Microsoft PowerPoint - Chap4_ Wafer fabrication

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1 Chapter 4 Wafer Fabrication 1

2 Why Silicon? Abundant, cheap Silicon dioxide is very stable, strong dielectric, and it is easy to grow in thermal process. Large band gap, wide operation temperature range. 2

3 Crystal ( 晶體 ) 晶體 (Crystal): 一種具有周期性結構的物質, 其局部原子做有規則有次序的排列, 且在三度空間中重覆出現數百億次, 形成長程規律性 (Long range order) 構成晶體的最小單位體積叫做晶胞 (unit cell, u.c.) 依結晶情況區分單晶體 (single-crystal) 複晶體 (poly-crystal) 非晶體 (non-crystalline, amorphous) (a) 單晶,(b) 多晶,(c) 非晶的比較示意圖晶粒長大示意圖 ( 箭頭爲晶界移動方向 ) 3

4 Crystal Planes ( 結晶平面 ) 結晶平面乃以米勒指標 (Miller indices) 來標示, 而所謂的米勒指標便是平面切割三軸形成截距之倒數比, 且化為簡單整數比平面的標示方法是在小括弧內 (hkl) 填寫米勒指標 4

5 Imperfection ( 缺陷 ) 缺陷分類結構的成份的常見的缺陷晶格空位 (vacancy) 雜質 (impurity) 原子錯位 (Frenkel defect) 陰陽離子成對消失 (Schottky defect) 差排 (dislocation) 晶界 (grain boundary) 域界 (domain boundary) 雙晶 (twins) 非晶體 (amorphous) 固溶體 (solid solution) 非化學量比 (nonstoichiometric) 5

6 Point Defects ( 點缺陷 ) 最簡單的點缺陷是晶格空位或叫空孔 (vacancies), 是指在規則的原子排列之中, 某原子在它應該出現的位置上沒有出現, 而成為一個空虛的位置 6

7 Line Defects ( 線缺陷 ) 在晶體中, 最常見的線缺陷是插排 (dislocation) 7

8 Plane Defects ( 面缺陷 ) 結晶缺陷可擴張到兩度空間而形成界面 (boundary), 最明顯的界面是外表面 (external surface) 8

9 Crystal Growth 發展大直徑晶錠的理由 CZ 方法 CZ 晶體拉升器摻雜雜質控制浮動區域方法 9

10 在較大晶圓直徑裡, 晶粒數會增加 88 個晶粒 8" 晶圓 232 個晶粒 12" 晶圓 10

11 摻質矽中摻雜濃度的命名法材料類型濃度 (Atoms/cm 3 ) < ( 非常輕微地摻雜 ) to ( 輕摻雜 ) to ( 摻雜 ) >10 19 ( 重摻雜 ) 5 價的元素 n n - n - n n + 3 價的元素 p p - p - p p + 晶圓直徑與屬性直徑厚度面積重量 (mm) (µm) (cm 2 ) (grams/lbs) 25 塊晶圓重 (lbs) ± / ± / ± / ± /

12 晶圓準備的基本製程步驟晶體成長晶圓研磨和邊緣磨光清潔整形蝕刻檢驗晶圓切片拋光封裝 12

13 Process Flow From Starting Material To Polished Wafer 13

14 From Sands to Si Crystals Si contained materials SiC ( solid) + SiO2( solid) Si( solid) + SiO( gas) + CO( gas) Metallurgical-grade : impurity~98% 300 C Si( solid) + 3HCl( gas) SiHCl3( gas) + H 2 SiHCl 3( gas) + H2( gas) Si( solid) + 3HCl( gas) ( gas) Electronic-grade : impurity~ppb Crystal growth - Ingots Si wafers 14

15 Charge Preparation & Loading Materials gathered per specification Seed Dopant Poly-Si Quartz Crucible Graphite Charge Placed in Puller 15

16 Crystal Puller Evolution 16

17 CZ Crystal Pulling Source: 17

18 Puller Configuration 晶體拉升器晶種單晶矽石英坩鍋碳加熱電熱絲熔解的多晶矽加熱擋板水保護罩 18

19 Melting/Stabilization Polysilicon is melted. Melt temperature brought to thermal equilibrium for crystal growth. The melting point of silicon is 1415 ºC. A typical 200mm charge size can be over 100 Kg and takes several hours to melt. 19

20 Czochralski Method for Crystal Growth Neck is grown to eliminate dislocations generated by thermal shock to seed crystal when it touches the melt. The neck is only 4-6 mm thick for a modern large diameter crystal but can still support the weight of the crystal. 20

