Plasma Deposition (Overview) Lecture 1

Plasma Deposition (Overview) Lecture 1

Material Processes Plasma Processing Plasma-assisted Deposition Implantation Surface Modification Development of Plasma-based processing Microelectronics needs (fabrication of Integrated Circuits) Reduced feature sizes, more functionality and performance at reduced cost

Generic Processing steps Plasma sputter thin film deposition Photoresist application Photoresist Exposure IC Fabrication Photoresist development (plasma/wet chemistry) Plasma etching Plasma stripping/ashing

Semiconductor Device Device Structure Sensitive to temperature High temperature processing can damage the underlying layers Solution : Plasma processing Deposition of interconnects (Al/Cu or Al/Si) at room temperature Deposition of encapsulation layers (SiO 2, SiN, SiOxNy) at T ~ 300 o C.

Ion Implantation Si doping/implantation (boron, phosporus, arsenic) Surface hardening (using nitrogen and carbon)

Surface Modification Plasma Cleaning of Substrates.

2 Areas of Plasma Deposition Processes Plasma-enhanced Chemical Vapor Deposition (PECVD) CVD : Increase substrate temperature for chemical reaction to take place (high temperature) PECVD : chemical effect of plasma allow reaction at lower temperature Sputter Deposition Physical sputtering (e.g. deposition of Al or Cu film interconnect) Reactive sputtering (e.g. deposition of TiN thin film)

Plasma Enhanced CVD The plasma is sustained by applying a voltage between 2 electrodes (e.g. AC, RF) The plasma forms "sheaths" with solid surfaces. The RF voltage appears mostly across these sheaths as if they were the dielectric region of a capacitor, with the electrode and the plasma forming the two plates.

Low Pressure Plasma processing Operating pressure 100 mtorr and 10 Torr. Cylindrical electrodes Separation bet. 2 electrodes small compared to the electrode diameter. The electrode "gap 0.5 cm - 10 cm (smaller for higher pressure operation). Typical gaps few hundred times the mean free path Electrons undergo many collisions but do not have time to transfer their energy to the neutral gas.

Film synthesis using PECVD Film uniformity affected by : High pressure High flow rate Short mean free path High gas-phase reaction rates High surface sticking probabilities Disadvantages Use of toxic gases/toxic byproducts High equipment cost (need for vacuum chamber,pumps etc) Advantages of film grown using PECVD Lower temperature processing (less temperature dependent) Lower chance of cracking deposited layer Good dielectric properties Good step coverage

Amorphous Silicon (Thin Films) Applications Solar cells Thin film transistors (TFT) for flat panel display Advantages over crystalline Si Can be deposited on large area substrates (i.e. PECVD) Can be doped to form p or n type layers (electronic device) a-si can be deposited at very low temp (as low as 75 o C) Polymeric substrates can be used Good candidate for roll-to-roll processing Lower quality than c-si but cheaper to produce, suitable for high volume applications (e.g. RFID tags) Film Type Density Deposition Rate Activation energy Substrate Temp Other Remarks a-silicon (PECVD) ~2.2 g/cm 3 50-500Angstrom/ min 0.025-0.1 ev 25-400 o C Using silane (SiH 4 ), 0.2-1 Torr

Main Reactions for SiH 4 Discharges SiH 4 : hazardous gas that reacts with air or H 2 O vapor SiH 4+ : stable or weakly stable SiH 3+ : typically observed Bond Energies SiH 2 H : 3.0 V SiH-H : 3.4 V Si-H : 3.0 V

Precursors for film growth SiH 3, SiH 2 radicals important for film growth SiH 4 Also participates in the surface reactions SiH 3 + Ion bombardment plays a critical role in film growth

Surface Coverage Model Surface (e.g. Silicon) Passive sites Active sites (1 dangling bond) Ion bombardment Also removes Si and/or hydrogen from the surface (site activation) Create dangling bonds SiH 2 Also insert itself into lattice upon impact with the surface (active or passive) Film deposition Adsorbed SiH 3 radicals can diffuse along the surface (only at active sites) SiH 4 adsorbed upon impact at active sites lose an H atom (site passivation) Si passivated activated substrate Si Si Si Si Si Si Si OR H Si Θ a +θ p = 1 Θ a = fraction of the surface covered by the active site Θ p = fraction of the surface covered by the passive site Si Si Si Si

Surface Reactions Kx : rate constants; function of sticking coefficient and velocity Yi : yield of H atoms removed per incident ion

Rate of Creation Rate of Creation of active sites Typical discharge n is /n 4s ~ 10-4 Θ a ~ 10-2 n 2S ~10-2 n 3s Deposition rate (SiH 2, SiH 3 ) 1 st term 10X larger than 2 nd term 1 st term responsible for deposition of good film

Silicon Dioxide formation (CVD) SiO 2 growth condition (using bare Si) 850-1100 o C (using O 2 or H 2 O gas) 600-800 o C (using SiH 4 /O 2 or TEOS/O 2 gas) Reference : www.icknowledge.com

