Atoms impinging on a surface attach to the surface either with chemical bonds (≈1 eV; chemisorption) or by short-range van der Waals forces (≈0.3–0.4 eV;
physisorption).
These adatoms are able to move because of their own initial energy or by substrate-supplied energy or because they receive energy from the impinging particles.
There are two main modes of film growth: 2D and 3D (Figure 7.2). Two-dimensional growth, also called layer-by-layer growth, is the preferred mode. It
is encountered in many epitaxial depositions. Three- dimensional growth is also known as island growth.
Island growth is common when metals are deposited on insulators where the bonds between film atoms are stronger than the bonds between film atoms and the substrate. A third mode, called Stranski–Krastanov, is a mixture of 2D- and 3D-modes. Understanding of growth mechanisms is elusive and it is difficult to predict which growth mode would take place.
If we measure the early stages of thin-film growth by surface-sensitive techniques, for example, Auger
Thin-film Growth and Structure 75
(a) (b)
Figure 7.2 Thin-film growth modes: (a) 2D (layer-by-layer) and (b) 3D (island) growth. Early stage and coalescence
Zone 3 Zone 2
Zone 1 Zone 1
30 20
10
1 0.10.20.30.40.50.60.70.80.91.0 Substrate
temperature (T /Tm) Argon
pressure (mTorr)
Figure 7.3 A zone model of sputtered thin-film microstructure. Reproduced from Thornton, J.A. (1986), by permission of American Inst of Physics
electron spectroscopy or X-ray photoelectron spec- troscopy (XPS) (which probe 1 or 2 nm deep), we can distinguish the mechanisms: in 2D-growth mode;
the signal from the substrate quickly dies out because the whole surface becomes covered by the deposited layer. In 3D-mode, the substrate signal slowly decreases as the proportion of open substrate area is dimin- ished.
In the initial stages of 3D-growth, numerous small nuclei are formed on the surface. This is a transformation from vapour phase to solid phase. These small nuclei are mobile, and they grow by merging with other nuclei, but they can also incorporate atoms from the vapour phase. Some of the impinging atoms re- evaporate immediately and do not contribute to growth, and some small nuclei also re-evaporate. The nuclei grow in size to become islands, but remain separate, and more nuclei can form on the area between the islands. Coalescence is driven by surface energy (and surface area) minimization, like the droplet movement
on a surface. Islands merge eventually to form a continuous layer. For PVD metal films this happens at ca. 10 to 20 nm thickness (100–200 atomic layers).
Films thinner than this are optically transparent but they can be electrically conductive (percolated). Such films have applications as permeable electrodes in gas sensors and as top metals in optical devices.
Zone models of PVD explain the structure of thin films (Figure 7.3). The first question is which materials will form amorphous films and which will result in (poly) crystalline films. Silicon and other covalently bonding materials often end up as amorphous films, and many compounds and metal alloys with dissimilar-sized atoms similarly result in amorphous films. Elemental metal deposition usually results in polycrystalline films.
The crystallinity of the sputtered films is determined by complex interactions between the substrate (its chemical and structural features and temperature) and the growing film. In the zone-model, pressure and temperature are the main variables to explain film
76 Introduction to Microfabrication
microstructure (temperatures are normalized to melting point temperatures, T /Tm, in K). Zone 1 is small- grained and porous. Zone 2 has larger columnar grains and Zone 3 exhibits still larger grains. The intermediate region is termedZone-T (for transition).
Z1 is the region where the low momentum of the impinging specie is combined with slow chemical processes due to low temperature: the film atoms come to rest almost immediately and do not move. This leads to a porous structure with columnar grains (see Figure 3.6 for simulated columnar-grain structure). Such a structure is under moderate tensile stress. The voids between the grains are nanometre-sized, which leads to measurable density reduction and poor stability because of the absorption of moisture and oxygen. Impurities such as oxygen can change the intrinsic stress from tensile to compressive and complicate the simple model described above.
At lower pressure, ion bombardment induces densifi- cation of the film, and the film stress is highly tensile. A further increase in ion bombardment (at lower pressure or higher sputtering power) leads to the disappearance of voids and conversion to compressive stress. Higher temperature leads to enhanced surface diffusion that can be calculated from Equation 7.1:
x2=√
4Dt (7.1)
where D=D0 exp (−6.5Tm/T) and surface diffusion constant D0 is of the order of 10−7m2/s and t is the time it takes to deposit the next atomic layer. For atoms to diffuse distances similar to void sizes (∼nanometre), Equation 7.1 can be used to estimate temperatures where transition from Z1 to Zone T takes place.
