Chapter 8 Ion Implantation

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) HV Accelerator (voltage > 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 source 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 m/q=(B2R2)/(2V) Or Faraday cup Acceleration energy = voltage x charge on ion

http://www.bpc.edu/mathscience/chemistry/images/periodic_table_of_elements.jpg

ION IMPLANTATION High energy ion enters crystal lattice and collides with atoms and interacts with electrons Types of collisions: Nuclear and electron Each collision or interaction reduces energy of ion until it comes to rest Amount of energy loss is dependent on ion, the energy it has at the time of the scattering event, and the type of scattering.

From Handbook of Semiconductor Manufacturing Technology by Yoshio Nishi and Robert Doering

From Handbook of Semiconductor Manufacturing Technology by Yoshio Nishi and Robert Doering

Channeling Deep penetration by the ion because it traveled along a path where no semiconductor atoms are situated Process is used for materials characterization: Rutherford backscattering To prevent channeling Implantation is performed at an angle of about 8° off the normal to the wafer surface. The wafer surface is amorphorized by a high dose, low energy implantation of a nonelectrically active ion. Hydrogen, helium, and silicon are common ions used

Determining the Dose The implanted dose can be accurately measured by monitoring the ion beam current using a Faraday cup The integrated current during the implant divided by the charge on the ion is the dose.

Post Implantation Anneals An annealing step is required to repair crystal damage (recrystallization) and to electrically activated the dopants. Dislocations will form during the anneal so times and temperatures must be chosen to force dislocations disappear. If the anneal time is long and the temperature is high, a drive of the implanted ions may occur.

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 Cp = peak concentration Rp = range Rp = straggle

MODEL FOR ION IMPLANTATION For an implant contained within silicon, the dose is

ION IMPLANTATION MODEL 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

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)

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)

Si SiO2 AZ-7500 resist Si3N4 http://www.iue.tuwien.ac.at/phd/hoessinger/node22.html

http://www.ensc.sfu.ca/~glennc/e495/e495l7j.pdf

http://www.ensc.sfu.ca/~glennc/e495/e495l7j.pdf

Xox = RP + RP (2 ln(10CP/CBulk))0.5 SiO2 AS A BARRIER The minimum oxide thickness for selective implantation: Xox = RP + RP (2 ln(10CP/CBulk))0.5 An oxide thickness equal to the projected range plus six times the straggle should mask most ion implants.

Other Materials 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. Nickel is one of the most commonly used metal masks

ADVANTAGES Low temperature process Wider range of barrier materials The wafer is cooled from the backside during high energy, high current diffusions are performed Less change of stress-induced dislocations due to thermal expansion issues Wider range of barrier materials Photoresist Wider range of impurities No concern about solid solubility limitations Implantation of ions such as oxygen, hydrogen, helium, and other ions with low solid solubility is possible.

Advantages over Diffusion Better control and wider range of dose compared to predep diffusions Impurity concentration profile controlled by accelerating voltage Very shallow layers Lateral scattering effects are smaller than lateral diffusion.

Complex-doping profiles can be produced by superimposing multiple implants having various ion energies and doses. 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

Recrystallization Radiation damage can be removed by annealing at 800-1000oC for 30 min. After annealing, a significant percentage of the impurities become electronically active. Point defects coalesce into line dislocations Line dislocations merge into loop dislocations Loop dislocations slowly disintegrate as interstitial Si atoms move on to lattice sites

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: