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Coating Materials News Vol 9 Issue 3

 

September, 1999

 

Thin Film Microelectronic Components on Semiconductors

Thin Film Conductors
Thin Film Resistors

Specially prepared solid materials are evaporated or sputtered to deposit thin film layers with desired properties. The layers can be grown from dielectric or metallic materials depending on the intended function. Past issues of Coating Materials News (CMN) have discussed deposition parameters of various metals and their applications specifically as reflecting layers for optical mirrors, compact disks, and decorative coatings. We have also discussed transparent conductors such as ITO. In this issue we discuss the widespread use of metal layers as conductors and thin film resistors for the semiconductor industry.

 

Thin Film Conductors

Metallization applied to circuitry must provide high conductance and stable interconnects and be capable of being patterned by photolithography and etching. Pure metals provide the highest conductance because electrons move freely through them (this property also makes them high reflectors of radiation from UV to IR). Typical resistivities for these conductors are approximately 2 E-6 ohm-cm. The choice of the metal to be used for a particular application depends on several properties: electrical and thermal conductivities, adhesion, surface reactivity, and processing ease. Metal traces are deposited on dielectric or semiconductor surfaces and then defined and etched using photolithographic processes. In modern semiconductor device manufacture, the metals are sputter deposited as opposed to chemical electro-deposition. The nature of the substrate-interface bond influences the current-passing contact. In selecting the metal, one must consider such issues as interdiffusion, which can affect resistance, adhesion, bondability to wire leads, and long term stability. Metals with the highest conductivities, namely gold, copper, and silver, do not form adherent bonds to dielectric, ceramic or polymer substrates. Interface metal layers of thicknesses between 10 nm and 100 nm must be deposited prior to the conductor to improve contact and adhesion properties.

Metals are generally deposited at low substrate temperature in high vacuum at high rates. The low substrate temperature is an advantage when photoresist patterning is required. Sputtering is the preferred method for large volume coating and is processed in load-lock systems in wafer or CD manufacturing lines, for example. With sputtering, of course, the vacuum is not as high and gas impurities will influence the ultimate conductance attainable. The following are the most frequently used single metals for conductor traces.

Gold
Gold has excellent resistance to corrosion and good conductivity. It does not adhere to dielectrics, however, and requires a bonding layer of titanium or chromium to achieve acceptable adhesion and electrical contact. Soldering to gold requires special solders to prevent the formation of alloyed interfaces that tend to be mechanically and electrically compromised. Electrical or ultrasonic welding is the preferred method for making connection to gold traces.

Copper
Copper also forms poor bonds to dielectric surfaces and requires an adhesor layer of titanium, aluminum, or chromium. On silicon, a barrier layer, often TaN, is required to prevent diffusion at elevated process temperatures. In addition, the surface requires passivation to prevent oxidization. Rather than use pure copper, it is often alloyed with Cr or Ti to overcome some of the mentioned limitations. Copper can be soldered to, if the surface is properly prepared.

Aluminum
Aluminum adheres well to most surfaces and provides reasonably good conductance and stability. A high degree of purity is required to preserve high conductance and corrosion resistance. However, alloying with Cu or Si is done to control diffusion or the thermal expansion coefficient. Alternatively, TiW or TiN is used to prevent diffusion. A freshly deposited aluminum surface will form aluminum oxide self-limiting to a thickness of ~40-50 Å in a matter of minutes upon exposure to air. Aluminum surfaces cannot be soldered to. Aluminized polymer film is used in the manufacture of capacitors.

Silver
Silver is a slightly better electrical conductor and poorer thermal conductor than copper. Its disadvantages in production are reactivity to sulfur-bearing gasses and chlorine. The reactant compounds have poor electrical and mechanical properties. Passivation of silver is not easily achieved, but a thin copper layer has been used for this purpose.

The resistances of copper, gold, silver and alloys of these metals can be decreased by heating to temperatures up to 300° C. In this case, defect reduction and lattice reordering is responsible for the observed change. At temperatures in excess of ~400° C, agglomeration can occur in very thin films of some metals, leading to island formation rather than film continuity, and thus resulting in a steep increase in resistance.

Sometimes two or more metals are stacked to make contact to a semiconductor surface and to provide adhesion, diffusion barrier, and conductance properties. One such system is Ti-Pd-Au, where Ti is the adhesion material and Pd prevents diffusion of Au and Ti. For silicon metal-oxide and VLSI devices, Ti -Ni contacts are often used because Ni can be easily soldered to and Ti bonds well to silicon. Problems with stress in the Ni and the formation of resistive titanium silicide, however, arise under certain conditions of temperature and sputtering parameters.

Nickel films are hard and corrosion and scratch resistant but can have high intrinsic stress depending on deposition pressure, etc. A native oxide coating provides some protection from corrosion to the Ni surface. The electrical conductivity of nickel films is about 4 times lower than that of copper. Alloys of Ni with Cu, Cr, and Fe have improved corrosion resistance to specific chemicals.

Chromium also forms hard film layers, but its electrical conductivity is 8 times lower than copper. Alloys of Ni and Cr to form nichrome have much higher resistivities and are used to make thin film resistors. CERAC offers several compositions for making different resistance values. NiCr is also used as an adhesor layer for metals.

The resistance, hardness, yield strength, chemical reactivity, and interface properties of all the above metals can be modified by altering the impurity content. 



Thin Film Resistors

When a metal is deposited as a thin film, its electrical properties differ from those of the bulk form. Resistivity is generally higher, and thermal conductivity is lower. The resistance of a film is characterized by its sheet resistance (ohms / sq.) that refers to the volume resistivity normalized to film thickness. Sheet resistance, Rs, can therefore be established by film thickness as well as by composition. Thicknesses of resistors can range from tens of nm to several µm. Rs ranges from 10 ohms/ sq. to greater than 2000 ohms /sq. Thin film resistance values can be trimmed to very precise values by varying the width of the trace using laser scribing and ablation processes. The stability of the sheet resistance of a film to temperature changes is characterized by its temperature coefficient of resistance (TCR), a parameter important in some circuits that must operate under environments with large temperature variations.

Since resistance can be increased by the addition of impurities that cause electrons to be scattered within these film structures, metal-metal and metal-dielectric systems have been developed with deliberate "impurity" alloying. The substrate surface also plays an important role in thin film resistor stability not only for its composition, but also for its stability, smoothness, and thermal and electrical properties. Rough substrates can influence chemical reactivity; composition can determine diffusion rate; thermal conductivity and expansion coefficient can influence mechanical stress.

Nichrome (80% Ni, 20% Cr) is a popular resistor material with low and controllable TCR. A few problems are encountered in the deposition of Ni-Cr resistors. The vapor pressure of Cr is greater than that of Ni, and this difference is exaggerated at evaporation temperatures near 1000° C compared to 1300° C. With repeated evaporation, the composition of the source material changes, and control is lost. Therefore, it is important to devise compensation measures for these effects if the source material cannot be easily refreshed. Sputtering appears to produce better composition control with Ni-Cr films. Another problem is resistance increase due to oxidation of the chromium component during annealing subsequent to deposition. This is especially problematic for film thicknesses near 10 nm, and less so for thicker films.

In addition to Ni-Cr thin film resistor material, Cr, Ta, tantalum oxynitride, tantalum nitride, and other materials are used to manufacture thin film resistors. The possibility of impurity contamination of these materials during sputtering is high, and precautions or allowances must be made accordingly. The impurities can be sputtered chamber-structure materials or background gasses such as argon or residual oxygen and nitrogen, any of which can affect the electrical and mechanical properties of the finished product. The TCR of metals that oxidize (tantalum, for example) can be modified by heating in oxygen. Negative TCRs can be created; this phenomenon is due to oxidation of the grain boundaries in the film.

Cermets are also used to fabricate thin film resistors. Cermets are solid solutions of metal particles in a dielectric matrix. A common cermet consists of Chromium and Silicon Monoxide mixtures in which the atomic % of Cr varies from 50% to 70%. CERAC has several compositions available in sintered form. The Cr grain size is near 3 nm in the film matrix. Flash evaporation preserves the composition, but resistance-heated and e-beam evaporation are more commonly used. Interestingly these Cr-SiO materials are also used to deposit highly absorbing films used in masking and in stray light control in optical systems.

Resistances up to 1E5 and Rs ~30 kohm / sq. are possible in films 20 nm thick using the 50:50 composition. These values can be lowered by annealing at temperatures approaching 400° C [1]. These films provide high stability and TCRs that are not excessively large. The TCR of Cr-SiO films can be controlled by composition and temperature. The films made with 70% Cr behave as semi-conductors at low temperature, and as metals at high temperature as evidenced by their positive TCRs at high temp. Heating in air can, however, cause resistance increases because of oxidation, therefore Cr-SiO resistors are often overcoated with SiO or epoxy for protection.

The interesting conducting and resistive behaviors of metals and metal-dielectric combinations briefly introduced here teach us various insights into film microstructure and reactivity. The use of thin deposited films in electronics provides a different perspective on materials and processes than does the optical applications of coatings.



Reference

1. E. Schabowski and R. Scigala, Thin Solid Films 135 (1986) 149.



(S.F. Pellicori is available for private consulting on matters concerning optical thin films. Please contact him directly for more information)

Dr. Mitchell C. Colton, Editor
CERAC, inc.
P.O. Box 1178 | Milwaukee, WI 53201
Phone: 414-289-9800 | FAX: 414-289-9805
e-mail: marketing@cerac.com

Samuel Pellicori, Principal Contributor
Pellicori Optical Consulting
P.O. Box 60723 | Santa Barbara, CA 93160
Phone/FAX: 805-682-1922
e-mail: pellopt@silcom.com

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