Z2 occurs atT /Tm>0.3, so the surface diffusion is significant. The grains grow larger, and the defects are eliminated. Z3 occurs atT /Tm>0.5, and the diffusion process is very fast. Elimination of the voids enhances diffusion. The films are annealed during deposition. The grains are more isotropic and the films ‘lose memory’
of the deposition-process details.
The final grain size is determined by subsequent annealing steps. The sputtered aluminium grain size is ca. 0.5àm, similar to a typical film thickness. In 3àm lines, there are always many grains across the line, but in 0.5àm lines, the situation changes dramat- ically: there are practically no three-grain boundaries and the grains are end-to-end, known asbamboo struc- ture. All processes that depend on grain boundaries, such as diffusion and electromigration, are strongly affected.
Film structure can change not only continuously as described above but also abruptly. Tantalum films
sputtered under different conditions can end up in either body centred cubic (bcc) structure or as tetragonal β-Ta. Resistivity of bcc-Ta is ca. 20àohm- cm with temperature coefficient of resistivity (TCR) 3800 ppm/◦C. Values for β-Ta are ca. 160àohm-cm and 178 ppm/◦C, respectively (see Figure 2.8 for another tantalum deposition experiment). In Chapter 19, TiSi2 phase transformation upon annealing will be discussed.
Grains in polycrystalline films can have any crystal orientation, but in practice, films are often strongly textured: the distribution of grain orientations are along one or two main crystal planes. For example, aluminium films usually have a (111) texture, that is, (111) planes are parallel to the wafer surface. For undoped LPCVD, polysilicon (110)-orientation crystals dominate, but for in situ phosphorus doped poly (311) is the dominant orientation.
The texture is established during deposition, and it is not much affected by subsequent annealing steps below (2/3) Tm even though the grain size is. Texture inheritance is common: subsequent films easily acquire the same texture as the underlying film. Thin seed layers can therefore be used to modify the thick layers. This is true for CVD and electrodeposition too.
7.2.1 Characterization of PVD films
PVD films, especially sputter-deposited films, can be modified by a number of parameters. System configu- ration and geometry come to play via target-substrate distance, base pressure/gas phase impurities and power coupling scheme/bias voltage; and process parame- ters such as pressure and power affect the momen- tum of the impinging atoms and ions, and substrate temperature is important for desorption, diffusion and reactions.
Collimated sputtering is a technique in which a mechanical grid is placed between the anode and the cathode, and off-angle atoms do not contribute to the flux arriving at the wafer, but are deposited on the collimator walls. Collimated sputtering is better in filling the bottoms of holes and trenches. In Table 7.1, a collimated system is compared with a conventional system, and analysed for an extensive range of film parameters. These characterization measurements relate to R&D phase, and in manufacturing sheet resistance will be used for quick monitoring.
Electrical characterization described in Chapter 2 and above has been DC, but circuits that operate at gigahertz frequencies must be measured at proper frequencies. The same applies to dielectric films too.
Thin-film Growth and Structure 77
Table 7.1 Sputtered titanium nitride (TiN) film characterization: collimated vs. standard Film property Analytical technique Collimated TiN Standard TiN Thickness (nm) RBS (density=4.94 g/cm−3) 81 nm 161 nm
TEM cross section 82 nm 178 nm
Sheet resistance Four-point probe 13.7 ohm/sq 7.4 ohm/sq
Rs uniformity Four-point probe 3.3% 5%
Resistivity (àohm-cm) Rs by four-point probe, 112 132
Thickness by TEM
Density Thickness by TEM & RBS, 4.88 g/cm−3 4.47 g/cm−3
Density by RBS 93% of bulk 86% of bulk
Stoichiometry (Ti/N) RBS 1.31 1.00
Phase Glancing angle XRD TiN (38–1420) TiN (38–1420)
(JCPDS card #) Electron diffraction TiN (38–1420) TiN (38–1420)
Preferred orientation θ−2θ XRD (220) (220)
Electron diffraction
Net stress Gpa Wafer curvature 2.7 3.1
(tensile) (tensile)
Grain structure Cross-section TEM Columnar Columnar
Plane view TEM 2D equiaxial 2D equiaxial
Average grain size TEM 19.2 nm 18.3 nm
Average roughness AFM 0.43 nm 1.23 nm
Min/max roughness 8 nm 18.7 nm
Specular reflection Scanning UV 248 nm: 142% 145%
(% of Si reference) 365 nm: 55% 95%
440 nm: 57% 123%
Impurities Auger O<1% O<1%
(atom %) C<0.5% C<0.5%
Source: Wang, S.-Q. & J. Schlueter: Film property comparison of Ti/TiN deposited by collimated and uncollimated physical vapor deposition techniques,J. Vac. Sci. Technol.,B14(3) (1996), 1837.