EE143 S06 Lecture 7 Ion Implantation C x y x as implant depth profile Blocking mask Si Equal Concentration contours Depth x Concentration Profile versus Depth is a single peak function Reminder Reminder During Duringimplantation implantation temperature temperatureisisambient ambient However However o post implant post implantannealing annealingstep step 900 900oC C isisrequired requiredto toanneal annealout outdefects defects Professor N Cheung U C Berkeley 1 EE143 S06 Lecture 7 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 dose uniformity 1 variation across 12 wafer Application example self aligned MOSFET source drain regions As As As Poly Si Gate n Professor N Cheung U C Berkeley p Si n SiO2 2 EE143 S06 Lecture 7 Monte Carlo Simulation of 50keV Boron implanted into Si You can download this program from http www srim org index htm Professor N Cheung U C Berkeley 3 EE143 S06 Lecture 7 1 Range and profile shape depends on the ion energy for a particular ion substrate combination 2 Height i e Concentration of profile depends on the implantation dose C x in cm3 3 Conc Conc of ofatoms cm atoms cm3 2 dose dose of ofatoms cm atoms cm2 dose C 0 x dx Depth x in cm Professor N Cheung U C Berkeley 4 EE143 S06 Lecture 7 Mask layer thickness can block ion penetration photoresist SiO2 Si3N4 or others Thick Mask Complete blocking Thin mask No blocking Incomplete Blocking SUBSTRATE Professor N Cheung U C Berkeley 5 EE143 S06 Lecture 7 Ion Implanter e g AsH3 As AsH H AsH2 3 4M implanter Magnetic Mass separation Ion source As wafer Translational wafer holder motion Professor N Cheung U C Berkeley 60 wafers hour Accelerator Voltage 1 200kV Dose 1011 1016 cm2 Accuracy of dose 0 5 Uniformity 1 for 8 wafer Accelerator Column ion beam stationary spinning wafer holder 6 EE143 S06 Professor N Cheung U C Berkeley Lecture 7 7 EE143 S06 Lecture 7 Eaton HE3 High Energy Implanter showing the ion beam hitting the 300mm wafer end station Professor N Cheung U C Berkeley 8 EE143 S06 Lecture 7 Implantation Dose For singly charged ions e g As Ion Beam Current in amps q Dose Ion Beam Scanning Implant time Area cm 2 Over scanning Over scanningof ofbeam beamacross acrosswafer waferisiscommon common In Ingeneral general Implant Implantarea area Wafer Waferarea area Professor N Cheung U C Berkeley 9 EE143 S06 Lecture 7 Practical Implantation Dosimetry aperture for dose monitoring Wafer holder wheel ions Faraday cup e Secondary electron effect eliminated V A bias applied to Faraday Cup to collect all secondary electrons Cup current Ion current Charge collected by integrating cup current cup area dose Professor N Cheung U C Berkeley 10 EE143 S06 Lecture 7 Meaning of Dose and Concentration Dose area looking downward how many fish per unit area for ALL depths Concentration volume Looking at a particular location how many fish per unit volume Professor N Cheung U C Berkeley 11 EE143 S06 Lecture 7 Ion Implantation Energy Loss Mechanisms Nuclear stopping Si Si Crystalline Si substrate damaged by collision e Electronic stopping Si e Electronic excitation creates heat Professor N Cheung U C Berkeley 12 EE143 S06 Lecture 7 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 Electronic stopping dominates B Electronic stopping dominates As Nuclear stopping dominates Professor N Cheung U C Berkeley 13 EE143 S06 Lecture 7 Stopping Mechanisms B into Si P into Si As into Si E1 keV 3 17 73 Professor N Cheung U C Berkeley E2 keV 17 140 800 14 EE143 S06 Lecture 7 Sn dE dx n Se dE dx e Depth x E 0 Se Surface E Eo Substrate A Se Sn Sn Eo incident kinetic energy x Rp More crystalline damage at end of range Sn Se Professor N Cheung U C Berkeley Less crystalline damage S e Sn 15 EE143 S06 Professor N Cheung U C Berkeley Lecture 7 16 EE143 S06 Gaussian Approximation of OneDimensional Implant Depth Profile C x Lecture 7 Uniform implantation at all lateral positions Cp y 0 61 Cp Rp x 0 Depth x Rp C x Cp e 2 2 R p 2 x R p x Note This is for no masking R p projected range R p longitudinal straggle Professor N Cheung U C Berkeley 17 EE143 S06 Lecture 7 Projected Range and Straggle Rp and Rp values are given in tables or charts e g see pp 113 of Jaeger Note this means 0 02 m Professor N Cheung U C Berkeley 18 EE143 S06 Lecture 7 Rp and Rp values from Monte Carlo simulation see 143 Reader for other ions Rp 51 051 32 60883 E 0 03837 E2 3 758e 5 E3 1 433e 8 E4 Projected Range Straggle in Angstrom Rp 185 34201 6 5308 E 0 01745 E2 2 098e 5 E3 8 884e 9 E4 B1 1 into Si 10000 Rp 1000 Rp 100 10 100 1000 Ion Energy E in keV both theoretical expt values are well known for Si substrate Professor N Cheung U C Berkeley 19 EE143 S06 Lecture 7 Dose Concentration Relationship Using Gaussian Approximation Cp Dose C x dx Gaussian 0 C x dx C p 2 R p Cp 2 Rp Professor N Cheung U C Berkeley negligible x 0 4 Rp 20 EE143 S06 Lecture 7 Junction Depth xj C x Shallow Implant Deep Implant C x CB xj CB xj1 x xj2 x C x x j CB Bulk Concentration If Gaussian approx for C x is used Cp exp xj Rp 2 2 Rp 2 CB we can solve for xj Professor N Cheung U C Berkeley 21 EE143 S06 Lecture 7 Sheet Resistance RS of Implanted Layers Example n n type dopants implanted into p type substrate RS x 0 p sub CB 1 xj 0 x xj q x C x C B dx x Needs numerical integration to get Rs value C x log scale n p 1017 Professor N Cheung U C Berkeley 1019 CB Total doping conc xj x 22 EE143 S06 Lecture 7 Approximate Value for RS If C x CB for most depth x of interest and use approximation x constant Rs Rs 1 q C x dx xj 0 1 q This expression assumes ALL implanted dopants are 100 electrically activated 1 q R ohm s use the for the highest doping region which carries most of the current or ohm square Professor N Cheung U C Berkeley 23 EE143 S06 Lecture 7 Example Calculations 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 m Rp 0 0775 m a Find peak concentration Cp 0 4 x 1013 0 0775 x10 4 5 2 x1017 cm3 b Find junction depths b Cp exp xj 0 254 2 2 Rp2 N CBB with …
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