Patrick W. Leech

Universal error corrections for finite semiconductor resistivity in cross-Kelvin resistor test structures

The Cross-Kelvin Resistor test structure is commonly used for the extraction of the specific contact resistance of ohmic contacts. Analysis using this structure are generally based on a two-dimensional model that assumes zero voltage drop in the semiconductor layer in the direction normal to the plane of the contact. This paper uses a three-dimensional (3-D) analysis to show the magnitude of the errors introduced by this assumption, and illustrates the conditions under which a 3-D analysis should be used. This paper presents for the first time 3-D universal error correction curves that account for the vertical voltage drop due to the finite depth of the semiconductor layer.

Reactive ion etching of piezoelectric materials in CF4/CHF3 plasmas

The reactive ion etching of substrates used in piezoelectric devices (quartz, fused silica, LiNbO3, LiTaO3, and sapphire) has been characterized in CHF3/CF4-based plasmas. For quartz and fused silica, a regime of ion-enhanced chemical etching similar to that established by Steinbruchel [Ch. Steinbruchel, J. Electrochem. Soc. 130, 648 (1983)] for CF4 was indicated over the range of compositions from CF4=1 to CHF3=1. In this regime, the etch rate was dependent on the square root of the rf bias voltage (V1/2). The etch rate of both the quartz and fused silica was at a maximum in a CF4 plasma and decreased continuously with an increase in the ratio of CHF3/CF4 gases in the mixture. In comparison, the etch rates of LiNbO3, LiTaO3, and sapphire were invariant with changes in the ratio of the CHF3/CF4 gases, the flow rate, and the chamber pressure. The constancy of etch rate in these substrates has been attributed to a predominance of etching by a physical process of sputtering.

Understanding the sheet resistance parameter of alloyed ohmic contacts using a transmission line model

Abstract The electrical characterization of alloyed ohmic contacts is commonly undertaken using a Transmission Line Model (TLM) network to model and experimentally determine two parameters—the specific contact resistance ϱ c and the sheet resistance R sk beneath a planar ohmic contact. This paper describes the use of a recently reported modification to the TLM network [the Tri-Layer Transmission Line Model (TLTLM)] to interpret measurements of the sheet resistance parameter. The TLTLM network models the composite alloyed ohmic contact as three layers (metal layer, alloyed semiconductor layer and the unalloyed semiconductor layer) and two interfaces between the three layers. By assigning appropriate parameters to the TLTLM network, it is possible to calculate a value for the sheet resistance R sk that has been experimentally derived using the standard TLM. The new TLTLM model predicts that values of R sk greater and less than R sh (the unmodified sheet resistance of the epitaxial layer) are possible, in agreement with experimentally reported observations.

Electrical modelling of Kelvin structures for the derivation of low specific contact resistivity

