S.E. Swirhun

Modeling and measurement of contact resistances

This paper presents a generalized model of ohmic contacts and a unified approach for the accurate extraction of specific contact resistivity (ρc) for ohmic contacts from measured contact resistance using the cross bridge Kelvin resistor, the contact end resistor, and the tranmsission line tap resistor test structures. A general three-dimensional (3-D) model of the contacts has been developed from the first principles and has been reduced to 2-D, 1-D, and 0-D (one lump) models with the necessary approximations. It is shown that the conventional I-D models overestimate the value of ρcbecause of the parasitic resistance due to 2-D current flow around the periphery of the contact window. Using 2-D simulations, we have accurately modeled the current crowding effects and have extracted accurate values of ρcindependent of contact size and the test structure type. A theory of scaling of contacts has been developed and is applied to commonly used structures. A universal set of curves has been derived for each particular contact resistance test structure and, given the geometry of the structure, these allow accurate determination of ρc, Without the actual use of the 2-D simulator. Experimental and theoretical accuracy of the three test structures has been compared. Accurate values of ρcfor various contact materials to n+and ρ+Si have been determined. The data confirm that in the past researchers have overestimated ρc, and that ρcwill not limit device performance even with submicrometer design rules.

Simultaneous measurement of hole lifetime, hole mobility and bandgap narrowing in heavily doped n-type silicon

The hole diffusion length, hole lifetime, hole mobility, and hole equilibrium concentration in epitaxial heavily phosphorus-doped silicon have been measured by a combination of steady-state and transient techniques. Steady state measurements were performed on bipolar transistors in which the base was epitaxially grown. The transient measurement relied on the observation of the decay of the photoluminescence radiation after laser excitation. Significant findings are: 1) the hole mobility is about a factor of two larger in heavily doped n-type silicon than in p-type silicon; 2) the apparent bandgap narrowing is smaller than previously thought, with a value of about 90 meV at a doping level of 1020cm-3.

Measurement of hole mobility in heavily doped n-type silicon

Accurate measurements of the mobility (and diffusion coefficient) of minority-carrier holes in Si:P with doping in the 1019cm-3range have been done. The technique employed the measurement of diffusion length by means of lateral bipolar transistors of varied base widths, and the measurement of minority-carrier lifetime on the same wafers from the time decay of luminescence radiation after excitation with a short laser pulse. Minority-carrier hole mobility is found to be about a factor of two higher than the mobility of holes as majority carriers in p-type Si of identical doping levels.

Experimental investigation of the temperature dependence of GaAs FET equivalent circuits

Accurate 13-element temperature-dependent RF equivalent circuits have been extracted from on-wafer S-parameter measurements of ion implanted and epitaxially grown recess-gate MESFETs and HEMTs at many biases for temperatures from -70 to +110 degrees C. The variations in each equivalent circuit element are expressed by a linear function of temperature. The temperature coefficients are bias- and technology-dependent. These data can be used to predict RF circuit performance variations with temperature. It is used to deduce the temperature dependence of physical factors such as electron mobilities and saturated velocities and the Schottky-barrier height. >

An accurate method to extract specific contact resistivity using cross-bridge Kelvin resistors

The cross-bridge Kelvin resistor structure is used to extract true interfacial specific contact resistivity (ρ c ). Two-dimensional (2-D) simulations demonstrate that the sublinear behavior of the measured contact resistance versus contact area on a log-log plot is due to current crowding around the contact which results from the contact window size being smaller than the diffusion width. The effect is more pronounced for low values of ρ c . Excellent agreement has been found between the simulations and measured data of contact resistances. An accurate value of ρ c has been extracted for the case of PtSi to n+polysilicon contacts.

VIB-4 temperature dependence of minority electron mobility and bandgap narrowing in p + Si

Measurements of the temperature dependence of minority electron mobility mu /sub n/ and band gap narrowing Delta E/sub G/ for moderate to heavily doped p-type Si are reported. Bipolar transistor test structures to measure these have been fabricated on uniformly doped bulk and epi Si:B. Preliminary results indicate that mu /sub n/ increases strongly with decreasing temperature for the most heavily doped layers. Delta E/sub G/ for these samples decreases with temperature. In the moderately doped samples mu /sub n/ and Delta E/sub G/ are roughly constant. The results suggest that higher performance npn bipolars than were previously thought possible can be made for cryogenic operation with high-mobility p+ base regions; the reduced band-gap narrowing, however, suggests that a low-concentration emitter is essential. >Measurements of the temperature dependence of minority electron mobility mu /sub n/ and band gap narrowing Delta E/sub G/ for moderate to heavily doped p-type Si are reported. Bipolar transistor test structures to measure these have been fabricated on uniformly doped bulk and epi Si:B. Preliminary results indicate that mu /sub n/ increases strongly with decreasing temperature for the most heavily doped layers. Delta E/sub G/ for these samples decreases with temperature. In the moderately doped samples mu /sub n/ and Delta E/sub G/ are roughly constant. The results suggest that higher performance npn bipolars than were previously thought possible can be made for cryogenic operation with high-mobility p+ base regions; the reduced band-gap narrowing, however, suggests that a low-concentration emitter is essential.<<ETX>>

Two-dimensional numerical analysis of latchup in a VLSI CMOS technology

The latchup behavior of a VLSI CMOS technology using hybrid Schottky-ohmic contact sources and drains and a high resistivity substrate has been extensively studied via two dimensional numerical simulation. The modeling allows quantitative explanation of the triggering and sustaining behavior of such structures, as well as an accurate characterization of the influence of the various process and geometrical parameters on the resistance to latchup. The technology is compared to a corresponding low resistivity substrate (epi) CMOS technology.

Contact resistance of LPCVD W/Al and PtSi/W/Al metallization

The sensitivity of measured specific contact resistivity to surface doping concentration has been investigated for selectively deposited LPCVD W contacts to n+ and p +Si with surface concentrations from 1018to 1020cm-3. W contact resistance to n+ Si is about a factor of 20 lower than that of Al; W contact resistance to p +Si is comparable to that of Al. Ultralow resistance, stable contacts with self-aligned PtSi, and W to p +Si are demonstrated.

Majority and minority carrier transport in polysilicon emitter contacts

The minority hole transport in polysilicon emitter contacts has been studied with a novel pnp test transistor. Segregated arsenic at the polysilicon/silicon interface is mostly responsible for the base current reduction in polysilicon-contacted npn transistors. This improvement comes at a price of a higher emitter resistance. This resistance was measured with a Kelvin resistor structure and the base current/ emitter resistance trade-off is quantified.

A VLSI-suitable Schottky-barrier CMOS process

Trenched Schottky-barrier (TSB) contact PMOS devices for use in latchup-free CMOS are examined in detail, and compared to Schottky-contact PMOS. Measurements and simulations show that the TSB structure has significant advantages in gain and current leakage over the Schottky-contact structure. CMOS using TSB PMOS may be made unconditionally free of latchup. The tradeoffs involving PMOS source-drain implant dose are made explicit and correlated to latchup measurements.