After wet After setting process over -3000 in a stepper +30",+35 1 (metal stage) After N2 After gas blow over -3000 exposure +55 .l). .l). Before dry After setting baking over +3000 in a carrie... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
cation and mask combination exposure processes, charges are considerably lower. This is because the wafer and contact body are ground conductors.
Table 11.2 gives the charged potentials of internal materials of the cleanroom and manufacturing equipment surfaces. Walls and windows, consisting of conductive materials, ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
is very low. Furthermore, the charge potentials of an acrylic acid resin box and clean paper exceed -3 kV.
In short, in most of the process steps during semiconductor manufacturing the wafer charge potential is above the kV level. Furthermore, concerning the charge on the glass substrate in liquid crystal display man... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Neutralization by way of irradiation is a new technology developed to overcome the weaknesses of the ionizer [7-9]. Ions and electrons required for neutralization are generated by ionization of gas molecules in the vicinity of a charge substrate. The wavelength chosen for the irradiation depends on the neutralization a... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
However, with transmission by air flow, many ions are eliminated due to recombination, so the neutralization capability is limited (Fig. l1.4a).
With neutralization using soft X-ray radiation, large air ion pairs can be generated by exposing the ambient surrounding the charged object with soft X-rays, and as a result... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
144 Hitoshi Inaba
, , ionizer' -
is employed close to the charged substance, and in this case extremely high neutralization capability was achieved. Neutralization times adjacent to the neutralization equipment are ten times faster than previo... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Figure 11.6 shows the neutralization capability assessment results of soft X-ray neutralization equipment on the insulation-glass substrate, which is more difficult to neutralize than the Si wafer.
When neutralization is not performed, the electric potential of the glass substrate exceeds the surface potential scale ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
These ions are, for the wafer, undesirable impurities that accelerate oxide film formation and cause carbon contamination. However, the density of contamination was extremely low, indeed far below the level at which these kinds of damage manifest (under 1010 cm-2), so that there were very few problems. Contamination de... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 11.4.2 Neutralization With Uv Irradiation In Vacuum [7-9]
Neutralization through UV irradiation in vacuum is a new and extremely effective charge prevention technology that uses reduced-pressure N2 and argon gas ambients. It can be applied to neutralization within manufacturing equipment, or to a closed manufacturi... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
The unique feature of vacuum UV irradiation is its dramatic rise in neutralization capability under low pressure. Figure 11.8 shows the dependence of neutralization capability on pressure in N2 gas.
The neutralization time became much faster as the ambient pressure was reduced, wh... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
The great effectiveness of soft X-ray neutralization in open air has already been discussed. In contrast, the neutralization capability of vacuum UV irradiation is strongly dependent on oxygen concentration. Figure 11.9 shows the measurement results of decay time and ozone concentration when highly pure oxygen was adde... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
K. Takahashi: Applied Aerosol Engineering, Yoken-do, 144, 1984 (in Japanese).
H. Inaba, T. Ohmi, and T. Takenami: American Association for Aerosol Research, Proceedings of 1989 Annual Meeting, 196 (1989).
T. Ohmi, H. Inaba, and T. Takenami: Microcontamination 7(10), 29-32, 86-97
(1989).
H. Inaba: Journal of Japa... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
H. Inaba and S. Iwanami: Proc. of 30th Annual Technical Meeting of the Japan Health Physics Society, 3 (1995).
T. Ohmi and T. Shibata: Automated Integrated Circuits Manufacturing PV91-5, 3-64, The Electrochemical Society, Pennington, NJ (1991).
[IJ
[2J
[3J
[4J
[5J [6J [7J
[8J
[9J
[10J
[11J
[12J
Part IV
A... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 12.2 Principles And Theory Of Light Scattering
Particles can be detected by scanning the wafer surface with a laser beam and detecting the position and intensity of the scattered light. The diameter of particles to be detected is 1 *J-Lm* or less, which is of the order of the light wavelength or even smaller. The t... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
$$S(\theta,\phi)=S_{\perp}(\theta)\sin(\phi)+S_{\parallel}(\theta)\cos(\phi)\quad.$$
$$S_{\perp}=\sum_{n=1}^{\infty}\frac{2n+1}{n(n+1)}\{a_{n}\pi_{n}(\cos\theta)+b_{n}\tau_{n}(\cos\theta)\}$$ $$S_{\parallel}(\theta)=\sum_{n=1}^{\infty}\frac{2n+1}{n(n+1)}\{a_{n}\pi_{n}(\cos\theta)+b_{n}\tau_{n}(\cos\theta)\}\quad.$$ $... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
The intensity of scattered light at the unit solid angle *1(8,* cf;) is given by
$$\left(5\right)$$
$$(6)$$
$$I=I_{0}\frac{\lambda^{2}}{2\pi}\sum_{n=1}^{\infty}(2n+1)\{\mid a_{n}\mid^{2}+\mid b_{n}\mid^{2}\}=I_{0}C_{s c a t}\quad.$$
Here 10 is the illuminance of the incoming light. When we integrate (6) over the wh... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Figure 12.2 shows polar plots of scattered intensity for the distribution of intensity of scattered light calculated with the Mie theory for small particles.
