03 wafer prep
TRANSCRIPT
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© 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian Serda
Chapter 3
Silicon and Wafer Preparation
Chapter 3
Silicon and Wafer Preparation
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© 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian Serda
Objectives
After studying the material in this chapter, you will be able to:
1. Describe how raw silicon is refined into semiconductor grade silicon.
2. Explain the crystal structure and growth method for producing monocrystal silicon.
3. Discuss the major defects in silicon crystal.
4. Outline and describe the basic process steps for wafer preparation, starting from a silicon ingot and finishing with a wafer.
5. State and discuss seven quality measures for wafer suppliers.
6. Explain what is epitaxy and why it is important for wafers.
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© 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian Serda
Semiconductor-Grade Silicon
Steps to Obtaining Semiconductor Grade Silicon (SGS)
Step Description of Process Reaction
1Produce metallurgical gradesilicon (MGS) by heatingsilica with carbon
SiC (s) + SiO2 (s) Si (l) + SiO(g) + CO (g)
2
Purify MG silicon through achemical reaction to producea silicon-bearing gas oftrichlorosilane (SiHCl3)
Si (s) + 3HCl (g) SiHCl3 (g) + H2 (g) + heat
3
SiHCl3 and hydrogen react ina process called Siemens toobtain pure semiconductor-grade silicon (SGS)
2SiHCl3 (g) + 2H2 (g) 2Si (s) + 6HCl (g)
Table 4.1
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Crystal Structure
• Amorphous Materials
• Unit Cells
• Polycrystal and Monocrystal Structures
• Crystal Orientation
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SiHCl3
Polycrystalline silicon rod
Siemens Reactor for SG Silicon
Figure 4.1
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Atomic Order of a Crystal Structure
Figure 4.2
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Amorphous Atomic Structure
Figure 4.3
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Unit Cell in 3-D Structure
Unit cell
Figure 4.4
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Faced-centered Cubic (FCC) Unit Cell
Figure 4.5
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Silicon Unit Cell: FCC Diamond Structure
Figure 4.6
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Polycrystalline and Monocrystalline Structures
Polycrystalline structure Monocrystalline structure
Figure 4.7
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Axes of Orientation for Unit Cells
Z
X
Y
1
1
1
0
Figure 4.8
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Miller Indices of Crystal Planes
Z
X
Y
(100)
Z
X
Y
(110)
Z
X
Y
(111)
Figure 4.9
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Monocrystal Silicon Growth
• CZ Method– CZ Crystal Puller– Doping– Impurity Control
• Float-Zone Method
• Reasons for Larger Ingot Diameters
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Crystal seed
Molten polysilicon
Heat shield
Water jacket
Single crystal silicon
Quartz crucible
Carbon heating element
Crystal puller and rotation mechanism
CZ Crystal Puller
Figure 4.10
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Silicon Ingot Grown by CZ Method
Photograph courtesy of Kayex Corp., 300 mm Si ingot
Photo 4.1
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CZ Crystal Puller
Photograph courtesy of Kayex Corp., 300 mm Si crystal puller
Photo 4.2
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Dopant Concentration Nomenclature
Concentration (Atoms/cm3)
DopantMaterial
Type< 1014
(Very Lightly Doped)1014 to 1016
(Lightly Doped)1016 to 1019
(Doped)>1019
(Heavily Doped)
Pentavalent n n-- n- n n+
Trivalent p p-- p- p p+
Table 4.2
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Float Zone Crystal Growth
RF
Gas inlet (inert)
Molten zone
Traveling RF coil
Polycrystalline rod (silicon)
Seed crystal
Inert gas out
Chuck
Chuck
Figure 4.11
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300 mm
200 mm
150 mm
125 mm
100 mm
75 mm
3 4 5 6 8 12
Wafer Diameter Trends
Figure 4.12
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Wafer Dimensions & Attributes
Table 4.3
Diameter(mm)
Thickness(m)
Area(cm2)
Weight(grams/lbs)
Weight/25Wafers (lbs)
150 675 20 176.71 28 / 0.06 1.5
200 725 20 314.16 53.08 / 0.12 3
300 775 20 706.86 127.64 / 0.28 7
400 825 20 1256.64 241.56 / 0.53 13
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88 die200-mm wafer
232 die300-mm wafer
Increase in Number of Chips on Larger Wafer Diameter
Figure 4.13
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Developmental Specifications for 300-mm Wafer Dimensions and Orientation
Parameter Units NominalSome Typical
Tolerances
Diameter mm 300.00 0.20
Thickness(center point)
m 775 25
Warp (max) m 100
Nine-Point ThicknessVariation (max)
m 10
Notch Depth mm 1.00 + 0.25, -0.00
Notch Angle Degree 90 +5, -1
Back Surface Finish Bright Etched/Polished
Edge Profile Surface Finish Polished
FQA (Fixed Quality Area –radius permitted on the
wafer surface)mm 147
Table 4.4
From H. Huff, R. Foodall, R. Nilson, and S. Griffiths, “Thermal Processing Issues for 300-mm Silicon Wafers:Challenges and Opportunities,” ULSI Science and Technology (New Jersey: The Electrochemical Society, 1997), p. 139.
