Simon S. Cohen

The technology of laser formed interactions for wafer scale integration

Restructurable VLSI wafer-scale circuits have been built using two methods, both using laser energy to create low resistance connections between bus lines on already existing circuits. In one technique verticle connections of about 10 Omega are made up from top metal, through silicon nitride, to first metal lines. The other involves melting of silicon in the gap between two implant regions, with the lateral diffusion of dopants creating connections of about 100 Omega . Details of the linking structures, their characteristics, and the apparatus used to interconnect them are described.<<ETX>>

A novel double-metal structure for voltage-programmable links

A novel metal-insulator-metal (MIM) structure has been developed for use in field-programmable gate arrays (FPGAs) as a voltage-programmable link (VPL). The present structure relies on a combination of a refractory metal and aluminum as the lower electrode, and aluminum alone as the top electrode. The insulator, prepared by means of plasma-enhanced chemical vapor deposition, comprises a sandwich of nearly stoichiometric silicon dioxide interposed between two layers of silicon-rich silicon nitride. This MIM structure has displayed characteristics desirable for use in the emerging FPGA technology.<<ETX>>

Laser-induced diode linking for wafer-scale integration

Diodes formed by ion implantation and diffusion in a conventional CMOS process are positioned such that when desired they may be used to obtain an electrical link between two otherwise separate sections of the integrated circuit. Electrical connections so obtained enable the realization of wafer-scale ICs as demonstrated in recent applications. They theory of laser-beam application to silicon is discussed and it is shown how the various beam and substrate parameters effect the properties of the diode links. Particular attention is paid to the important issue of the reflectivity from the composite system. Careful analytical examinations of the resulting molten zone properties have been performed to fully qualify the use of laser radiation in this technology. Both scanning electron microscopy and secondary-ion mass spectrometry were used to examine such parameters as the lateral and in-depth extension of the molten zone. In addition, electrical measurements were carried out. The results for the various observables compare well with the theoretical predictions. >

A novel metal-insulator-metal structure for field-programmable devices

A metal-insulator-metal (MIM) capacitor structure has been developed for use in field-programmable gate arrays (FPGAs) as a voltage-programmable link (VPL). The structure relies on a combination of a refractory metal and aluminum as the lower electrode, and either a similar combination or aluminum alone as the top electrode. The insulator is prepared by means of plasma-enhanced chemical vapor deposition (PECVD). It comprises a sandwich of nearly stoichiometric silicon dioxide interposed between two like layers of silicon-rich silicon nitride. The structure has displayed characteristics desirable for use in emerging FPGA technology, including high density, low on-resistance, reduced capacitance, and low programming voltage. >

A flat-aluminum based voltage-programmable link for field-programmable devices

A new metal-insulator-metal (MIM) structure has been developed for use in field-programmable gate arrays (FPGAs) as a voltage-programmable link (VPL). The present capacitor structure relies on aluminum metallization; hence, it should be amenable to immediate application. The addition of minute amounts of titanium or molybdenum has been found to suppress hillock formation. The insulator, prepared by means of plasma-enhanced chemical vapor deposition (PECVD), comprises a sandwich of a nearly stoichiometric silicon dioxide interposed between two like layers of silicon-rich silicon nitride. This MIM structure has displayed characteristics desirable for use in the emerging FPGA technology including high density, very low on-resistance, reduced capacitance, low programming voltage, and the potential for further scaling to the sub-micron regime. >

The effect of multiple laser pulses on damage to thin metallic films

The mechanical effects due to the application of multiple laser pulses to thin metallic films are studied. Particular attention is paid to systems involving thin aluminum films deposited on an insulating substrate such as silica. This film/substrate combination is widely employed in silicon semiconductor technology. In building such devices laser energy is sometimes used for the purpose of cutting conducting lines, while in other applications it is used to effect linking between two levels of metallization. Both processes have been greatly facilitated by employing a multiple‐pulse scheme. The mechanism responsible for this effect is discussed and it is shown how the present model leads to a good agreement between the measured and calculated quantities.

Chapter 2 - Laser Beam Processing and Wafer-Scale Integration

Publisher Summary This chapter provides an overview of the applications for laser-beam processing and wafer-scale integration (WSI). It describes certain emerging applications of laser that hold promise for the fabrication of ever more dense integrated circuits. It also discusses the laser processing of devices necessary for the realization of the WSI concept. The theory of laser melting of a semiconductor substrate is basically that derived from the heat diffusion equation and as such is common to all applications involving heating or melting a substrate. The type of laser used, however, depends on the particular application and in turn dictates certain theoretical and practical considerations. The chapter also addresses the issues of reflectivity from a combined liquid/solid system. It outlines the theoretical considerations that form the basis for modeling the WSI applications, indeed most other laser heating applications. In WSI, exact characterization of the laser-semiconductor interaction is required. Details related to the physical and geometric features of the hot zone that forms are of paramount importance. This is because the laser is applied at a stage where most of the device processing has already been completed. The chapter discusses the models involved by appropriate calculations of the thermal profiles in the substrate.

Metal wire cutting by repeated application of low‐power laser pulses

Thermomechanical fatigue, induced by the repeated application of a sufficiently powerful laser pulse, will eventually cause a metallic film to fail. This principle is proposed to cut metal lines for deletive redundancy in microelectronics. Aluminum alloy wires were cut with a series of 5 μs pulses from an argon ion source. The power required to cut 4‐μm‐wide, 0.75‐μm‐thick lines was reduced by as much as 87% from that of a single 5 μs laser pulse. By comparison to a single pulse of equal total time, a 70% reduction in power has been realized.

Laser-induced melting of thin conducting films. I. The adiabatic approximation

The authors explore the thermal characteristics of an isolated metallic film which is subjected to a short pulse of laser radiation. The main feature of such an adiabatic system is that no steady-state solution is possible. This means that the molten zone dimensions depend on the pulse duration length and also on the temperature dependence of the influencing parameters (essentially, the thermal diffusivity). The authors use available models for the temperature-dependent conductivity and diffusivity to compare the theoretical results with experimental data obtained from a quasi-adiabatic system. >

A model for the reflectivity in laser‐substrate interactions

We study the issue of reflectivity in interactions of laser beams with condensed matter substrates. In particular, we discuss the effect that the reflectivity has on the melting of silicon. The theory of the laser‐solid interaction expresses the properties of the hot zone that forms in the solid in terms of the beam parameters, such as the power and the spatial profile. The value of the reflectivity determines the actual value of the deposited power, and hence greatly influences the properties of the hot zone. The reflectivity is temperature dependent, and hence it constantly varies during the period in which the laser power rises to its maximum value. Therefore, a meaningful model is required in order to properly account for its effects. We have developed such a dynamical model for the reflectivity. The results of this model compare well with relevant experimental measurements.