Deposition and detection of particles during Integrated circuit manufacturing Faisal Walia, D. Martin Knotterb, John J. Kellyc, and Fred G. Kupera,b a) University of Twente, b) NXP semiconductors, c)University of Utrecht, Gerstweg 2 (FB 0.050), 6534 AE Nijmegen, The Netherlands Phone: +31 (0) 24 353 3212 Fax: +31 (0) 24 353 3323 E-mail:
[email protected]
Abstract—Deposition mechanism of silica particles on silicon wafers was investigated by depositing specially prepared mono-dispersed particles (mean diameter = 330 nm). To measure particles of the size below the detection limit of our particle measurement tools, silica particles with luminance core were prepared. Three phenomena of deposition were discerned: sedimentation (diffusion driven), printing due to hydrodynamic forces at gas-solidliquid interface, and dry in of carry-over layer. Keywords— Deposition, mono-disperse suspensions, silica particles, and luminance core
I. INTRODUCTION Semiconductor industry has received a great deal of attention in recent years due to advancement and shrinkage of technological features into nano-scales. The degree of manufacturing success is measured by yield, which is defined as the average ratio of the number of usable devices that pass different tests after completing processes to the number of potentially usable devices before starting processes. By determining the probabilities of failure and all type of defects on the critical areas, it is possible to predict yield. It is believed that most of the defects are due to the particle contamination coming on the wafer during different process steps [1,2]. Most of the manufacturing facilities employ some sort of wafer inspection scheme within the wafer fabrication area in order to detect particulate defects in the process line [3]. Usually, automated inspection equipment is used to detect the defects, while a specialized microscope station is used to review and classify selected subsets of the defects after detection. Later it analyzed if these particle contaminations are resulted in “killing defects”.
Most of inspection tools are based on the light scattering principle in which a laser beam is scanning the surface of the wafer and is scattered on the defect points [3,4,5]. The critical size of the defects decreases with the same pace as circuit features. The conventional defects inspection technology based on the imaging optics ultimately reaches diffraction limits imposed by the wavelength of incident laser beam. So it becomes impossible to use them for defect detections. Our goal is to study the removal of particles smaller than the limit of typical optical detection tools, i.e., 60 nm [3]. By using model particles we can explore the use of alternative methods to detect particles on the wafers. One of the methods is to synthesize particles with luminance core [6]. The core of the particle uses fluorescence emission to facilitate detection by using an UV spectrometer. A benefit of these particles is that they are distinguishable from particles not under study. To reduce the number of particle defects on the wafer surface it is essential to comprehend the particle deposition mechanism during production. In the last decade, studies have been published on deposition of particles by immersions of wafers in a bath [7,8]. In our study we focused on preparation of nano-sized silica particles with and without luminance core and the deposition of these silica particles on bared wafers by using a single wafer spinner.
II. MATERIALS AND METHODS A. Chemicals The water used in all experiments was ultra pure water (UPW). The pH of the solutions was adjusted with 20 % ultra pure hydrochloric acid (HCl) from Tema Chemical
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Co. Ltd (Tamapure-AA-10) with a purity of 1 ppb of metals. For preparation of silica particles, ammonia and ethanol from Merck and TEOS from Fluka were used. B. Preparation of silica particles Mono-disperse colloidal pure silica spheres with and without luminance core were prepared by following a published procedure [6, 9, 10, 11]. The following reaction took place during mixing of the reactants: , ethanol Si (OC2 H 5 ) 4 +4 H 2O ⎯ammonia ⎯⎯⎯ ⎯ ⎯→ Si (OH ) 4 + 4C2 H 5OH , ethanol Si (OH ) 4 ⎯ammonia ⎯⎯⎯ ⎯ ⎯→ SiO2 ↓ +2 H 2O
Fluoroisothiocynate (FITC) was used as fluorescence dye and aminopropyltriethoxysilan (APS) was used to bound the fluorescent material to the silica matrix. C. Deposition methods For the deposition of the particles, a single wafer spinner/dryer from SPS (type ATP polos) was used. The speed of the spinner is adjustable up to 2000 rpm. A 150mm silicon wafer was placed on the spinner, 10 mL of silica particles suspension with concentration of 5⋅106 particles / mL was applied on the surface, and after a variable time interval each wafer was spun dry. Process is repeated on different wafers with different pH and with other particle concentrations.
Figure 3.1: Mono disperse pure silica particles with mean particle size 500 nm
The size of silica particles with luminance core was also measured by using SEM. Two batches with different sizes of silica particles were prepared with smaller and larger sized particles:
D. Detection of particle Inspection tools used for detecting particle contaminations on a wafer are KLA Surfscan SP1 and KLA Tencor Surfscan 6200 [4, 5]. Both tools are designed for detecting, counting and sizing particles, pits and scratches on bare wafers. The tools gave the measurements in Light point defects (LPDs).
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Mean diameter of 75 nm was prepared from TEOS (0.5 mol/ L), Ethanol (20 mol / L), and ammonia (2.5 mol / L). Figure 3.2 shows picture taken by SEM.
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Mean diameter of 350 nm was prepared from TEOS (0.5 mol/ L), Ethanol (20 mol / L), and ammonia (5 mol / L). Figure 3.2 shows picture taken by SEM.
Scanning Electron Microscope (SEM) is used as well to detect particles on the surface of the wafer after deposition. III. RESULTS
Silica particles are prepared from TEOS (0.5 mol / L), Ethanol (20 mol / L), and ammonia (5 mol / L). Figure 3.1 shows a SEM micrograph. The mean particle size of the particle was 500 nm noted with KLA Tencor and SEM. The size of the silica particles can be controlled with controlling the concentration of ammonia in the mixture.
Figure 3.3: Mono disperse silica particles with luminance core and mean particle size 350 nm
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To study the deposition mechanisms in more detail, the wafer was first wetted with DI water and subsequently silica suspensions were applied. In figure 3.6, the results are shown that the deposition rate (slope of curves) is identical for depositing particles on wet surface and dry surface. However Y intercepts of the wetted surface is much lower than for the dry surface. In case of wet surfaces, the water layer on the wafer surface inhibits the particle suspension from direct contact with surface. Particles deposited only by diffusing through this water layer.
Figure 3.4: Mono disperse silica particles with luminance core and mean particle size 75 nm
A suspension (10 mL) of pure silica particles of size 330 nm with concentration of 5⋅106 particles / mL is applied on the surface of a 150-mm bared silicon wafers. After a sedimentation period, the wafer was spin dried.
Amount of particles / wafer
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Deposition on wet wafer Deposition on dry wafer
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y = 236x - 231.4
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Time (min) y = 220x + 703
Amount of particles
y = 234.2x + 623.8
Figure 3.6: Effect of sedimentation time and amount of particles deposited per wafer (pH = 2)
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Time (min)
The sedimentation process is dominated by diffusion. The diffusion rate is dependent on pH [12]. In the figure 3.7, the deposition rate as function of the pH is shown. A significant change is noticed in slope and Y intercept of the curves. The deposition rate increased with decreasing pH value of the particle suspensions. Notable, the Y-axis intercept is also changing with pH, which is not expected from a dry-in mechanism only.
Figure 3.5: Effect of sedimentation time and amount of particles deposited per wafer (pH =2)
In the figure 3.5, the number of deposited particles versus sedimentation time is shown. By increasing the sedimentation time there is a linear increase of the amount of particles on the surface. The slope of the curve appears to be showing the deposition rate of particles and the Y-axis intercept could correspond to dry-in of carry-over layer. However, from other experiments out of the scope of this publication, we established that the carry-over layer is
