ECE/ChE 4752: Microelectronics Processing Laboratory Ion Implantation ECE/ChE 4752: Microelectronics Processing Laboratory Gary S. May March 4, 2004

Outline Introduction Ion Distribution Ion Stopping Ion Channeling Implant Damage Annealing

Basics Definition: penetration of ionized dopant atoms into a substrate by accelerating the ions to very high energies Allows precisely controlled doses of dopant atoms to be injected (1011 cm-2 to 1018 cm-2) Used instead of pre-deposition in industry, but we don't use because of: cost ($2M for one machine) equipment maintenance limited throughput safety

Equipment

Equipment Description (1) Gas source of material, such as BF3 or AsH3 at high accelerating potential; valve controls flow of gas to ion source (2) Power supply to energize the ion source (3) Ion source containing plasma with the species of interest (such as +As, +B, or +BF2), at pressures of ~ 10-3 torr (4) Analyzer magnet: allows only ions with desired charge/mass ratio through (5) Acceleration tube through which the beam passes (6) Deflection plates to which voltages are applied to scan the beam in x and y directions and give uniform implantation (7) Target chamber consisting of area-defining aperture, Faraday cage, and wafer feed mechanism

Outline Introduction Ion Distribution Ion Stopping Ion Channeling Implant Damage Annealing

Implanted Ions Implantation energies typically 1 keV - 1 MeV Ion distributions have depths of 10 nm - 10 µm Doses vary from 1012 ions/cm2 for threshold voltage adjustment in MOSFETs to 1018 ions/cm2 for formation of buried insulating layers. Advantages of ion implantation: (1) Precise control and reproducibility (2) Lower processing temperature

Range Energetic ions lose energy through collisions with electrons and nuclei in substrate Total distance an ion travels = range (R) Projection of this distance along axis of incidence = projected range (Rp). # collisions per unit distance and energy lost per collision are random variables; standard deviation in the projected range = projected straggle (sp) Statistical fluctuation along an axis perpendicular to the axis of incidence = lateral straggle (s┴)

Projected Range

where S is the ion dose per unit area Ion Profile Implanted profile can be approximated by a Gaussian distribution function: where S is the ion dose per unit area Maximum concentration is at Rp

Example For a 100 keV boron implant with a dose of 5 × 1014 cm-2, calculate peak concentration. Solution: From Fig. 6a, Rp = 0.31 mm and sp = 0.07 µm → n(Rp) = 2.85 × 1019 cm-3

Outline Introduction Ion Distribution Ion Stopping Ion Channeling Implant Damage Annealing

Mechanisms Nuclear (Sn)– transfer of energy from incoming ion to target nuclei (collisions) Ionic (Se) – interaction of incident ion with electrons surrounding target atoms (Coulombic interaction)

where: E0 = initial energy Stopping and Range Average energy (E) loss per unit distance: Total distance traveled: where: E0 = initial energy

Nuclear Stopping When spheres collide, momentum is transferred Deflection angle (q) and velocities, v1 and v2, can be obtained from conservation of momentum and energy Maximum energy loss in a head-on collision:

where ke is a weak function of atomic mass and number Electronic Stopping Proportional to velocity of incident ion: where ke is a weak function of atomic mass and number ke ~ 107 (eV)1/2/cm for silicon

Total Stopping Power

Outline Introduction Ion Distribution Ion Stopping Ion Channeling Implant Damage Annealing

Definition Channeling occurs when incident ions align with crystallographic direction and are guided between rows of atoms.

Results Only loss mechanism is electronic stopping, and range can be significantly larger than it would be in an amorphous target. Ion channeling is particularly critical for low-energy implants and heavy ions.

Minimizing Channeling Can be minimized by : (a) A blocking amorphous surface layer (b) Misorientation of the wafer (c) Creating a damage layer in wafer surface

Outline Introduction Ion Distribution Ion Stopping Ion Channeling Implant Damage Annealing

Cause of Damage Nuclear collisions transfer energy to the lattice so that host atoms are displaced resulting in implant damage (also called lattice disorder). Displaced atoms may in turn cause cascades of secondary displacements of nearby atoms to form a “tree of disorder” along ion path. When displaced atoms per unit volume approach the atomic density, material becomes amorphous.

Tree of Disorder Most energy loss for light ions (e.g., 11B+) is due to electronic collisions→ little damage, most occurs near final ion position. For heavy ions, most energy loss is due to nuclear collisions → heavy damage.

Outline Introduction Ion Distribution Ion Stopping Ion Channeling Implant Damage Annealing

Basics Process of repairing implant damage (i.e., “healing” the surface) is called annealing Also puts dopant atoms in substitutional sites where they will be electrically active 2 objectives of annealing: 1) healing, recrystallization (500 - 600 oC) 2) renew electrical activity (600 - 900 oC)

Boron Annealing Annealing depends on dopant type and dose. For a given dose, annealing temperature is temperature at which 90% of the implanted ions are activated by a 30 minute annealing in a conventional furnace. For boron, higher annealing temperatures are needed for higher doses.

Phosphorus Annealing At lower doses, P annealing is similar to B. When the dose is greater than 1015 cm–2, the annealing temperature drops to about 600 °C. At doses greater than 6 × 1014 cm–2, silicon surface becomes amorphous, and semiconductor underneath amorphous layer is a seeding area for recrystallization. A 100 – 500 nm amorphous layer can be recrystallized in a few minutes. Full activation can be obtained at relatively low temperatures.

Rapid Thermal Annealing Wafer heated to 600 – 1100°C quickly under atmospheric conditions Advantages: Short processing time Less dopant diffusion and contamination Disadvantages: Temperature measurement/control Wafer stress and throughput