Masanori Murakami

Effects of interfacial microstructure on uniformity and thermal stability of AuNiGe ohmic contact to n‐type GaAs

As part of the investigation of the use of AuNiGe as the ohmic contact to n‐type GaAs at a high integration level, cross‐sectional transmission electron microscopy was used to explore the uniformity at the metal/GaAs interface and the thermal stability of the AuNiGe contact after the ohmic contact formation. A close relation between spread of the contact resistance and nonuniformity of the interfacial microstructure of the contact was found. Deposition of 5‐nm‐thick Ni as the first layer of the AuNiGe ohmic contact significantly reduced the spread of the contact resistance and led to the formation of a uniform interface without large protrusions. The improvement in uniformity of compound distribution and the reduction of interface roughness are believed to be due to a change in the sequence of alloying reactions, compared to those in the contact without a Ni first layer. This suggests an ideal interface structure for a low resistance AuNiGe ohmic contact after alloying to be a uniform two layer structure: a high density of the NiAs(Ge) grains contacting the GaAs substrate, and a homogeneous β‐AuGa phase close to the top surface. However, due to the existence of β‐AuGa phases with a low melting point of around 375u2009°C, the thermal stability of the contact at 400u2009°C is of serious concern. Segregation of the NiAs(Ge) grains was observed after annealing at 400u2009°C for 10 h, which reduced the contact areas between the NiAs(Ge) grains and GaAs. During subsequent annealing at this temperature for up to 90 h, liquidlike flow of the β‐AuGa phase was observed which deteriorated the interface uniformity, causing an increase in contact resistance. A typical contact edge slide distance after contact alloying at 440u2009°C for 2 min was measured to be 0.2 μm and the longest distance among specimens examined was 0.47 μm. This edge deterioration could limit the use of the AuNiGe contact in GaAs submicron devices.

Residual strains of Pb thin films deposited onto Si substrates

Abstract An investigation has been carried out of the strain and strain-relaxation in (111) oriented Pb films deposited onto oxidized Si substrates at 300 K and then cooled down to temperatures as low as 4.2 K. The strain in the Pb caused by differences in thermal expansion of the Pb and Si was measured using X-ray diffraction techniques. For film thicknesses of ~1400 A, the magnitude of the observed strain at 4.2 K was found to approximate the strain value calculated from the difference in thermal expansion coefficients using the biaxial strain model. For greater film thicknesses, the value of the strain measured immediately after cooling was much lower, indicating that significant strain-relaxation occurred during the cooling process. It is probable that most of the strain was relaxed by a dislocation-slip mechanism. During isothermal annealing above 50 K a second, slow relaxation process was observed. Nonuniformity of the strain normal to the film-surface was found by analyzing the asymmetric X-ray line broadening that occurred during cooling. A strong dependence of strain on grain orientation was observed, and interpreted in terms of the biaxial strain model.

Low temperature strain behavior of Pb thin films on a substrate

The microstructural changes occurring during cooling from 300 to 100 K in a 0.2 μm thick polycrystalline Pb film deposited on a Si3N4 substrate were studied by a combination of transmission electron microscopy and X-ray diffraction technique. The tensile strain induced in the film upon cooling due to the thermal expansion coefficient mismatch between the film and the substrate was observed to be relaxed by dislocation glide. Most of the dislocations were observed to glide across the grains on 111 planes that are inclined at an angle of ∼ 70 deg to the film surface, and on 111 planes that are nearly parallel to the film surface. All the dislocation motions are confined in each grain by the surface oxide, the substrate, and grain boundaries. Some observations suggested that these dislocations emanate from grain boundaries. At 100 K, the density of dislocations introduced in the grains with diameters larger than ∼0.6 μm was found to be roughly constant (about 1010/cm2), while no dislocations were observed in grains smaller than ∼0.6 μm. The observed dislocation density can account for the amount of strain relaxed which was measured by the X-ray diffraction technique. It was also found that almost all the dislocation glide events involved in the thermal cycling process are reversible. This explains a previous X-ray diffraction result that no work hardening effect was observed in Pb films during the thermal cycling at low temperature. The yield stresses of Pb films as determined by the strain measurements are about three times higher than those expected by a simple dislocation pinning model. Based on the dislocation motion observed in this work, the yield stresses of the films were re-evaluated as a function of film thickness and grain size using an energy criterion model. This model took into account the effects of the surface oxide and substrate on dislocation

Thermal strain in lead thin films I: Dependence of the strain on crystal orientation

Abstract The dependences on crystal orientation of strains normal to a film surface and of the resolved shear stress for dislocation glide were calculated for Pb thin films strained by an underlying substrate using the biaxial strain model. These results are plotted on (111) stereographic projections. The calculated values compare favorably with experimental values obtained by X-ray diffraction on Pb films deposited onto oxidized Si substrates at room temperature and subsequently cooled to lower temperatures.

