CHAPTER 5 ION IMPLANTATION In ion implantation, ionized impurity atoms are accelerated through an electrostatic field and strike the surface of the wafer. A high-voltage particle accelerator produces a high-velocity beam of impurity ions. The complete implanter system is operated under vacuum Advantages (1)The dose can be tightly controlled by measuring the ion currents. (2)The penetration depth of the impurity ions can be precisely controlled ranging from 1 to 200 keV. (3)Wider range of impurity species than possible compared with diffusion (4)Relatively low temperatures to prevent undesired spreading of impurities by diffusion  Therefore, most silicon processes were fully implanted. Drawbacks (1) The incident ions damage the semiconductor lattice (can be partially repaired by high temperature process) (2) Very shallow and very deep profiles are difficult. (3) Ion implantation equipment is expensive

(1) Ion source: Operate at a high voltage (25 kV) Produce a plasma containing the desired impurity & other undesired species Gas sources such as AsH 3, PH 3, B 2 H 6 if available If not, solid charge can be heated to obtain vapor phase. In arc chamber, the source is broken into atomic & molecular species, then ionized. (2) Mass Spectrometer: To select the desired impurity ion using an analyzing magnet (3) High-Voltage Accelerator: Accelerated by the accelerator column up to 5 MeV Up to several meters long under high vacuum to avoid collisions during accelerations (4) Scanning System (x- and y-axis deflection system) To produce Uniform implantation & to build up the desired dose The beam is bent to prevent neutral particles from hitting the target. (5) Target Chamber

Scanning systems: Near the end of the tube, the deflection plate is located. The beam is rastered back & force and up & down, writing uniformly across the wafer. Total dose: The wafer is placed in a Faraday cup, which is a cage that captures all of the charge that enter it. Si wafer should be grounded. Electron current can be calculated by integration over time to measure the total dose Q I: the beam current in amperes, A: the wafer area, m=1 for singly ionized ions m=2 (a doubly ionized species) is used to increase the ion energy since E=mqV. Selection Analyzer magnet is used to select the desired impurity ions.  Only one mass will have the correct radius of curvature to exit the source F=q(v  B) : a charged particle moving with velocity through a magnetic field F=mv 2 /r where qV(voltage)=0.5mv 2 Mass separation stage of an ion implanter showing perpendicular magnetic field and ion trajectory (A) Electrostatic rastering commonly used in medium current machines. (B) Semielectrostatic scanning used on some high current machines because individual wafers does not have a large thermal load

5.3 Vertical projected range The total distance that an ion travels in the solid is the range. The projected range, Rp (=average penetration depth) is the projection of this distance along the depth axis. Incident ion Implantation Once introduced, ions collides with atoms in the lattice and interact with electrons When a energetic ion enters a solid, it will begin to lose energy.  finally stopped Two energy loss mechanisms (1)ion-electron interactions (electronic stopping): continuous energy loss - The space in the crystal is made up of the electron clouds. Since the mass ratio btw. the ion and an electron is of ~ 10 5, any single electron-ion interaction will not dramatically alter the momentum of the incident ion. Similar to a particle moving through a fluid. (2) nuclear stopping (ion-lattice interaction): non-continuous (discrete) energy loss - The incident & target ions have masses of the same order of magnitude. Less chance. Non-continuous (discrete) energy loss Statistical process (MC simulation)

Implanted impurity profile: Gaussian distribution R P : projected range (= the average distance an ion travels before it stops); depends on the mass, energy of the ion, atomic # of both the ion & target material) multiple implant steps (square type: uniform doping) N P : peak concentration (at x = R P ) ΔR P : straggle (standard deviation) Implanted dose, Q For an implant completely contained within the Si The implanted dose can be controlled within a few percent. For diffusion, >20% Mask Photoresist (PR) is commonly used as the implant mask  PR may flow or be baked due to high temperature during an implant since PR is a poor thermal conductor.  The ions striking the surface break apart the organic molecules in the PR leading to the formation of gaseous H 2 that evolves from the surface, leaving behind involatile & hardended carbonized layer that is difficult to remove. SiO 2, Si 3 N 4 (more effective than SiO 2 ), Al, Cr…(more effective than SiO 2 ), PR (less effective than SiO 2 )

The Hope Diamond contains trace amounts of boron and will conduct electricity, unlike a pure diamond, which is an insulator. Since the acceptor- level energy is so small, even the thermal energy at RT can produce this change, and the resulting holes in the valence band can move.

5.4 Channeling and Lateral projected range Penetration depth model is based on amorphous with a completely random order In single crystalline materials, channeling can occur when the ion velocity is parallel to a major crystal orientation. Some ions may travel considerable distance with little energy loss. Channel can produce a significant tail on the implant distributions. To avoid this tail, most implantation is done off axis (~ 7 degree). Diamond structure along a major axis & along a random direction

Deviations from the Gaussian Theory Light ions, B  Backward scattering Heavy ions, As  Forward scattering A common problem for implantation of wafers with topology: shadowing. Any non-planar masking layer will cast an implant shadow because of the tilt angles. Simple shadowing example for deeply scaled MOSFET.

5.5 Implantation damage Due to collisions with lattice nuclei when a high energy ion enters a wafer, atoms are ejected from the lattice during the process  displacement damage The implantation process produces considerable damage that must be repaired. Impurity activation: the process of moving a large fraction of the implanted impurities onto lattice sites. Both implant damage repair & implant activation are done by heating the wafer (thermal annealing, typically  C, 30 min) after implant. Si atoms can move back into lattice sites. Impurity atoms can enter substitutional site in the lattice.  can be electrically active. Diffusion during HT annealing process  considerable spreading by diffusion Overdoses  amorphorization B: too light to form amorphous layer at high doses Heavier impurity  less doses for amorphorization At high temperatures, the substrate self-anneals.  The threshold dose becomes very large.