Chapter 8 Ion Implantation Instructor: Prof. Masoud Agah

ION IMPLANTATION Two problems associated with diffusion especially for IC fabrication: High-temperature process Unable to provide shallow junction depths Ion implantation is a relatively simple means to place a known number of atoms in a wafer. Ion implantation process: Ionization of the dopant source to form positive ions Acceleration of ions through a high voltage field to reach the required energy Projection of high-energy ions towards the wafer surface (target) Collision of ions with silicon atoms resulting in energy loss End of penetration of ions in the substrate (coming to rest)

SYSTEM REQUIREMENTS May achieve better control of distribution of dopants versus depth with ion implantation Process can be faster Process does not require as much thermal processing

ION IMPLANTATION SYSTEM Ion implanter is a high-voltage accelerator of high-energy impurity ions Major components are: Ion source (gases such as AsH3 , PH3 , B2H6) Mass Spectrometer (selects the ion of interest. Gives excellent purity control) HV Accelerator (voltage up to 1 MeV) Scanning System (x-y deflection plates for electronic control) Target Chamber (vacuum)

ION IMPLANTATION SYSTEM Cross-section of an ion implanter 2 1 3 90o analyzing magnet 25 kV Ion souce Resolving aperture R R R C 0 to 175 kV Acceleration tube Focus Neutral beam trap and beam gate Neutral beam Beam trap y-axis scanner x-axis 4 5 Wafer in process chamber Integrator Q

ION IMPLANTATION SYSTEM Cross-section of an ion implanter

ION IMPLANTATION High energy ion enters crystal lattice and collides with atoms and interacts with electrons Each collision or interaction reduces energy of ion until it comes to rest Interactions are a complex distribution. Models have been built and tested against observation

ION IMPLANTATION To prevent channeling, implantation is normally performed at an angle of about 8° off the normal to the wafer surface. An annealing step is required to repair crystal damage and to electrically activated the dopants. The implanted dose can be accurately measured by monitoring the ion beam current. Complex-doping profiles can be produced by superimposing multiple implants having various ion energies and doses. Lateral scattering effects are smaller than lateral diffusion. Expensive $$$$$

ION IMPLANTATION Projected range (RP): the average distance an ion travels before it stops. Projected straggle (RP): deviation from the projected range due to multiple collisions. http://eserver.bell.ac.uk

MODEL FOR ION IMPLANTATION Distribution is Gaussian Np = peak concentration Rp = range Rp = straggle

MODEL FOR ION IMPLANTATION The implanted impurity profile can be approximated by the Gaussian distribution function. For an implant contained within silicon: Q = (2π)0.5 NP RP

MODEL FOR ION IMPLANTATION Model developed by Lindhard, Scharff and Schiott (LSS) Range and straggle roughly proportional to energy over wide range Ranges in Si and SiO2 roughly the same Computer models now available at low cost for PCs

MODEL FOR ION IMPLANTATION Range of impurities in Si 10 100 1000 Acceleration energy (keV) Rp 0.01 0.1 1.0 B P As Sb Projected range (mm)

MODEL FOR ION IMPLANTATION Straggle of impurities in Si B Sb As P DRp DR 0.10 0.01 0.002 10 100 1000 Acceleration energy (keV) Normal and transverse straggle (mm)

SiO2 AS A BARRIER SiO2 serves as an excellent barrier against ion-implantation SiO2 Silicon Np NB N(X0) Rp X0 Depth, x

Xox = RP + RP (2 ln(10NP/NBulk))0.5 SiO2 AS A BARRIER The minimum oxide thickness for selective implantation: Xox = RP + RP (2 ln(10NP/NBulk))0.5 An oxide thickness equal to the projected range plus six times the straggle should mask most ion implants. A silicon nitride barrier layer needs only be 85% of the thickness of an oxide barrier layer. A photoresist barrier must be 1.8 times the thickness of an oxide layer under the same implantation conditions. Metals are of such a high density that even a very thin layer will mask most implantations.

ADVANTAGES Advantages over diffusion: low temperature process allows wider range of barrier materials permits wider range of impurities better control of dose wider range of dose can control impurity concentration profile can introduce very shallow layers

PROFILE CONTROL Various shapes of profiles can be created by varying the energy of the incident beam 200 KILOELECTRON VOLTS FINAL PROFILE 100 50 20 10 15 5 150 200 250 300 350 DEPTH (NANOMETERS) NITROGEN CONCENTRATION (ATOMIC PERCENT)

RADIATION DAMAGE Impact of incident ions knocks atoms off lattice sites With sufficient dose, can make amorphous Si layer

RADIATION DAMAGE Critical dose to make layer amorphous varies with temperature and impurity 1018 1017 1016 1015 1014 1013 Temperature, 1000/T (K-1) B P Sb Critical dose (atom/cm2) 0 1 2 3 4 5 6 7 8 9 10 Radiation damage can be removed by annealing at 800-1000oC for 30 min. After annealing, almost all impurities become electronically active.

Ion Implantation Implanting through a sacrificial oxide layer: Large ions (arsenic) can be slowed down a little before penetrating into the silicon. The crystal lattice damage is suppressed (at the expense of the depth achieved). Collisions with the thin masking layer tends to cause the dopant ions to change direction randomly, thereby suppressing channeling effect. The concentration peak can be brought closer to the silicon surface.

Ion Implantation For deep diffusion (>1µm), implantation is used to introduce a certain dose, and thermal diffusion is used to drive in the dopants. The resulting profile after diffusion can be determined by:

Ion Implantation A boron implantation is to be performed through a 50nm oxide so that the peak concentration is at the Si-SiO2 interface. The implant dose in silicon is to be 1013/cm2. What are the energy of the implant and the peak concentration at the interface? Peak at Si-oxide interface  RP = 0.05µm  Energy = 15keV (RP=0.023µm) Implanted dose in silicon = 1013  Q=2x1013 NP = Q/2.5RP = 3.5x1018/cm3 How thick should the oxide layer be to mask the implant if the background concentration is 1016/cm3? Xox = 0.05 + 0.023(2 ln(10 x 3.5 x 1018/1016))0.5 = 0.14µm If the oxide layer is 50nm, how much photoresist is required on top of the oxide to completely mask the implant? PR thickness = 1.8 x (oxide thickness) = 1.8 x (0.14 – 0.05) = 0.16µm