Roland A. - PVD for microelectronics (779636), страница 55
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As apractical matter, it is not deposition rate that matters but the deposition ratenormalized to the sputter cathode power, or specific deposition rate (SDR).Figure 9.7 shows SDR values (,~/sec-kW) as a function of power to anA1 planar magnetron cathode showing a flattening above ~ 5 kW. As thepower applied to the cathode increases, the number of sputtered A1atoms in the volume between target and wafer also increases, and the increased AI-AI gas-phase collisions scatter AI away from the wafer, limiting the gain in AI deposition rate below that expected from the increased sputter erosion rate of the target. Using the data in Fig.
9.7, wesee that while 3 kW gives an AI deposition rate of 5220 A/min (SDR =29 ~/sec-kW), one must go to 9 kW (SDR - 21 ~/sec-kW) to double thedeposition rate to ~ 1.1 /~m/min. With regard to cathode size, the SDRtends to decrease linearly with increasing target area. For example, theSDR values in Fig. 9.7 taken with a 12-inch-diameter cathode (VarianQuantum TM source) were empirically found to be ~ 50% higher when an8-inch-diameter magnetron was used (Varian Mini-Quantum TM source).On the other hand, coating uniformity of 200-mm wafers was not nearlyas good when the smaller cathode was used.9.3.3 DEPOSITION TEMPERATURE AND MICROSTRUCTURESince chemical reaction rates and physical diffusion phenomena depend strongly (often exponentially) on temperature, it is not surprisingthat the deposition temperature of PVD A1 has a strong influence on itsR.
POWELL AND S. M. ROSSNAGEL29630'~"28-Jr(/)~26-24-er22-tO20-u~oQ.18-Q16"2ll1tiIl345678910Power (kW)FIG. 9.7Deposition rate of AI as a function of magnetron cathode power.microstructure and therefore on its electrical, optical, and mechanicalproperties. In addition, as discussed in Chapter 5, outgassing from theheated wafer and the indirectly heated chamber walls and fixtures can release oxidants (H20, 02) and other contamination that degrade film properties. For example, high specular reflectivity is often used as a measure offilm quality, with milky-looking, rough AI films indicative of oxidationduring sputter deposition.Grain Size PVD AI and AI alloy films are polycrystalline in nature withthe dominant orientation, grain size, and grain size distribution dependenton a variety of process conditions, but strongly influenced by temperature.
Figure 9.8 shows the dramatic increase in average grain size for aPVD AI-Si-Cu film as a function of wafer temperature (20-400~duringdeposition onto a thermal oxide-coated Si wafer. The grain size distribution can be visualized from Fig. 9.9, where a dark field optical micrograph( ~ 1000• of the A1 alloy film surface is shown at the low- (20~ andhigh-end (400~ temperatures.Reflectivity Even though the use of A1 in microelectronics is driven byits electrical properties, its optical properties are routinely measured be-PVD MATERIALS AND PROCESSES297FIG.
9.8 Grain size of PVD AI-Si-Cu alloy as a function of wafer temperature during depositiononto SiO,.cause they directly impact subsequent lithographic patterning steps and indirectly indicate film purity and microstructure. The reflectivity of AI isprobably the most common optical property measured even though the realand imaginary parts of the complex dielectric constant N - n + ik are themore fundamental physical parameters. ( N o t e : The refractive index n isFIG. 9.9 Grain size distribution for the film of Fig. 9.8 for (a) very low (20~(400~ process temperature.and (b) very high298R.
POWELL AND S. M. ROSSNAGELsometimes called out on thin film spec data sheets as "RI".) The specularreflectivity of A1 is typically measured at a wavelength used for opticallithography (such as 440 nm) and is given in absolute units or relative tothat of Si. As with grain size, reflectivity also depends on temperature butin a rather complicated way that is related to changes in both grain size andfilm morphology (see Fig. 9.10).