Tetraethoxysilane (TEOS) Si(OC 2 H 5 ) 4 rel. inert at RT Bond Energies C-O : 3.7 V Si-O : 4.7 V N 2 or Ar as gas carriers 1%TEOS/99%O 2 Gas phase kinetics dominated by O 2 (pure O 2 discharge) O 2 rich mixture to produce good quality film O2 burns C and H in TEOS forming CO 2 and H 2 O gases Must be efficient to control C and H contaminants

Silane with carrier gases SiH 4 /Ar/oxidants Oxidants : N 2 O, NO, O 2 Deposition rate ~ 200 nm/min N 2 O produces more oxygen atoms; best film quality. Deposition precursors SiH 3, SiH 2, O radicals Created by e - impact dissociation of SiH 4 and oxidants Initial Steps in Film formation 2SiH 3 + O(s) (SiH 3 ) 2 O SiH 3 OH + H 2 Further oxygenation to burn H atoms to form H 2 O (g) Final Film : 2-9% H atoms

SiH 3 and SiH 2 precursors High sticking probability s~ 0.35 Lead to nonconformal deposition (e.g. different deposition rates on trenches) Small s Big s

Nonconformal deposition Maximum deposition rate at near the top of trench Formation of void/keyhole within the trench Conformal deposition Small s (precursor reflection) Big s precursor should have diffusion rate along the surface Deposition flux on the sidewall

Silicon Nitride Used as encapsulating layer for IC Resistant to water vapor, other chemical contaminants SiH 4 /NH 3 Precursors SiH 3, SiH 2, NH radicals (electron impact) Other precursors : Si 2 H 6, Si(NH 2 ) 4, Si(NH 2 ) 3 Parameters Deposition pressure : 0.25-3 Torr Temp: 250-500 o C Deposition rate : 20-50 nm/min Stoichiometry (SiN x H y ) x~ 1-1.2 y~ 0.2-0.6 Film quality dependent on H content H atoms in lattice At high temp/high RF power flux : low H content Thermal or ion induced desorption of H in lattice

Sputter Deposition Ions incident on a target Target components deposited on a substrate Wide variety of materials can be deposited (metals, metal alloys, ceramics, etc.)

Multicomponent target Multicomponent target Different composition of deposited film Difference in sputtering yield for different materials Altered layer form on surface (due to SY difference) Flux of atom sputtered from layer will have the same stoichiometry with the bulk

Deposition Rate Γ i : incident ion (cm -2 s -1 ) n f : film density (cm -3 ) A t : target area sputtered A s : substrate area sputtered γ sput : Sputtering yield Film Morphology Influenced by substrate temperature (T/T m ) Low T, low P : tapered column, domed head, voids between column Zone 1 : T/T m ~ 0.3 Higher T Zone T : 0.3 T/T m 0.5 Fibrous structure, less voids, smooth surface, denser Result of ion bombardmentinduced mobility of deposited atoms on the substrate Zone 2: 0.5 T/T m 0.8 T-activated surface diffusion Columnar grains Zone 3 : 0.8 T/T m 1 Volume diffusion of atoms Smooth, randomly-oriented polycrystalline film

Gas dissociated from by plasma Reacts with target Reactive Sputtering Deposition of compound film from sputtered material Components have different vapor pressures, sticking probabilities Used to synthesize dielectrics (oxides, nitrides, carbides, silicides) Also used to synthesize superconducting films (e.g. YBaCuO using O 2 reactive gas) Reactive gases O 2, H 2 O, N 2, NH 3, CH 4, C 2 H 2, SiH 4 target SiO 2 SiO 2 Si Ar Si-rich Ar/O 2 stoichimetric O 2 reactive substrate SiO 2 SiO 2 SiO 2

Modes of Operation Chemical reaction at both substrate and target Low ion flux, high gas flux : target covered by compound High ion flux, low gas flux : metallic target Metallic target : higher deposition rate Fixed ion flux/ reactive gas varied Transition between covered and metallic mode Transition flux for increasing flux to pass from metallic to covered mode is higher (vs. decreasing flux to pass from covered to metallic mode)

Reactive Sputtering Model A t,a s : target and substrate areas Θ t & Θ s : fraction of target/substrate areas covered by compound film γ m, γ m : sputtering yield for metal/compound Γ i, Γ r : incident and reactive fluxes s r : sticking coefficient of a reactive molecule on a target i: # of atoms/molecule of reactive gas (e.g. i=2 for O 2 ) Total number of reactive gas molecules/ second - Consumed to form compound and deposited on substrate Formation rate of compound on target Target sputtering flux Accounting for ratio of target and substrate areas

Film synthesis (GaN) CVD, PECVD, MBE Plasma sputtering (using liquid Ga and nitrogen) Plasma deposition at low temperature Equilibrium N 2 pressure over GaN. Dashed lines indicate the maximum pressure and temperature available in the experimental system 1 bar = 1 x 10 5 Pa

Transparent Conducting Oxides (TCO)

TCO Industrialization

Non-uniform resistivity, ρ