The characterisation of semiconductor processing steps requires the use of various test structures and devices. In developing low resistance ohmic contacts, test structures such as the Cross Bridge Kelvin Resistor which allow the extraction of the specific contact resistance, ρ c are commonly used. This paper describes the results of modelling undertaken on a Kelvin Resistor. The results suggest that the use of multiple structures allows the determination of very low values of ρ c. Introduction Continuing developments in semiconductor process technology have resulted in significant reductions in the specific contact resistance, ρc(Ω.cm2) of ohmic contacts. As device dimensions decrease, then so also must ρc in order not to compromise scaled device performance. Thus the derivation of ρc is an important part of device technology. The four terminal Cross Bridge Kelvin Resistor(CBKR) of fig.1 is commonly used to measure the contact interfacial resistance and so obtain an extracted value of specific contact resistance ρ’c. Error correction curves are then used to compensate for the semiconductor parasitic resistance and thus yield the true value of specific contact resistance ρc. However these correction curves, which are based on two dimensional models[1,2], become increasingly inaccurate due to the high correction factors occurring when low ρc values ( 1 for d/w=1. The difference between the two models is due to the 3-D model taking the vertical voltage drop in the semiconductor layer into account. Figures 4(a) and (b) give a good indication of of the improvement in accuracy in modelling the Kelvin structure in 3-D rather than 2-D. Fig. 4(a). Comparison of 2-D and 3-D models for the derivation of extracted ρ’c vs d/w for circular contacts. Fig. 4(b). Normalised values of ρ’c vs d/w for 2-D and 3-D models using square contacts. The variation of ρ’c/ρc with RSH=50Ω/o, is given in fig.5 where data for four RSH values at ρc=10-8 Ω.cm2 is plotted for circular contacts. A slight degradation in the linearity is observed as RSH increases to 200Ω/o, while an improvement occurs at lesser values. However, the linearity is more sensitive to the width of the test structure arms w, as illustrated by the data of fig. 6. Fig. 5. Normalised ρ’c values versus d/w (w=1μ m) for four RSH (Ω/o) values (circular contacts, ρc=10-8 Ω.cm2). Fig. 6. Normalised ρ’c values versus d/w for three test structure arm widths w, (circular contacts, ρ c=10-8 Ω.cm2). For example, extrapolation of the w=2μm curve in the 0.2<d/w<0.4 range yields a ρ’c/ρc value of ~0.65. Thus the ρc value obtained would be 6.5x10-9Ω.cm2, significantly less than the true value of 10-8 Ω.cm2. Note that this technique relies on the averaging of data from several test structures for the determination of ρc, whereas in conventional application of the CBKR test structure, one test structure followed by the application of a correction factor will yield a result. The data presented in this paper is derived from L-type CBKR models. A second type of Kelvin structure, the D-type structure, reduces the parasitic resistance contribution to the total resistance measured by further reducing the test structure arm widths. When the D-type structure is modelled, the data shows a greater non-linearity in ρ’c/ρc versus d/w despite a reduction in the magnitude of ρ’c/ρc compared with the equivalent L-type structure.

Specific contact resistance of indium ohmic contacts to n‐type Hg1−xCdxTe

The specific contact resistant ρC of In contacts ton‐Hg1−xCdxTe has been measured as a function of the Cd mole fraction and annealing treatment of the Hg1−xCdxTe. Transmission line model measurements were performed on the In/Hg1−xCdxTe junctions with the Hg1−xCdxTe doped n type in the range 3.5×1016 to 1.6×1018 cm−3 (77 K). A linear dependence of In ρC on x was obtained, with values of ρC ranging from 2.0×10−5 Ω cm2 at x=0.30 through to 2.6×10−2 Ω cm2 at x=0.68. Hg annealing of the epitaxial Hg0.38Cd0.62Te layers resulted in a decrease in ρC from 5.9×10−3 Ω cm2 unannealed to 1.2×10−3 Ω cm2 after a 300 °C anneal, corresponding to equivalent changes in the resistivity of the Hg1−xCdxTe. Isothermal annealing of the In/n‐Hg1−xCdxTe contacts for x=0.30, 0.40, and 0.62 produced an enhanced indiffusion of In but with only a minor reduction in ρC.

Properties of Schottky diodes on n‐type Hg1−xCdxTe

The electrical properties of Schottky diodes formed by Ag, Au, Cu, Pd, Pt, Sb, and Ti on n‐Hg1−xCdxTe (x=0.6–0.7) have been measured using current‐voltage and capacitance‐voltage techniques. Both LPE grown (111) Hg0.3Cd0.7Te and metal‐organic chemical‐vapor deposition grown (100) Hg1−xCdxTe epitaxial layers were used with surface preparation either by chemical etching or by air exposure. The current–voltage characteristics of contacts fabricated on chemically etched surfaces have been described by a thermionic emission‐recombination model. For these etched surfaces, the barrier heights produced by the metals Ag, Au, Cu, and Sb on (111) Hg0.3Cd0.7Te were in the range φ’b=0.74 V to 0.79 V while for Pd and Pt, φb=0.69±0.01 V. In close agreement, Pt contacts on etched (100) surfaces exhibited a barrier height of φ’b=0.70±0.01 V for stoichiometries of x=0.6–0.68. The effects of air exposure on diode characteristics were most significant for the Au contacts due to an inability of the metal to reduce interfacial...