The effect of temperature on A1 film morphology is conveniently summarized in Fig. 9.11 using the structure zonemodel first proposed by Movchan and Demchishin [9.10] whereby thestructure of a film deposited on a substrate at temperature T depends universally on the normalized temperature ratio T/T, where T is the meltingpoint of the film in degrees Kelvin (this ratio is also referred to as the homologous temperature). The initial work of Movchan and Demchishin wasbased on e-beam evaporated films and did not consider the structure ofPVD films per se.
The model was later amended by John Thornton for application to sputter deposition by addition of another independent variable m the pressure of the inert sputter gas in the deposition chamber.Thornton then introduced the amended model to the semiconductor industry in the early 1970s [9.11, 9.12]. As a result, the three-dimensional pictogram shown in Fig. 9.11 relating zones of PVD film morphology to bothsputter gas pressure (x-axis) and normalized temperature T/T, (y-axis) ispopularly referred to as a Thornton diagram.100-~.AZone 2 for AI80-C(B60-4-v>mG:Zone T for AI40-20O..i0'i.... *100i200....
~i300""I'400~i500Deposition T e m p e r a t u r e (~FIG. 9.10Reflectivity of a PVD AI film as a function of deposition temperature.PVD MATERIALSAND PROCESSES2991.0.930'"~:."Pressure(mTorr)1:0.10.60.2"uOs,ra,eTemperature (T/Trn)Visualization of PVD film morphology versus process pressure and temperature can bemade using a Movchan-Demchishin diagram [9.10], also referred to as a "Thornton" diagram [9.11 ].(Reprinted with permission from J.
A. Thornton, J. Vac. Sci. & Tech. All(4): 666 (1974). Copyright1974 American Institute of Physics.)FIG. 9.11The structure zone model graphically shows how PVD film microstructure evolves with increasing deposition temperature from highly porous andcolumnar (Zone 1), to densely columnar (Zone 2), and finally to a recrystallized dense grain structure (Zone 3).
Given the range of PVD AI sputterpressure ( ~ 3-5 mTorr) and deposition temperature (20-550~used formicroelectronics, the relevant regions of the diagram are the "transition"Zone T and Zones 2 and 3. Zone T films are characterized by small grains.The surface is flat relative to the wavelength of the incident radiation so thatthe entire film surface acts as one large reflector, and film reflectivity ishigh m R ~ 90% absolute at 440 nm. As temperature increases, the filmmorphology moves into Zone 2, where the grains are larger and comparable to the wavelength of incident light.
The surface angle of the individualgrains differ from those surrounding a random grain and the reflectivity isreduced. At sufficiently high temperature ( > 450~a Zone 3 film with recrystallized, larger grains is formed. The individual grains are now largeenough to act as individual reflectors and R increases slightly.Resistivity The bulk resistivity of a PVD A1 film typically decreasesslightly with deposition temperature because the larger individual grainslead to a reduced number of grain boundaries per unit length, leading toreduced grain boundary scattering of the conduction electrons. This is300R.
POWELL AND S. M. ROSSNAGELillustrated in Fig. 9.12, which shows the bulk room-temperature resistivity for a t = 1-/xm-thick PVD A l - l % S i - 2 % C u film as a function ofdeposition temperature. The measured sheet resistance of the film wouldhave been R s = p / t = ( 3 - 4 / z l ) - c m ) / ( 1 /xm) = 0.03-0.04 fl/sq.T h e r m a l S t r e s s Highly stressed films are not desirable in IC processingsince this can lead to reliability problems, particle generation, and even thepossibility of delamination of the film from the substrate or underlayer. Ingeneral, the total stress in a PVD film results from the sum of three components: (1) external stress, (2) intrinsic stress, and (3) thermal stress.External stress is usually not important given the small weight of a SiFIG. 9.12Bulk resistivity of PVD AI- 1%Si-2%Cu film as a function of deposition temperature.PVD MATERIALS AND PROCESSES301wafer and the subatmospheric pressure of PVD processing (one notable exception is the Forcefill TM method described in Chapter 7 in which extremely high external pressure ( > 600 atm) is applied to cause an A1 filmto flow into fine structures.) Intrinsic stress is related to the detailed microstructure of the film (e.g., lattice defects and impurities) and by the mismatch in lattice spacing between film and substrate.
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