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Crystal Defects in Silicon
A crystal defect (microdefect) is any interruption in the repetitive nature of the unit cell crystal structure.
Three general types of crystal defects in silicon:
1. Point defects - Localized crystal defect at
the atomic level
2. Dislocations - Displaced unit cells
3. Gross defects - Defects in crystal structure
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Yield of a Wafer
Figure 4.14
Yield =66 good die88 total die
= 75%
Reduction in defect density is a critical aspect for increasing wafer yield.
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Point Defects
Redrawn from Sorab K. Ghandi, VLSI Fabrication Principles: Silicon and Gallium Arsenide, 2nd edition, New York, Wiley, 1994, page 23
(a) Vacancy defect
(b) Interstitial defect (c) Frenkel defect
Figure 4.15
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Dislocations in Unit Cells
Figure 4.16
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Crystal Slip(Gross Defects)
(a) (b) (c)
Redrawn from Sorab K. Ghandi, VLSI Fabrication Principles: Silicon and Gallium Arsenide, 2nd edition, New York, Wiley, 1994, page 49
Figure 4.17
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Crystal Twin Planes(Gross Defects)
X X’
Redrawn from Sorab K. Ghandi, VLSI Fabrication Principles: Silicon and Gallium Arsenide, 2nd edition, New York, Wiley, 1994, page 55
Figure 4.18
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Crystal GrowthCrystal Growth
ShapingShaping
Wafer SlicingWafer Slicing
Wafer Lapping and Edge GrindWafer Lapping and Edge Grind
EtchingEtching
PolishingPolishing
CleaningCleaning
InspectionInspection
PackagingPackaging
Basic Process Steps for Wafer Preparation
Figure 4.19
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Flat grind
Diameter grind
Preparing crystal ingot for grinding
Ingot Diameter Grind
Figure 4.20
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Wafer Identifying Flats
P-type (111) P-type (100)
N-type (111) N-type (100)
Figure 4.21
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Wafer Notch and Laser Scribe
1234567890
Notch Scribed identification number
Figure 4.22
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Internal diameter wafer saw
Internal Diameter Saw
Figure 4.23
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Polished Wafer Edge
Figure 4.24
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Chemical Etch of Wafer Surface to Remove Damage
Figure 4.25
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Double-Sided Wafer Polish
Upper polishing pad
Lower polishing pad
Wafer
Slurry
Figure 4.26
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Quality Measures
• Physical dimensions
• Flatness
• Microroughness
• Oxygen content
• Crystal defects
• Particles
• Bulk resistivity
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Improving Silicon Wafer RequirementsYear
(Critical Dimension)1995
(0.35 m)1998
(0.25 m)2000
(0.18 m)2004
(0.13 m)
Wafer diameter(mm)
200 200 300 300
Site flatnessA (m)Site size (mm x mm)
0.23(22 x 22)
0.17(26 x 32)
0.1226 x 32
0.0826 x 36
MicroroughnessB of frontsurface (RMS)C (nm)
0.2 0.15 0.1 0.1
Oxygen content(ppm)D
24 2 23 2 23 1.5 22 1.5
Bulk microdefectsE
(defects/cm2) 5000 1000 500 100
Particles per unit area(#/cm2)
0.17 0.13 0.075 0.055
EpilayerF thickness( % uniformity) (m) 3.0 ( 5%) 2.0 ( 3%) 1.4 ( 2%) 1.0 ( 2%)
Adapted from K. M. Kim, “Bigger and Better CZ Silicon Crystals,” Solid State Technology (November 1996), p. 71.
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Improving Silicon Wafer Requirements
Notes:
A. Flatness is the linear thickness variation across the wafer or a site on a wafer (see below).
B. See below for a description of microroughness.
C. RMS is a method for determining the best estimate of group of measurements – in this case, the surface finish measurements (see below). It is calculated by taking the root-mean-square (square root of the average of all measurements squared). Surface finish measurements are obtained by measuring the highest point relative to the lowest point on a surface.
D. ppm is part per million.
E. Bulk microdefects represents all defects within a square centimeter.
F. See below to define epilayer.
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Positive deviation Negative deviation
Vacuum chuck
Wafer
Reference plane
Wafer Deformation
Figure 4.27
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Flatness of Wafer Front Surface
Figure 4.28
Silicon wafer surface
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Monocrystalline layers
Epitaxial layer
Silicon wafer
Formation of Epitaxial Silicon Layers
Figure 4.29
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Chapter 4 Review
• Summary 88
• Key Terms 88
• Review Questions 89
• Selected Industry Web Sites 90
• References 90