Ion Implantation Topic covered: Process and Advantages of Ion Implantation Ion Distribution and Removal of Lattice Damage Simulation of Ion Implantation
Range of Implanted Ions Ion Implantation is the introduction of energetic, charged particles into a substrate such as silicon. Required energy ranges from 1keV to 1MeV, resulting to average depth of 10nm to 10um. Ion doses vary from 10 12 ions/cm 2 (threshold voltage adjustment) to 10 18 ions/cm 2 (formation of buried insulating layers). 2
Basic Concepts Ions enter the substrate (target) with high energy (velocity) and lose energy through collisions with (1) target nuclei and (2) target electrons Depth depends on ion energy (velocity) and distributed collisions with target nuclei knock them off lattice sites damaging substrate High temperature anneal repairs the target damage and activates dopants (put dopants and placed target nuclie back in lattice sites). 3
Figure 1 Comparison of (a) diffusion and (b) ionimplantation techniques for the selective introduction of dopants into the semiconductor substrate. 4
Advantages Precise control of dose, depth profile and area uniformity Excellent reproducibility Wide choice of masks Low temperature process Small lateral spread of dopants Vacuum cleanliness Disadvantages Expensive & complicated equipment Very deep and very shallow profiles are difficult Not all the damage can be corrected by annealing. Typically has higher impurity content than does diffusion. Often uses extremely toxic gas sources such as arsine (AsH3), and phosphine (PH3). 5
Medium-Energy Implantor Figure 2 Schematic of a medium-current ion implantor. 6
Ion Distribution Range ( R ) total distance that the ion travels in coming to rest. Projected Range ( R p ) projection of this distance along the axis of incidence; average penetration depth below the surface Projected Straggle ( σ p ) the statistical fluctuations in projected range ; standard deviation of projected range Straggle ( σ ) the statistical fluctuation along an axis perpendicular to the axis of incidence; Standard deviation in plane perpendicular to the surface. 7
Figure 3-1 (a) Schematic of the ion range R and projected range R p. (b) Two-dimensional distribution of the implanted icons. S ( x R n( x) = exp 2 σ p 2π 2σ P p ) 2 (1) where S is the ion dose per unit area 8
Figure 3-2 Ion-Implanted Profiles 9
Ion Stopping The average rate of energy loss with distance is given by: where de dx = S ( E) S ( E) (2) n + S n (E) (de/dx) n is the nuclear stopping power and S e (E) (de/dx) e is the electronic stopping power e If the total distance traveled by ion before coming to rest is R, R = R dx = R de S ( E) + n Se ( E) 0 0 (3) 10
Figure 4 Collision of hard spheres. 11
1 2 M 2 v 2 2 4M1M ( M1 + M 2 = 2 2) E o (4) where E o is the initial ion energy Se ( E) = k e E (5) where k e is relatively weak function of atomic mass and atomic number. 12
Figure 5 Nuclear stopping power, S n (E) and electronic stopping power, S e (E) for As, P, and B in Si. The points of intersection of the curves correspond to the energy at which nuclear and electronic stopping are equal. 13
Once S n (E) and S e (E) are known, we can compute the range from Eq.3. This is in turn can give us the projected range and projected straggle with the help of the following approximate equations. R R p 1+ ( M 2 / 3M1) (6) σ 2 M M 1 2 P R P 3 M1 + M 2 (7) 14
Figure 6 Projected range, projected straggle, and lateral straggle for (a) B, P, and As in silicon and (b) H, Zn, and Te in gallium arsenide. 15
Figure 7 Impurity profile obtained in a purposely misoriented target. Ion beam is incident 7 C from the <111> axis. 16
Implant Damage and Annealing What is lattice damage? It is the displacement of the lattice atoms by energy Both electronic and nuclear interactions are important in determining range of ions; however, only nuclear collisions can transfer enough energy to target atoms to produce damage The energy to displace a silicon atom just far enough from its lattice site to create a stable separated Si interstitial and Si vacancy (a Frenkel pair) is the displacement energy E d ( 15 ev) Heavy ions (As, Sb, In) stop primarily by nuclei collisions, hence they cause more damage than light ions (B, P) which stop primarily by electronic interactions. Nuclear stopping dominates at low energy, hence we should expect most of the damage associated with B implant to occur where the ions stop. 17
Figure 8 Model for a diamond structure, viewed along a <110> axis. 18
Figure 9 Minimizing channeling. (a) Implantation through an amorphous oxide layer. (b) Misorientation of the beam direction to all crystal axes. (c) Pre-damage on the crystal surfaces. 19
Figure 10 Implantation disorder caused by (a) light ions and (b) heavy ions. 20
Annealing Because of the damaged region and disorder cluster that result from ion implantation, semiconductor parameters such as mobility and lifetime are severely degraded. In addition, most ions as implanted are located ins substitional sites. To activate the implanted ions and restore mobility and other material properties, we must anneal the semiconductor at an appropriate combination of time and temperature. 21
Figure 11 Annealing temperature versus dose for 90% activation of boron and phosphorus. 22
Figure 12 Rapid thermal annealing system that is optically heated. 23
Technology Comparison ================================================================= Determinant Conventional Rapid Thermal Furnace Annealing ========================================================== Process Batch Single-wafer Furnace Hot-wall Cold-wall Heating rate Low High Cycle time High Low Temperature Monitor Furnace Wafer Thermal Budget High Low Particle problem Yes Minimal Uniformity and Repeatability High Low Throughput High Low 24
Implant-Related Process 25
Figure 13 Composite doping profile using multiples implants. 26
Figure 14 Minimum thickness of SiO 2 ( ), Si 3 N 4 (----), and a photoresist (- - - -) to produce a masking effectiveness of 99.99%. 27
Figure 15 60-keV arsenic implanted into silicon, as a function of beam tilt angle. Inset shows the shadow area for tilt-angle ion implantation. 28
Figure 16 Threshold voltage adjustment using boron ion implantation. 29
Example 1 Assume 100keV boron implants on a 200-mm silicon wafer at a dose of 5x10 14 ions/cm 2. Calculate the peak concentration and the required ion beam current for 1 minute of ion implantation. 30
Example 2 Simulation Problem Suppose we want to simulate the ion implantation of a 2 x 10 13 cm -2 dose of boron at 30keV into an n-type <100> silicon wafer. The implant is then followed by drive-in at 950 o C for 60 minutes. If the silicon substrate is doped with phosphorous at a level of 10 15 cm -3, use SUPREM to determine the boron doping profile and junction depth. 31
SUPREM Input Listing TITLE COMMENT INITIALIZE COMMENT IMPLANT COMMENT DIFFUSION PRINT PLOT STOP Implantation Example Initialize silicon substrate <100> Silicon Phosphor Concentration = 1e15 Implant boron Boron Energy =30 Dose=2e13 Diffuse boron Time=60 Temperature=850 Boron Solidsol Layers Chemical Concentration Phospor-Boron Net Active Net Cmin=1e14 End Implantation example 32
Junction depth = 0.4454um Figure 17 Plot of boron concentration as a function of depth into the silicon substrate, using SUPREM. 33