June 1992

Laser Metal Deposition for High-Density Interconnect

Y. S. Liu

Fabrication of interconnect structures using lasers has become in creasingly important as demands for discretionary interconnects grow. Patterned deposition of metals and dielectrics are key technologies of interconnect fabrication for microelectronic and optoelectronic applications. Lasers offer the flexibility of in-situ process monitoring, tuning, and control, and the capability of interfacing with CAD tools to allow on-the-fly design changes and testing.

Integrated Enhanced Epitaxy of Optoelectronic Materials

Integrated optical and electronic devices require the fabrication of many different components, each with its own material and structural requirements, on one chip. To achieve optimum performance of an integrated device, each individual device structure should fulfill its optimum material, thickness, and doping requirements. Current activities in device integration have relied mainly on the same multilayer structure to fabricate the different optical and electronic components. Thus, a single or multilayer structure that is optimum for one particular device—for example, a field-effect transistor (FET)— might not satisfy the device structure required for a detector or laser.

Laser Chemical Etching of Semiconductors

Many of today's important technologies require selective removal of small regions of a thin film to produce a particular pattern. Laser-driven chemical reactions have been extensively studied as alternatives to current ion-beam-based etching processes, such as ion-beam milling and reactive ion etching (RIE). The attraction of photon-based processes lies in their ability to selectively etch very small features with little or no substrate damage using a range of reactants, temperatures, and pressures. There are three general reaction classes: thermal chemical reactions, photochemical reactant generation, and carrier-driven reactions. The most appropriate type to use depends on the specific application. Although most research has employed lasers, one should always bear in mind that there is nothing magic about using a laser except when extremely monochromatic or highly coherent light is required. For many applications, high-intensity lamps might be a better light source.

Materials Patterning with Excimer Laser Ablation

When lasers were introduced three decades ago, initial materials applications were limited due to the narrow range of laser wavelengths that were available. In those early days, infrared and visible lasers (CO2, ruby, and Nd:YAG) were used for welding, cutting, and drilling of different metals. With the invention of the UV excimer laser in the mid-seventies, a host of new possibilities for material processing emerged. Today, high resolution direct patterning—down to the micron level—is possible for a wide range of materials. UV lasers offer distinct advantages for direct patterning. Most materials, especially organic polymers, have higher absorption at UV laser wavelengths than at longer wavelengths. The short pulse length, high absorptivity, and high energy density available with UV excimer lasers allow efficient ablation of material. An equally important advantage is the ability to pattern the light beam onto the substrate by projection systems, thereby allowing direct patterning and obviating the need for the solvents and other wet processing present in conventional photolithography.

Applications of Pulsed Laser Deposition to Optics

The laser, as a source of "pure" energy in the form of monochromatic and coherent photons, is enjoying ever increasing popularity in such diverse and broad applications as drilling micron-sized holes on semiconductor devices to guidance systems used in drilling a mammoth tunnel under the English Channel. In many areas, it has become an irreplaceable tool. Material processing technology takes no exception. Among the various material processing techniques driven by the advancement in laser performance and its uniqueness, pulsed laser deposition (PLD) has experienced a phenomenal growth since 1987. Because of its success in in-situ growth of high Tc superconductive thin films, PLD is often associated with this class of material. However, due to its simplicity and versatility, thin films of more than 100 different materials have been grown for various applications by PLD in the nearly three decades since its initial conception.

Laser Cleaning Techniques for Critical Surfaces

Organics, as exemplified by the fingerprint, and particulates or dust on optical surfaces are familiar problems for anyone in optics. For critical optical surfaces such as those exposed to high intensities, or where minor contamination significantly degrades performance, such contamination can be more than just a nuisance, causing optical component damage or extensive downtime for cleaning and part replacement. For state-of-the-art semiconductor devices, low level metal contamination also must be removed. The cleaning of critical surfaces must, therefore, address organics, inorganics—particularly metals, and particulates. Because of potential recontamination of cleaned surfaces by liquids, an ideal cleaning technique would utilize gas phase reactants to produce gas phase products that could be easily removed from the system. Promising optically based cleaning methods for organics and particulates are being developed and optical techniques for metal removal are possible.

Image Technology in Engineering Practice: An Example from Optical Testing.

Contemporary technology in optical testing involves the use of computers, software, and video data transfer in addition to establishing an appropriate vibration-free test arrangement. Many unique problems arise in applying these techniques when more than one optical element is required in the test setup. This article discusses some of the realities and frustrations involved in carrying out a very difficult optical testing task. The intent here is to provide some tips and suggestions that will be of use to others attempting similar work.

Good Reason for Microscope Users to Take Part in Standard Setting

An ISO/TC172/SC5 meeting on microscopes in Shanghai, China, May 18-22, prompts this month's topic. It has been difficult to get appropriate national representa tion at this international meeting on microscope standards due to the consolidation of microscope manufacturers. In the past, many countries were involved in manufacturing microscopes. Today the major manufacturers can be counted on the fingers of one hand.

Scintillate, Scintillate Little Star...

If you have a chance to get out with a youngster on a night when the stars are twinkling, it might be interesting to point out the phenomenon and ask why the stars twinkle. Is it a characteristic of the stars themselves? Some children may remember that on many nights the stars shine quite steadily. On a turbulent night, they should be able to observe that all of the stars, not just a few, twinkle. This may lead to the reasonable conclusion that the state of the atmosphere makes the difference. After I pointed out to my daughter that in our area the scintillation (twinkling) often occurs when the hot winds come off the desert, she asked if it could be caused by the heat waves. She said the heat waves quiver, so the light could quiver.


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