Thermal strain in lead thin films II: strain relaxation mechanisms

Abstract A deformation mechanism map was constructed to study the mechanisms of strain relaxation in lead thin films which were deposited on oxidized silicon wafers at room temperature and which were then thermally cycled between room temperature and liquid helium temperature. The stress level, which was calculated from the strain measured by an X-ray diffraction technique, was plotted on the map. By comparing the calculated and experimental stress levels the following observations were obtained. In the cooling process the strain was relaxed rapidly in a field of dislocation glide mechanism for films of greater than 0.2 μm thickness. In the heating process most of the strain was again believed to be relaxed by the glide mechanism. For a film 0.5 μm thick the stress (after the primary relaxation was completed) was found to be (1–1.5) × 10 9 dyn cm -2 for the cooling process and (0.17–0.24) × 10 9 dyn cm -2 for the heating process at temperatures around 200–280 K. Slow secondary relaxations were observed after the primary relaxations were completed. The measured compressive strain relaxation rate at room temperature was very close to the rate calculated on the assumption of grain boundary diffusion creep. This suggested that the secondary relaxation mechanism of compressive strain was grain boundary diffusion creep at temperatures near room temperature. These suggestions were supported by scanning electron microscopy observations in which dislocation slip lines were observed inside grains and hillocks were observed on grain boundaries.

Thermally stable ohmic contacts to n-type GaAs. III: GeInW and NiInW contact metals

Improvement in thermally stable, low‐resistance ohmic contacts to n‐type GaAs is reported for GeInW and NiInW contact metals. Coevaporation of In with Ge or In with Ni reduced the contact resistances by a factor of about 2 compared with those of the layered structures. The reduction is believed to be due to a uniform In distribution in the contact metals in the as‐deposited state which resulted in an increased area of InxGa1−xAs phases in direct contact with the GaAs substrate. Annealing the coevaporated GeInW contacts for a short time at temperatures between 900 and 980u2009°C resulted in a mean contact resistance of 0.5 Ωu2009mm. Similar annealing of the coevaporated NiInW contacts at temperatures between 800 and 1000u2009°C resulted in a contact resistance of 0.3 Ωu2009mm. Additionally, the thermal stability of these ohmic contacts at 400u2009°C after contact formation, which is required by subsequent integrated circuit process steps, was studied. Although a slight increase in the contact resistances was observed after an...

Uniform and thermally stable AuGeNi ohmic contacts to GaAs

An in situ rf sputter precleaning of the GaAs substrate before AuGeNi ohmic metal deposition yields contact resistance Rc=0.11 Ωu2009mm at a peak doping of ∼1018/cm3. Excellent uniformity and thermal stability are achieved across the wafer. The contacts do not deteriorate appreciably after themal treatment at 410u2009°C for 57 h.

Thermal strain in thin lead films III: Dependences of the strain on film thickness and on grain size

The effects of film thickness h or of average grain size g on strains e33 due to the mismatch of the thermal expansion coefficients were studied by an X-ray diffraction technique for thin lead films 0.03–1.0 μm thick evaporated onto silicon substrates. Films with the same average grain size and with different film thicknesses were prepared by a sputter-etch-thinning technique after the film depositions had been completed. Films with small grain sizes were prepared by deposition at liquid nitrogen temperature or by seeding a very thin layer of gold or palladium before the lead deposition. These films were cooled from 300 ro 4.2 K using a cold stage attached to an X-ray diffractometer. n nFor films with h g/5 the critical grain size gc was determined to be about 1 μm. When g >gc the e33 levels decreased with increasing g. When g < gc the e33 values were found to be independent of g and also of h. However, e33 did not reach the calculated maximum strain value. We propose that the difference between the calculated strain and the measured strain was due to an absorption of the strain at grain boundaries. The fact that gc was about six times larger than hc means that g exerted a stronger effect than h did on the inhibition of strain relaxation during cooling to 4.2 K. It was found that the intermetallic compounds Pb3Au or Pb2Pd which were formed in the binary films did not greatly affect the inhibition of strain relaxation. The e33 levels in these binary films were also determined by the grain sizes.

Development of ohmic contact materials for GaAs integrated circuits

GaAs is a very attractive material for special devices such as high-frequency microwave and optoelectronic devices which perform functions unattainable by Si devices. GaAs digital integrated circuits can operate at speeds beyond the capability of Si devices. In addition, compared with Si devices, the GaAs devices operate at lower power, are more radiation tolerant, and the device fabrication process is simpler. Two distinct types of contacts are fundamental components for GaAs devices: Ohmic (low-resistance) and Schottky (rectifying) type contacts. Performance of GaAs devices is strongly influenced by the electrical properties of these contacts. A variety of metallization systems for these contacts has been developed which provide promising device performance. With increase of the integration level of devices, thermal stability during device fabrication process and operation, control of the diffusion depth of the contact metals into the GaAs, and smooth surface morphology have become important issues as well as the electrical properties. The purpose of the present article is to review the development of Ohmic contact materials for GaAs devices prepared by conventional evaporation and annealing techniques and to discuss compatibility of these contact materials with highly integrated circuits.

Thermally stable, low-resistance NiInW ohmic contacts to n-type GaAs

A new thermally stable, low‐resistance NiInW contact metal to n‐type GaAs has been developed by depositing a thin In layer with Ni and W layers and annealing at elevated temperatures for a short time. Low resistances of ∼0.3 Ωu2009mm were obtained at annealing temperatures in the range of 800 to 1000u2009°C. The contact resistances were stable during subsequent annealings at 400u2009°C for 100 h and 500u2009°C for 10 h. The thermal stability of the contact resistance and the surface morphology of this contact are superior to those of the conventionally used AuNiGe contacts and this new contact is suitable for various device applications. Further reduction of the contact resistance can be achieved simply by reducing the sheet resistance of the contact metals.