EE143 Fall 2016 Microfabrication Technologies. Lecture 7: Ion Implantation Reading: Jaeger Chapter 5

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1 EE143 Fall 016 Microfabrication Technologies Lecture 7: Ion Imlantation Reading: Jaeger Chater 5 Prof. Ming C. Wu wu@eecs.berkeley.edu 511 Sutardja Dai Hall (SDH) 1 Ion Imlantation - Overview Wafer is target in high energy accelerator Imurities shot into wafer Preferred method of adding imurities to wafers Wide range of imurity secies (almost anything) Tight dose control (A few % vs. 0-30% for high temerature re-deosition rocesses) Low temerature rocess Exensive systems Vacuum system 1

2 Equiment Force on charged article Magnetic Field Imlanted Dose F = q B = 1 Q = nqa ( v x B) mv qr T ò 0 I ( t)dt 3 Ion Imlantation + C(x) as-imlant deth rofile y Blocking mask Si x Equal-Concentration contours Deth x During imlantation, temerature is ambient. Post-imlant annealing ste (> 900 o C) is required to anneal out defects. 4

3 Advantages of Ion Imlantation Precise control of dose and deth rofile Low-temerature rocess (can use hotoresist as mask) Wide selection of masking materials e.g. hotoresist, oxide, oly-si, metal Less sensitive to surface cleaning rocedures Excellent lateral uniformity (< 1% variation across 1 wafer) Alication examle: self-aligned MOSFET source/drain regions As + As + As + Poly Si Gate n + -Si n + SiO 5 Ion Imlantation Energy Loss Mechanisms Nuclear stoing Si + Si + Crystalline Si substrate damaged by collision Electronic stoing e e + Si + Electronic excitation creates heat 6 3

Ion Energy Loss Characteristics Light ions/at higher energy à more electronic stoing Heavier ions/at lower energy à more nuclear stoing EXAMPLES

4 Ion Energy Loss Characteristics Light ions/at higher energy à more electronic stoing Heavier ions/at lower energy à more nuclear stoing EXAMPLES Imlanting into Si: H + B + As + Electronic stoing dominates Electronic stoing dominates Nuclear stoing dominates 7 Simulation of 50 kev Boron imlanted into Si 8 4

5 Model for blanket imlantation Gaussian Profile R D R é N( x) = N exê - ê ë = Projected Range Dose = Straggle Q = ò 0 N ( x - R ) D R ù ú ú û ( x) dx = N D R 9 Projected Range and Straggle R and D R values are given in tables or charts e.g. see. 113 of Jaeger 0 nm 10 5

6 Selective Imlantation N F ( x, y) = N( x) F( y) ( y) D R ^ ( ) is one - dimensional solution N x 1 é æ y - a ö æ = ê erfcç - ç ç ê ë erfc ç è D R^ ø è = transverse straggle y + a D R ^ ö ù ú ø ú û Comlementary error function: erfc x = 1 erf x = A π = e>t dt x 11 Transverse (or Lateral) Straggle (D R t or D R^ ) ΔR t ΔR D > 1 D R t D R D R t 1 6

Feature Enlargement due to Lateral Straggle y x Mask x =

secies has lateral distribution, larger than mask oening

Desire imlanted imurity level under the mask should be

7 Feature Enlargement due to Lateral Straggle y x Mask x = R Lower concentrationhigher concentration Imlanted secies has lateral distribution, larger than mask oening C(y) at x = R y 13 Selective Imlantation Mask Thickness Desire imlanted imurity level under the mask should be much less than background doing N x 0 N x 0 or N B < N B

8 C(x) Transmission Factor of Imlantation Mask Mask material (e.g. hotoresist) Si substrate What fraction of dose gets into Si substrate? x=0 x=d C(x) Mask material with d= 15 x=0 x=d ò d ( ) ò ( ) T = C x dx - C x dx 0 Transmitted Fraction 1 ì erfc d - R ü = í ý î D R þ x - erfc( x) y = 1- ò e dy 0 0 Rule of thumb: Good masking thickness ( ) C x = d d = R D R ~ 10 C x R, D R are values of for ions into the masking material ( = R )

9 Junction Deth The junction deth is calculated from the oint at which the imlant rofile concentration = bulk concentration: N N x ( x ) j j = R = N ( x - R ) é j exê - ê ë D R B ± D R ù ú = N ú û æ N lnç è N B ö ø B 17 Channeling 18 9

Use of tilt to reduce channeling C(x) Random comonent channeled comonent Lucky ions fall into channel desite tilt x Random Planar Channeling Axial Channeling To minimize channeling, we tilt wafer by

10 Use of tilt to reduce channeling C(x) Random comonent channeled comonent Lucky ions fall into channel desite tilt x Random Planar Channeling Axial Channeling To minimize channeling, we tilt wafer by 7 o with resect to ion beam. 19 Prevention of Channeling by Preamorhization Ste 1 High dose Si+ imlantation to covert surface layer into amorhous Si Ste Imlantation of desired doant into amorhous surface layer Si + 1E15 /cm B + Si crystal Amorhous Si Si crystal Disadvantage: Needs an additional high-dose imlantation ste 0 10

11 Kinetic Energy of Multily Charged Ions With Accelerating Voltage = x kv Singly charged Doubly charged B + P + As + B ++ Kinetic Energy = x kev Kinetic Energy = x kev Trily charged B +++ Kinetic Energy = 3x kev Note: Kinetic energy is exressed in ev. An electronic charge q exeriencing a voltage dro of 1 Volt will gain a kinetic energy of 1 ev 1 Molecular Ion Imlantation Kinetic Energy = x kev BF + accelerating voltage + = x kv - B has 11 amu F has 19 amu Solid Surface B F F Molecular ion will dissociate immediately into atomic comonents after entering a solid. All atomic comonents will have same velocity after dissociation. Velocity v = v = v 1 K.E.of B = mb vb 1 K.E.of F = mf vb K.E.of B 11» = 0% + K.E.of BF B F F 11

12 Imlantation Damage 3 Amount and Tye of Crystalline Damage 4 1

13 Post-Imlantation Annealing Summary After imlantation, we need an annealing ste A tyical anneal will: 1. Restore Si crystallinity.. Place doants into Si substitutional sites for electrical activation 5 Deviation from Gaussian Theory Curves deviate from Gaussian for deeer imlants (> 00 kev) 6 13

14 Shallow Imlantation 7 Raid Thermal Annealing Raid Heating o C >50 o C/sec Very low doant diffusion (b) 8 14

15 Dose-Energy Alication Sace 9 15

Section 6: Ion Implantation. Jaeger Chapter 5

Section 6: Ion Implantation. Jaeger Chapter 5 Section 6: Ion Imlantation Jaeger Chater 5 Ion Imlantation - Overview Wafer is Target in High Energy Accelerator Imurities Shot into Wafer Preferred Method of Adding Imurities to Wafers Wide Range of Imurity