Chapter 12pPhysical Vapor Deposition (PVD)

PVD methods:

Evaporation condensation of metal vapor in high vacuum to deposit a thin film on a wafer; unable tohigh vacuum to deposit a thin film on a wafer; unable to cover severe topology (poor step coverage) which is beneficial when doing lift-off patterning; also hard to g p gdeposit alloys due to possible difference in melting points.

Sputtering - use of plasma and acceleration of ions towards a target; material sputtered from theions towards a target; material sputtered from the target and deposited on the wafer; extensively used in Si technology; moderate step coverage.gy; p g

Evaporation Process:*Chamber under high vacuum (low pressure and long mfp)*metal pieces placed in crucible (charge) and heated to Tmp p ( g ) m*substrates placed above the crucible, and*metal deposits on the substrates.

Not shown in picture is a shutter that would beNot shown in picture is a shutter that would be opened when the evaporation rate reaches a certain rate indicating pure metal deposition.

View factor associated with deposition (like RTP); deposition rate depends on location of wafer in chamber (above crucible will have dep rate).( p )

For better uniformity and step coverage, planetary sample holders are rotated during deposition and samples are heated.

Deposition rate monitored with quartz crystal ( ill f h hif h

Figure 12.1 A simple diffusion-pumped evaporator showing vacuum plumbing and

(oscillates at a resonance frequency that shifts when additional mass is deposited on the crystal).

p g p gthe location of the charge-containing crucible and the wafers.

For reasonable depositionFor reasonable deposition rates, the vapor pressure must be 10 mTorr. This

k it l i iblmakes it nearly impossible to evaporate some materials (refractory metals: Ta, W, Mo, Ti)

Figure 12.2 Vapor pressure curves for some commonlyevaporated materials (data adapted from Alcock et al.).

Deposition rate depends on position of wafer View factor, k depends on R, , Wafers directly above the crucible will be coated more heavily than wafers off to the side. Wafers can all be mounted on the surface of awafers off to the side. Wafers can all be mounted on the surface of a sphere (planetary) for more uniform deposition.

Figure 12.3 The geometry of deposition for a wafer (A) in an arbitrary position and (B) on the surface of a sphere.

Step Coverage AR = step heightstep diameter

If the substrate surface is not planar, the long mfp in evaporation means thatevaporation means that evaporated material will follow a straight line; some areas are shadowed

Helps to rotate and heat the psubstrates.

Planetary holders rotateFigure 12.5 (A) Time evolution of the evaporative coating of a feature with aspect ratio of 1.0, with little surface atom mobility (i e low substrate

Planetary holders rotate simultaneously around two axes.

little surface atom mobility (i.e., low substrate temperature) and no rotation. (B) Final profile of deposition on rotated and heated substrates.

Evaporator systems: crucible heating techniquesResistively Heated

Figure 12.7 Resistive evaporator sources. (A) Simple sources including heating the charge itself and using a coil of refractory metal heater coil and a charge rod. (B) More standard thermal sources including a dimpled boat in resistive media.

Evaporator systems: crucible heating techniquesInductively HeatedInductively Heated

Figure 12.8 Example of an inductively heated crucible used to create moderately charged temperatures.

Evaporator systems: crucible heating techniquesElectron beamElectron beamHeat of vaporization is

supplied by the impact of an electron beam focused on the charge and melts a region of the material to be

devaporated; Adv: possible to co-evaporate materials with dual targetsdual targets, Disadv: more expensive than resistance heated systems substrates may

Figure 12.9 Electron beam evaporative sources. (A) A simple low flux source using a hot wire electron source and a thin movable rod. (B) A popular source using a 2707 source arc in which the beam can be rastered

systems, substrates may have radiation damage

p p gacross the surface of the charge. The magnet must be much larger than shown to achieve the full 270 of arc.

Evaporation

Advantages: Simple process, relatively inexpensive, high purity films can be deposited.

Disadvantages: difficult to evaporate alloysDisadvantages: difficult to evaporate alloys due to different melting points, line of sight deposition results in poor surface

l th i t ti f thcoverage unless there is rotation of the samples; deposition of refractory materials is a problem due to high temperatures required.

Figure 12.10 A commercial evaporator. Inset shows a planetary (photographs courtesy of CHA Industries).

Evaporation of Alloys

Figure 12.11 Methods for evaporating multicomponent films include (A)single-source evaporation, (B) multisource simultaneous evaporation, and (C)multisource sequential evaporation.q p

Sputtering:Bombard the target (cathode) with energetic ions, usually Ar+, in a plasma. Target material, not the wafers, must be placed on the electrode with maximum ion flux.

*Chamber base pressure - high vacuum, *Argon flows into chamberArgon flows into chamber raising pressure to mTorr range,*power supplied to electrodes, *target material deposits on the*target material deposits on the wafers.

Allows deposition of refractory metals, alloys, dielectrics if an RF system is used. Figure 12.12 Chamber for a simple parallel-platey Figure 12.12 Chamber for a simple parallel plate

sputtering system.

Basic steps in sputter deposition:1) plasma generation glow discharge formed when inert gas ) p g g g gbecomes ionized by an E-field; an electron accelerated towards anode, ionizes Ar atoms upon collision2) ion bombardment Ar+ impacts the target with high energies and2) ion bombardment Ar impacts the target with high energies and transfer their momentum to the target material; these collisions disrupt the atomic surface causing target atoms, ions, and electrons to be ejected3) sputtered atom transport sputtered atoms, ions, are influenced by collisions they undergo during transport to the film (determined y y g g p (by background pressure); sputtered particles will lose their energy as number of collisions increases so important to control the pressure4) film growth - sputtered material leaves the target and deposits on4) film growth - sputtered material leaves the target and deposits on surrounding surfaces. The rate of diffusion is dependent upon the substrate material and Temp. Growth proceeds by diffusion and form

l i l i d ll f i l d i l dnuclei, nuclei grow and eventually form islands; islands grow together until a continuous film is formed.

