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 Secies (Almost Anything) Tight Dose Control (A few % vs. 0-30% for high temerature re-deosition rocesses) Low Temerature Process Exensive Systems Vacuum System
Equiment Force on charged article Magnetic Field Imlanted Dose F = q B Q = = ( v x B) 1 nqa mv qr T 0 I()dt t Ion Imlantation + C(x) as-imlant deth rofile y Blocking mask Si x Equal-Concentration contours Deth x Reminder: During imlantation, temerature is is ambient. However, ost-imlant annealing ste ste (>900 o o C) C) is is required to to anneal out out defects.
Advantages of Ion Imlantation Precise control of dose and deth rofile Low-tem. 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 Ion Imlantation Energy Loss Mechanisms Nuclear stoing Si + Si + Crystalline Si substrate damaged by collision Electronic stoing e e + Si + Electronic excitation creates heat
Ion Energy Loss Characteristics Light ions/at higher energy more electronic stoing Heavier ions/at lower energy more nuclear stoing EXAMPLES Imlanting into Si: H + Electronic stoing dominates B + Electronic stoing dominates As + Nuclear stoing dominates Stoing Mechanisms E1(keV) E(keV) B into Si 3 17 P into Si 17 140 As into Si 73 800
Simulation of 50keV Boron imlanted into Si Model for blanket imlantation Gaussian Profile R ΔR N( x) = N ex = Projected Range Dose = Straggle Q = 0 N ( x R ) ΔR ( x) dx = π N ΔR
Projected Range and Straggle R and ΔR values are given in tables or charts e.g. see. 113 of Jaeger Note: this means 0.0 μm. Selective Imlantation N F ( x, y) = N( x) F( y) ( y) ΔR ( ) is one - dimensional solution N x 1 y a = erfc erfc ΔR = transverse straggle y + a ΔR
Transverse (or Lateral) Straggle (ΔRt or Δ R ) ΔR t ΔR >1 ΔR t ΔR ΔR t Feature Enlargement due to lateral straggle y x Mask x = R Lower concentration Higher concentration Imlanted secies has has lateral lateral distribution, larger larger than than mask mask oening C(y) at x=r y
Selective Imlantation Mask thickness Desire Imlanted Imurity Level to be Much Less Than Wafer Doing or N(X 0 ) << N B N(X 0 ) < N B /10 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= - x=0 x=d
d () () T = C x dx C x dx 0 1 erfc d R = ΔR x y erfc() x = 1 e dy π 0 Rule of thumb Transmitted Fraction 0 : Good masking thickness ( = ) Cx d d = R +43Δ. R ~ 10 Cx R, ΔR are values of for ions into the masking material ( = R) 4 Junction Deth The junction deth is calculated from the oint at which the imlant rofile concentration = bulk concentration: N = N N x j ( x ) = j ( x R ) j ex ΔR R B ± ΔR = N ln N B N B
Channeling 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. To minimize channeling, EE143 we tilt Ali wafer Javey by 7 o with resect to ion beam.
Prevention of Channeling by Pre-amorhization Ste 1 High dose Si+ imlantation to covert surface layer into amorhous Si Si + 1 E15/cm Si crystal Ste Imlantation of desired doant into amorhous surface layer B + Amorhous Si Si crystal Disadvantage :: Needs an an additional high-dose imlantation ste ste 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
Kinetic Energy = x kev BF + accelerating voltage + = x kv - B has 11 amu F has 19 amu Molecular Ion Imlantation 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 11 + 19 + 19 B F F Imlantation Damage
Amount and tye of Crystalline Damage 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
Deviation from Gaussian Theory Curves deviate from Gaussian for deeer imlants (> 00 kev) Shallow Imlantation
Raid Thermal Annealing Raid Heating 950-1050 o C >50 o C/sec Very low doant diffusion (b) Dose-Energy Alication Sace
μ Sheet Resistance R S of Imlanted Layers Examle: n-tye doants imlanted into -tye substrate R S = 0 x j q μ μ n n 1 ( x) [ C( x) C ] -sub (C B ) B dx C(x) log scale x =0 x =x j x C B μ 10 17 10 19 Total doing conc x j x