Enhancement of the etch rate of LiNbO3 by prior bombardment with MeV O2+ ions

The implantation of LiNbO3 with 5 MeV O2+ ions has been examined as a means of increasing the rate of subsequent reactive ion etching in CF4/CHF3 plasmas. The etch rate of LiNbO3 implanted at a dose sufficient to amorphize the substrate (1×1015–5×1015 ions cm−2) was ⩽3 times higher than in unimplanted substrates. The measured etch rates in CF4/CHF3 plasmas were essentially independent of the implant dose below 3×1014 ions cm−2 but increased significantly at and above 1×1015 ions cm−2. This transition was correlated with the onset of amorphization. In an Ar plasma, the etch rate of LiNbO3 was unaffected by prior million electron volt implantation and was an order of magnitude lower than in the CF4/CHF3 plasmas. These results have suggested that the damage induced by million electron volt implantation acted to accelerate the process of ion-enhanced chemical etching in LiNbO3 but had no significant influence on the process of physical sputtering in an Ar plasma.

ELECTRICAL CHARACTERISTICS AND THERMAL STABILITY OF OHMIC CONTACTS TO P-TYPE IN0.47GA0.53/AS/INP

The electrical characteristics and thermal stability of Pd/Zn/Pd/Au, Pd/Au, Zn/Pd/Au, Au/Zn/Au, Ni/Zn/Ni/Au, and Pd/Mn/Sb/Pd/Au contacts to p‐type In0.47Ga0.53As/InP have been investigated. For all of the as‐deposited contacts, the specific contact resistance, ρc, was within the range between 1 and 3×10−5 Ω cm2. The thermal annealing of the contacts between 250 and 500 °C produced a differing effect on ρc for each of the metallization schemes. Based on ρc measurements, the thermal stability of the contacts at 400 °C showed an initial regime of low degradation rate with a subsequent transition to a higher rate regime. The exception to this trend was the Pd/Mn/Sb/Pd/Au contact for which no threshold was evident, and for which the dependence of degradation rate on time, t0.15, was lower than for the other configurations with t0.5. During aging at 500 °C, a single regime of high degradation rate was present. In both the low rate and high rate regimes, the type of interfacial metal was not a significant factor...

Graphitic Schottky Contacts to Si formed by Energetic Deposition

Carbon films deposited by filtered cathodic vacuum arc have been used to form high quality Schottky diodes on p-Si. Energetic deposition with an applied substrate bias of -1 kV and with a substrate temperature of 100 °C has produced carbon diodes with rectification ratios of ~ 3 × 106, saturation currents of ~0.02 nA and ideality factors close to unity (n = 1.05). Simulations were used to estimate the effective work function and the thickness of an interfacial mixed (C/SiO2) layer from the current/voltage characteristics of the diodes.

Thermal Stability in Pd-Based Contacts to p-Type In 0.53 Ga 0.47 as Characterized by Rbs

The resistivity and interfacial characteristics of Pd/Zn/Pd/Au and Pd/Zn/Au/LaBd 6 /Au contacts to p-In 0.53 Ga 0.47 As have been investigated. Annealing of the contacts at 375–425°C yielded a minimum in specific contact resistance, p,, of 2 × 10 -7 Δ cm 2 for the Pd/Zn/Pd/Au contacts and 1 Δ cm 2 for the Pd/Zn/Au/LaB 6 /Au configuration. This is the lowest reported measurement of p c for an ohmic contact to p-In 0.53 Ga 0.47 As doped to ≤1 × 10 19 cm −3 . In the Pd/Zn/Au/LaB 6 /Au scheme, the minimum in p c was the same irrespective of whether the Zn was incorporated as a structural layer or as Zn ions implanted into the interfacial Pd prior to metallization. The effect of thickness of the Zn layer on pc has been determined for the Pd/Zn/Au/LaB 6 /Au scheme. RBS measurements have shown that during annealing, the LaB 6 layer acted as a barrier to the indiffusion of Au and to the degradation of the In 0.53 Ga 0.47 As substrate.