Section 6: Ion Implantation Jaeger Chapter 5 EE143 – Ali Javey

Ion Implantation - Overview Wafer is Target in High Energy Accelerator Impurities “Shot” into Wafer Preferred Method of Adding Impurities to Wafers Wide Range of Impurity Species (Almost Anything) Tight Dose Control (A few % vs. 20-30% for high temperature pre-deposition processes) Low Temperature Process Expensive Systems Vacuum System EE143 – Ali Javey

Equipment EE143 – Ali Javey

Ion Implantation as-implant C(x) + depth profile y Blocking mask Si Equal-Concentration contours Depth x x Reminder: During implantation, temperature is ambient. However, post-implant annealing step (>900oC) is required to anneal out defects. EE143 – Ali Javey

Advantages of Ion Implantation Precise control of dose and depth profile Low-temp. process (can use photoresist as mask) Wide selection of masking materials e.g. photoresist, oxide, poly-Si, metal Less sensitive to surface cleaning procedures Excellent lateral uniformity (< 1% variation across 12” wafer) Application example: self-aligned MOSFET source/drain regions As+ As+ As+ Poly Si Gate SiO2 n+ n+ p-Si EE143 – Ali Javey

Ion Implantation Energy Loss Mechanisms Si Nuclear stopping Si + + Crystalline Si substrate damaged by collision e e + + Electronic stopping Si Electronic excitation creates heat EE143 – Ali Javey

Ion Energy Loss Characteristics Light ions/at higher energy more electronic stopping Heavier ions/at lower energy more nuclear stopping EXAMPLES Implanting into Si: H+ B+ As+ Electronic stopping dominates Nuclear stopping EE143 – Ali Javey

Stopping Mechanisms EE143 – Ali Javey

Electronic / Nuclear Stopping: Damage Sn  dE/dx|n Se  dE/dx|e Depth x Surface E ~ 0 E=Eo Substrate A+ Se Se Eo = incident kinetic energy Sn Sn More damage at end of range Sn > Se Less damage Se > Sn x ~ Rp EE143 – Ali Javey

Simulation of 50keV Boron implanted into Si EE143 – Ali Javey

Model for blanket implantation EE143 – Ali Javey

Projected Range and Straggle Rp and DRp values are given in tables or charts e.g. see pp. 113 of Jaeger Note: this means 0.02 m. EE143 – Ali Javey

Selective Implantation EE143 – Ali Javey

Transverse (or Lateral) Straggle (Rt or  R) >1 Rp Rt Rp Rt EE143 – Ali Javey

Feature Enlargement due to lateral straggle y Mask x x = Rp Lower concentration Higher concentration Implanted species has lateral distribution, larger than mask opening C(y) at x=Rp y EE143 – Ali Javey

Definitions of Profile Parameters (1) Dose (2) Projected Range: (3) Longitudinal Straggle: 1 (4) Skewness: ¥ ( ) 3 ( ) º ò M x - Rp C x dx , M > or < 3 3 f -describes asymmetry between left side and right side (5) Kurtosis: C(x) Kurtosis characterizes the contributions of the “tail” regions x Rp EE143 – Ali Javey

Selective Implantation – Mask thickness Desire Implanted Impurity Level to be Much Less Than Wafer Doping N(X0) << NB or N(X0) < NB/10 EE143 – Ali Javey

Transmission Factor of Implantation Mask Mask material (e.g. photoresist) C(x) What fraction of dose gets into Si substrate? Si substrate x=0 x=d C(x) Mask material with d= - x=0 x=d EE143 – Ali Javey

( ) ( ) ( ) ( ) ( ) ò ò ò Transmitted Fraction T = C x dx - C x dx ì 1 ¥ ( ) d ( ) T = ò C x dx - ò C x dx ì 1 d - R ü = p erfc are values of for ions into the masking material í ý 2 2 D R î þ p 2 ( ) x - y 2 erfc x = 1 - ò e dy p Rule of thumb Good masking thickness : ( ) C x = d d R p = + 4 3 D . - 4 ~ 10 ( ) C x = R p EE143 – Ali Javey

Junction Depth The junction depth is calculated from the point at which the implant profile concentration = bulk concentration: EE143 – Ali Javey

Sheet Resistance RS of Implanted Layers x =0 Example: n-type dopants implanted into p-type substrate p-sub (CB) n x =xj x x C(x) log scale xj CB  1017 1019 n Total doping conc p EE143 – Ali Javey

Approximate Value for RS If C(x) >>CB for most depth x of interest and use approximation: (x) ~ constant This expression assumes ALL implanted dopants are 100% electrically activated use the m for the highest doping region which carries most of the current or ohm/square EE143 – Ali Javey

200 keV Phosphorus is implanted into a p-Si ( CB= 1016/cm3) with a dose of 1013/cm2 . From graphs or tables , Rp =0.254 mm , Rp=0.0775mm (a) Find peak concentration Cp = (0.4 x 1013)/(0.0775 x10-4) = 5.2 x1017/cm3 (b) Find junction depths (c) Find sheet resistance Example Calculations CB EE143 – Ali Javey

Channeling EE143 – Ali Javey

Use of tilt to reduce channeling C(x) Random component Lucky ions fall into channel despite tilt channeled component x Axial Channeling Planar Channeling Random To minimize channeling, we tilt wafer by 7o with respect to ion beam. EE143 – Ali Javey

Prevention of Channeling by Pre-amorphization Si+ 1 E15/cm2 Si crystal Step 1 High dose Si+ implantation to covert surface layer into amorphous Si B+ Si crystal Amorphous Si Step 2 Implantation of desired dopant into amorphous surface layer Disadvantage : Needs an additional high-dose implantation step EE143 – Ali Javey

Kinetic Energy of Multiply Charged Ions With Accelerating Voltage = x kV B+ P+ As+ Kinetic Energy = x · keV B+++ B++ Kinetic Energy = 2x · keV Kinetic Energy = 3x · keV Singly charged Doubly charged Triply charged Note: Kinetic energy is expressed in eV . An electronic charge q experiencing a voltage drop of 1 Volt will gain a kinetic energy of 1 eV EE143 – Ali Javey

Molecular Ion Implantation Kinetic Energy = x keV Molecular ion will dissociate immediately into atomic components after entering a solid. All atomic components will have same velocity after dissociation. B F BF2+ accelerating voltage = x kV + - Solid Surface B has 11 amu F has 19 amu EE143 – Ali Javey

Implantation Damage EE143 – Ali Javey

Amount and type of Crystalline Damage EE143 – Ali Javey

Post-Implantation Annealing Summary After implantation, we need an annealing step. A typical anneal will: (1) Restore Si crystallinity. (2) Place dopants into Si substitutional sites for electrical activation EE143 – Ali Javey

Deviation from Gaussian Theory Curves deviate from Gaussian for deeper implants (> 200 keV) Curves Pearson Type-IV Distribution Functions (~sum of 4 Gaussians) EE143 – Ali Javey

Shallow Implantation EE143 – Ali Javey

Rapid Thermal Annealing Rapid Heating 950-1050o C >50o C/sec Very Low Dt (b) EE143 – Ali Javey

Dose-Energy Application Space EE143 – Ali Javey