Ohmic contact



An ohmic contact is a region on a photolithography. Low-resistance, stable contacts are critical for the performance and reliability of integrated circuits and their preparation and characterization are major efforts in circuit fabrication.

Theory

The screening length means that any electrical field extends only a short distance beyond the interface.

 

In a classical physics picture, in order to surmount the barrier, a carrier in the semiconductor must gain enough energy to jump from the Fermi level to the top of the bent conduction band. The needed barrier-surmounting energy φB is the sum of the built-in potential and the offset between the Fermi level and the conduction band. Equivalently for n-type semiconductors, φB = φM − χS where χS is the semiconductor's dopants in the semiconductor:

\nabla ^2 V = \frac{\rho}{\epsilon}

where in MKS units ρ is the net charge density and ε is the dielectric constant. The geometry is one-dimensional since the interface is assumed to be planar. Integrating the equation once, we get

\frac{dV}{dx} = \frac{\rho x}{\epsilon} + C_0

The constant of integration C_0 = \frac{-\rho W}{\epsilon} due to the definition of the depletion width as the length over which the interface is fully screened. Then

V(x) = \frac{\rho}{2 \epsilon}x^2 - \frac{\rho W}{\epsilon}x + V_{bi}

where the fact that V(0) = Vbi has been used to fix the remaining integration constant. This equation for V(x) describes the dashed blue curves in the right-hand panels of the figures. The depletion width can then be determined by setting V(W) = 0 which results in

W = \sqrt{ \frac{2 \epsilon V_{bi}}{\rho} }

For 0 < x < W, ρ = eNdopant is the net charge density of ionized donor or acceptors Ndopant in the completely depleted semiconductor and e is the electronic charge. ρ and Vbi have positive signs for n-type semiconductors and negative signs for p-type semiconductors giving the positive curvature V''(x) for n-type and negative curvature for p-type as shown in the figures.

Note from this crude derivation that the barrier height (dependent on electron affinity and built-in field) and barrier thickness (dependent on built-in field, semiconductor dielectric constant and doping density) can only be modified by changing the metal or changing the doping density. In general an engineer will choose a contact metal to be conductive, non-reactive, thermally stable, electrically stable and low-stress, and then will increase the doping density below the contact to narrow the width of the barrier region. The highly doped regions are termed n + or p + depending on the carrier type. Since the transmission coefficient in tunneling depends exponentially on particle mass, semiconductors with lower bandgaps more readily form ohmic contacts because their electron affinities (and thus barrier heights) tend to be lower.

The simple theory presented above predicts that φB = φM − χS, so naively metals whose work functions are close to the semiconductor's electron affinity should most easily form ohmic contacts. In fact, metals with high work functions form the best contacts to reconstruct leading to a new electronic state. The dependence of contact resistance on the details of the interfacial chemistry is what makes the reproducible fabrication of ohmic contacts such a manufacturing challenge.

Preparation and characterization of ohmic contacts

The fabrication of ohmic contacts is a much-studied part of materials engineering that nonetheless remains something of an art. The reproducible, reliable fabrication of contacts relies on extreme cleanliness of the semiconductor surface. Since a native oxide rapidly forms on the surface of silicon, for example, the performance of a contact can depend sensitively on the details of preparation.

The fundamental steps in contact fabrication are semiconductor surface cleaning, contact metal deposition, patterning and annealing. Surface cleaning may be performed by sputter-etching, chemical etching, reactive gas etching or ion milling. For example, the native oxide of silicon may be removed with an HF dip, while chemical vapor deposition (CVD). Sputtering is a faster and more convenient method of metal deposition than evaporation but the ion bombardment from the plasma may induce surface states or even invert the charge carrier type at the surface. For this reason the gentler but still rapid CVD is increasingly preferred. Patterning of contacts is accomplished with standard photolithographic methods such as lift-off, where contact metal is deposited through holes in a photoresist layer that is later dissolved away. Post-deposition annealing of contacts is useful for relieving stress as well as for inducing any desirable reactions between the metal and the semiconductor.

The measurement of transmission line method is typical.

Technologically important kinds of contacts

Modern ohmic contacts to silicon such as titanium-tungsten disilicide are usually oxygen in the native oxide. Silicides have largely replaced Al in part because the more refractory materials are less prone to diffuse into unintended areas especially during subsequent high-temperature processing.

Formation of contacts to compound semiconductors is considerably more difficult than with silicon. For example, GaAs surfaces tend to lose alloy contact layer as opposed to a heavily doped layer. For example, GaAs itself has a smaller bandgap than AlGaAs and so a layer of GaAs near its surface can promote ohmic behavior. In general the technology of ohmic contacts for III-V and II-VI semiconductors is much less developed than for Si.

Material Contact materials
Si Al, Al-Si, TiSi2, W, MoSi2, PtSi, CoSi2, WSi2
Ge In, AuGa, AuSb
GaAs AuGe, PdGe, Ti/Pt/Au
GaN Ti/Al/Ti/Au, Pd/Au
InSb In
ZnO InSnO2, Al
CuIn1-xGaxSe2 InSnO2
In

Transparent or semi-transparent contacts are necessary for active matrix LCD displays, optoelectronic devices such as indium tin oxide, a metal that is formed by reactive sputtering of an In-Sn target in an oxide atmosphere.

Significance

The RC time constant associated with contact resistance can limit the frequency response of devices. The charging and discharging of the leads resistance is a major cause of power dissipation in high clock rate digital electronics. Contact resistance causes power dissipation via Electromigration and delamination at contacts are also a limitation on the lifetime of electronic devices.

References

  • Sze, S.M. (1981). Physics of Semiconductor Devices. John Wiley & Sons. ISBN 0-471-05661-8.  Discussion of theory plus device implications.
  • Zangwill, Andrew (1988). Physics at Surfaces. Cambridge University Press. ISBN 0-521-34752-1.  Approaches contacts from point of view of surface states and reconstruction.

See also

  • The American Vacuum Society has an excellent short course on this topic.
  • Journal of the American Vacuum Society, Thin Solid Films and Journal of the Electrochemical Society are journals that publish current research on ohmic contacts.
 
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