The incoming light has a wavelength of 488 nm and is rectilinearly polarized.
The refraction index of the particles is 1.599 (PLS standard spheres). Below 0.1... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Because forward scattering is dominant for Mie scattering, and also exceeds 50% for Rayleigh scattering, it is plausible to assume that most of the detected light was reflected by the surface of the substrate after forward scattering. Therefore, qualitatively, the detected light intensity is equal to the product of the... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Because the wavelength, angle of incoming light, and polarization are determined by the equipment, if the actual material of the film (its index of refraction) is fixed it is possible to estimate theoretically the variation of the surface reflectivity. First, one determines the rela... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
It follows from the Mie theory that the particle detection sensitivity is first determined by the intensity and wavelength of the incoming light. With a laser:
1) it is easy to reach high energy densities of radiated light 2) variations of the beam characteristics are small over long periods 3) the design of the optic... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
For the laser light scanned on the sample, an elliptical beam in the direction of the pitch of normal scanning is used. The intensity distribution inside the laser peak follows a Gaussian distribution. In this case, in the interests of precision, it is best to maintain the intensity distribution of the irradiating ligh... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
However, if two or more particles are inside one pixel, only the largest particle will be recorded. Assuming that the particles are distributed homogeneously on the wafer surface, and that the average number of particles m inside one pixel is small (m « 1), then the number of particles x inside one pixel follows a Pois... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
As the signal from one particle reflects the intensity distribution of the laser beam, it is also Gaussian. At every sampling, the same shape of signal is reproduced; however, the intensity is maximal at the center, which can be identified with the place where the particle is. A series of signals obtained by
histogram and decreases the accuracy of measurement. This leads to a limitation on the possible distinction between particles of different sizes, which is called the particle size resolution ability. In the histogram shown in Fig. 12.9 the particle size resolution ability is given b... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 12.5 The Detection Of Particles On Patterned Wafers
From the point of view of yield analysis, control of the etching and photolithography process as well as examination of the correlation with the failure of product wafers seems practical.
However, the difficulty is that on a patterned wafer the background due to... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
As shown in Fig. 12.12b, the fact that the signal from the pattern is periodic, whereas the signal coming from the particles is random, can be used to remove the periodic signal in the image processing and to increase the *SIN*
ratio. As a resu... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
On the other side, the signals from particles or defects pass the filter and the optical image is projected on the screen. This makes it possible to extract the signal from particles and defects only [5].
In principle, when the pattern on the wafer is totally periodic, a sensitivity of 0.1-0.2 *J.Lm* can be achieved.... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
[1] M. Born and E. Wolf: Principles of Optics, Pergamon Press, Oxford (1975).
[2] J. Pecon, A. Neukermans, G. Kren, and L. Galbraith: Counting errors in particulate detection on unpatterned wafers, Solid State Technology, May, pp. 149154 (1987).
[3] R. Browning, I. Lincoln, and P. Stonestrom: Recent advances in aut... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Since chemical techniques need a preparation of sample solution and are a form of destructive analysis, they are in general reserved for off-line evaluation. However, the physical technique using total reflection X-ray fluorescence analysis is a non-destructive method for detection ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
fluorescence analysis covers a comparatively large area (about 1 cm2 ) and has a high sensitivity; therefore, this physical technique can well be applied to impurity analysis. Generally, in a way similar to that for particle measurement in the wafer process, an initial general evaluation of the gross contamination on t... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 13.2 Chemical Analysis
The chemical analysis consists in principle of (1) sample solution preparation by extraction of the impurities from the analyzed object, and (2) measurement of the impurities in the sample solution with an analytical tool. In order to obtain high sensitivity with chemical analysis, the analys... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 13.2.1 Metallic Impurities
For metallic impurities on silicon wafers, sample preparation determines the sensitivity and accuracy of the analysis.
13.2.1.1 Preparation Techniques. A surface analysis technique using HF
vapor is the technique that ideally satisfies the three points above. Figure 13.2 shows the prog... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers |
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