21 Crystal Parts 21

22 Seed Dip/Neck/Taper/Body Growth Seed placed in contact with melt. Neck growth. Taper grown to transition to required diameter. The diameter is typically a few millimeters more than 200 mm to allow for grinding. Typical growth rates for a 200 mm crystal are mm/min. Typical cooling rate during crystal growth are ºC/min Oxygen incorporation into crystal ~ 8-17 ppma. 22

23 Dopant Incorporation The doping concentration incorporated into the crystal is usually different from the doping concentration of the melt at the interface. 23

24 Dopant Incorporation 24

25 Carbon & Oxygen Sources Oxygen source: quartz (SiO 2 ) crucible dissolution. >98% of oxygen dissolved evaporates as SiO. Remainder is incorporated into the silicon crystal. Carbon source: graphite parts. Carbon concentrations normally ~ FTIR detection limit. Abnormal concentration is indicative of heater arcing or puller leak. 25

26 Intrinsic Gettering 26

27 Oxygen Control 27

28 Endcone/Cooling/Modification Endcone or bottom - Diameter reduced to a point to prevent thermal shockback when the crystal is removed from the melt. Crystal cooled prior to removal/handling. This takes several hours as the crystal has to cool below ºC before it can be handled. Crystal taper and endcone cropped. Control slugs taken to verify process control. 28

29 Grinding and Slicing 準備直徑研磨之晶錠直徑研磨平面研磨 29

30 Ingot Polishing, Flat, or Notch Flat, 150 mm and smaller Notch, 200 mm and larger 30

31 200 mm Wafer Thickness and Surface Roughness Changes After Wafer Sawing After Edge Rounding After Lapping After Etch After CMP 76 µm 914 µm 76 µm 914 µm 12.5 µm 814 µm <2.5 µm 750 µm Virtually Defect Free 725 µm 31

32 Float-Zone Process 氣體入口 ( 鈍氣 ) 平盤多晶柱 ( 矽 ) 熔解區 RF 活動式 RF 線圈晶種鈍氣出口平盤 32

33 Float-Zone Process 33

34 Comparison of the Two Methods CZ method is more popular Cheaper Larger wafer size (300 mm in production) Reusable materials Floating Zone Pure silicon crystal (no crucible) More expensive, smaller wafer size (150 mm) Mainly for power devices. 34

35 Wafer Specifications 項目參考規格影響 General characteristics Growth method Cz Orientation (100) ± 1 Oxidation Wafer type P-type boron doped Oxidation Electrical characteristics Resistivity (ohm-cm) 5-15 Oxidation, Device Radial resistivity gradient (R.R.G.) <= 10 % Uniformity (CTR-AVE)/CTR x 100 Minority carrier lifetime (sec) >=1E-4 Leakage current Chemical characteristic O content (ppma) 13 ± 1, (ASTM F =0.652 Defects, gettering effect ASTM F121-79) Radial oxygen gradient <= 5% Defects, gettering effect (CTR-AVE)/CTR x 100 C content (ppma) < 0.3 (ASTM F123-74) Defcts Structural characteristics Dislocation etch pit density (defect/cm 2 ) < 0.13, Center cross, Edge exclusion 3 Junction leakage, Carrier lifetime Crystal Oriented Particles (COP) mm, Width 0.1mm Slip/lineage None Wafer crack, Junction leakage, Carrier lifetime Swirl None Junction leakage, Carrier lifetime OSF (defect/cm 2 ) < 4.4, Center cross, Edge exclusion 3 Junction leakage, Carrier lifetime mm, width 0.1mm 35

36 Wafer Specifications 項目參考規格影響 Mechanical characteristics Diameter (mm) 200 ± 0.2 Alignment Notch Depth (mm) (SEMI Standard) Alignment Notch angle (degree) 90 ± 5 (semi standard) Alignment Notch location (011) ±1 (semi standard) Alignment Thickness (um) 725 ± 15 Wafer transfer, Wafer handling Edge profile Edge polished Bow (um) < 65 Wafer handling, DOF Warpage (um) < 30 Wafer handling, DOF TTV (um) < 3 DOF STIR (um) < 0.3 DOF (or SFPD) Reference plane : site best fit Site area 22x22 mm 2 Pua (%) 100 Standard mapping, partial site inactive Yield of flatness Medium Surface BOW Warpage 36

2 Miller Index (hkl) (1 00) X {hkl} {100} (100),(010),(001),(100),(0 10),(00 1) [hkl] (hkl) [100] (100) <hkl> 3 Characteristics of Etching Techniques

2 Miller Index (hkl) (1 00) X {hkl} {100} (100),(010),(001),(100),(0 10),(00 1) [hkl] (hkl) [100] (100) <hkl> 3 Characteristics of Etching Techniques 1 National Kaohsiung First University of Sci. & Tech. Silicon Anisotropic Wet Etching Department of Mechanical and Automation Engineering National Kaohsiung First University of Science and Technology MicroSystem