When an energetic ion strikes the surface of a material --1) Ions with low energies may bounce of the surface1) Ions with low energies may bounce of the surface2) Ions may adsorb to the surface, giving up its energy to

phonons (heat)3) I t t i t t i l d iti d i t th3) Ion penetrates into material, depositing energy deep into the

substrate

Figure 12.13 Possible outcomes for an ionincident on the surface of a wafer.

Sputter Yield (S)Determines rate of sputter depositionDetermines rate of sputter deposition

S = # target atoms ejected/number of ions incident

Depends on:pTarget materialMass of bombarding ionsEnergy of bombarding ionsEnergy of bombarding ions

Each target material has a threshold energy (below that energy no sputtering occurs), typically 10-30 eV.

Figure 12.14 Sputter yield as a function of ion f l i id i fenergy for normal incidence argon ions for a

variety of materials (after Anderson and Bay, reprinted by permission).

S versus Ion atomic number

Figure 12.15 Sputter yield as a function of the bombarding ion atomic number for 45-keV ions incident on silver copper andatomic number for 45 keV ions incident on silver, copper, and tantalum targets (after Wehner, reprinted by permission, AIP).

Magnetron Sputtering a magnetic field applied at right angles to the E-field; causes e- to follow spiral paths, increases probability of ; p p , p yionizing a gas atom, increases ionization efficiency, confines plasma resulting in a higher deposition rate; also able to form plasma at lower chamber pressuresplasma at lower chamber pressures

Figure 12.17 Planar and cylindrical magnetron sputtering systems T: target; P: plasma; SM: solenoid; M: magnet; E: electric field; B: magnetic field (after Wasa and Hayakawa, reprinted by permission, Noyes Publications).

Fig. 12.18 in text, p. 340 shows cross section of a planar magnetron target using permanent magnets to supply the fieldThe region of the target beneath the ring-shaped volume where the plasma density is highest is sputtered the most rapidly and thishighest is sputtered the most rapidly and this target erosion is called the race-track

Target showing erosion in the race-track

Figure 12.18 Detailed cross section of a l l i

From: Fundamentals of High Power ImpulseMagnetron Sputtering, dissertation by Johan Bhlmark

rectangular planar magnetron target using permanent magnets to supply the field (after Wasa and Hayakawa, reprinted by permission, Noyes Publications).

Film morphologyZone model (Zones 1, 2, 3, T) indicates the films finalZone model (Zones 1, 2, 3, T) indicates the films final characteristics based on the substrate temperature and ion energy; T-region is characterized by very small grains

Zone1-low T, low ion energy yields amorphous, porous materials; Raise T or lower P moves to T-zoneZone2-Increase T and/or increase ion energy will increase grain size tallincrease grain size - tall columnar grainsZone3-Increase T, film has large 3-D grains surface maylarge 3 D grains surface may be rough and hazy

Figure 12.21 The three-zone model of film deposition as proposed by Movchan and Demchishin (after Thornton, reprinted by permission, AIP).

Step CoverageApplication of substrate heat will dramatically improve the stepApplication of substrate heat will dramatically improve the step coverage due to surface diffusion; High AR can be a problem otherwise.

Figure 12.22 Cross section of the time evolution of the typical step coverage for unheated sputter deposition in a high aspect ratio contact.

Can improve step coverage by collimated sputtering or application of a bias to the waferapplication of a bias to the wafer.

Figure 12.24 In collimated sputtering a disposable collimator is placed close to the

f t i di ti litwafers to increase directionality.

Figure 12.26 In bias sputtering, the ions incident on the surface of the wafer redistribute the deposited film to improve step coverage.

Ionized Metal Plasma (IMP) sputter deposition ejected atoms pass through a second plasma; IMP process produces p g p ; p pnear-vertical deposition.

Figure 12.25 The Endura system by Applied Materials uses a number of PVD or CVD chambers fed by a central robot. For conventional and IMP sputtering, targets are hinged to open upward. Two open chambers are shown, along with the load lock (from Applied Materials).

Reactive sputtering: use of reactive gases (O2, CH4, NH3, N2) rather than inert gases to sputter oxides, carbide, nitrides. g p , ,Example below is for TiN

Figure 12.28 Resistivity and composition of reactively sputtered TiN as a function of the N2flow in the sputtering chamber (after Tsai, Fair, and Hodul, reprinted by permission, The Electrochemical Society, and Molarius and Orpana, reprinted by permission, Kluwer Academic Publishing).

Figure 12.29 Cross section electron micrograph of a moderately high aspect ratio contact that has been sputter-deposited with TiN (after Kohlhase, Mndl, and Pamler, reprinted by permission, AIP).

A thin film deposited on a substrate can be either in tensile stress or compressive stress; if stress is too large, film may peel away from the surface; implications in reliability.

Figure 12.30 The change in wafer deflection may be used to measure the stress in a deposited layer. This is typically measured using a reflected laser beamusing a reflected laser beam.

SputteringSputtering

Advantages: Moderately good step coverage; preferred g y g p g ; ptechnique for deposition of alloys, can sputter a wide variety of materials

Disadvantages: may have some Argon incorporation in the film; could have some damage to substrate although not as much as in e-beam evaporation