ECE 6450 - Dr. Alan DoolittleGeorgia TechLecture 12Physical Vapor Deposition: Evaporation and SputteringReading:Chapter 12ECE 6450 - Dr. Alan DoolittleGeorgia TechEvaporationEvaporation and Sputtering (Metalization)For all devices, there is a need to go from semiconductor to metal. Thus we need a means to deposit metals. Many of these techniques used for metal deposition can be used to deposit other materials as well.Several methods are currently used for deposition of metal layers.Physical Vapor Deposition techniques (PVD)1.) Evaporation2.) Sputtering3.) Chemical Vapor Deposition (CVD)4.) Electrochemical techniques1.) Evaporation:Advantages: Highest purity (Good for Schottky contacts) due to low pressures.Disadvantages: Poor step coverage, forming alloys can be difficult, lower throughput due to low vacuum.Evaporation is based on the concept that there exists a finite “vapor pressure” above any material. The material either sublimes (direct solid to vapor transition) or evaporates (liquid to vapor transition).ECE 6450 - Dr. Alan DoolittleGeorgia TechTwo main types of evaporators: E-beam and filament.Evaporator SystemsRough PumpCryo or Turbo PumpAir InletValveValveValveWafersHeated MaterialVacuum ChamberECE 6450 - Dr. Alan DoolittleGeorgia TechPxTePeevaporationHNkToEakT=≈−⎛⎝⎜⎞⎠⎟⎛⎝⎜⎞⎠⎟31012 3 2 1 2σ//∆JPkTm=22πRmkPTAreardevaporation=2422πρ πFor evaporation, the vapor pressure is,where σ is the surface tension of the liquid, N is avagadro’s number, and ∆H is the enthalpy of evaporation (the energy required to convert from a liquid to gas phase).We can define the number of molecules crossing a plane per unit time as,where P is pressure in Pascals, k is Boltzsmans constant, T is absolute temperature and m is the atomic or molecular mass.If the liquid is assumed to be at a constant temperature, and the crucible (container holding the liquid) has a constant area opening, the wafer is located on a sphere as defined by figure 12-3, the the deposition rate is,where ρ is the mass density (kg/m2), Area is the area of the wafer, and r is the radius of the sphere defined in figure 12-3.Evaporator PhysicsVirtual Source: Point in free space where the pressure drops enough to result in molecular flow. Closer to the source viscous flow applies.ECE 6450 - Dr. Alan DoolittleGeorgia TechThe step coverage of evaporated films is poor due to the directional nature of the evaporated material (shadowing) (see figure 12-5). Heating (resulting in surface diffusion) and rotating the substrates (minimizing the shadowing) help with the step coverage problem, but evaporation can not form continuous films for aspect ratios (AR=step height/step width or diameter) greater than 1.We need a less directional metalization scheme====> Higher pressures!Evaporation Step CoverageECE 6450 - Dr. Alan DoolittleGeorgia Tech2.) Sputtering:Advantages: Better step coverage, less radiation damage than E-beam evaporation, easier to deposit alloys.Disadvantages: Some plasma damage including implanted argon. Good for ohmics, not Schottky diodes.A plasma at higher pressure is used to “knock” metal atoms out of a “target”. These energetic atoms deposit on a wafer located near the target. The higher pressure produces better step coverage due to more random angled delivery. The excess energy of the ions also aids in increasing the surface mobility (movement of atoms on the surface).SputteringECE 6450 - Dr. Alan DoolittleGeorgia TechEHeat of VaporizationwhereMMMMthreshold=−=+γγγ()()1412122The sputter yield (ratio of target atoms expelled to incident atoms impinging on the target) increases with increasing energy (plasma power or DC bias). (See 12-13).A threshold energy for the release of an atom from the target exists, below which the atom is not “sputtered”. This threshold energy is,SputteringECE 6450 - Dr. Alan DoolittleGeorgia TechFilm MorphologyDeposited films can be: (See 12-20).1.) Porous and/or Amorphous —> Results from poor surface mobility =low temperature, low ion energy (low RF power/DC bias or higher pressures=less acceleration between collisions).2.) “T-zone”: Small grain polycrystalline, dense, smooth and high reflectance (the sweet spot for most processes) Results from higher surface mobility =higher temperature or ion energy3.) Further increases in surface mobility result in columnar grains that have rough surfaces. These rough surfaces lead to poor coverage in later steps.4.) Still further increases in surface mobility result in large (non-columnar) grains. These grains can be good for diffusion barriers (less grain boundary diffusion due to fewer grains) but pose problems for lithography due to light scatter off of large grains, and tend to be more rigid leading to more failures in electrical lines.Sputtering Details12 or T34ECE 6450 - Dr. Alan DoolittleGeorgia TechOften, it is needed to sputter alloys instead of pure elemental metals (Al+Si+Cu). Consider the problem ofelectromigration in AluminumCauses:Electron momentum transfer to the ions in high current density lines.Solution: Add a small number (typically <3-5%) larger atoms such as copper that “anchor” the aluminum atoms in place or replace the entire metal line with larger atoms such as copper so that each atom is more difficult to move.When sputtering Aluminum and Copper alloys, the film on the wafer has more copper than the target. Reason: At the target, the argon has achieved high enough energy to sputter the Al and Cu evenly. However, in the gas (lower electric field), the heavier atoms are less effected by light sputter gas. The light Al can gain enough energy to be above it’s evaporation temperature when it hits the wafer. Thus, the Cu sticks, but the Al does not.The target material must be tailored to the sputter conditions to get the desired film composition!Sputtering DetailsECE 6450 - Dr. Alan DoolittleGeorgia TechStressYoungs Modulus of filmPoissons s R atio of filmdTthermal mismatch filmTTwaferroomdeposition=−−∫1( ' )()ααStressEDtvR=−δ2213()Film stress:Film stress can result in wafer bowing (problems with lithography), film cracking or peeling. There is 2 kinds of films stress:1.) Extrinsic Stress (forces acting on the wafer due to sources external to the deposited film)Example: Thermal induced stress:where α is the thermal expansion coefficient [1/degree].2.) Intrinsic Stress (forces acting on the wafer due to sources internal