Steven A. Cordes

Enhanced thermoelectric cooling at cold junction interfaces

We describe a thermoelectric device structure that confines the thermal gradients and electric fields at the boundaries of the cold end, and exploits the reduction of thermal conductivity at the interfaces and the poor electron-phonon coupling at the junctions. The measured temperature–current and voltage–current characteristics of a prototype cold point-contact thermoelectric cooler based on a p-type Bi0.5Sb1.5Te3 and n-type Bi2Te2.9Se0.1 material system indicate an enhanced thermoelectric figure-of-merit ZT in the range of 1.4–1.7 at room temperature.

Lithographically fabricated fiber guides for optical subassemblies

A new optoelectronic packaging technology is presented which permits highly accurate fiber-to-chip alignment at low cost. Instead of the more common methods employed in fabricating optical subassemblies, which use leadframe, precision plastic-molding, or silicon-optical-bench technology, here fiber guides are fabricated in a photoresist by use of standard photolithographic procedures. By this means the fiber guides are directly created on an entire wafer of either VCSELs or receivers, resulting in structures with very tight dimensional tolerances fabricated at very low cost. After dicing, a fiber is interfaced with a chip under computer control using a very simple semiautomatic tool to insert the fiber into the fiber guide. This new technology may be used in the fabrication of a wide variety of single or multi-channel optoelectronic transceivers.

Pellicles for x-ray lithography masks

The feasibility of using thin films of organic material as a protective cover for x-ray lithography masks has been demonstrated. A pellicle structure that fits unobtrusively inside the mask-wafer gap and on the x-ray lithography NIST standard ring was developed. The pellicle solves several problems: 1) protects the mask absorber from direct contact to external sources of possible damage without itself contacting the absorber, 2) allows effective cleaning of the x-ray mask by preventing dust or debris particles from getting imbedded in the absorber pattern, 3) reduces contamination of the stepper and wafers by absorber metal debris in the event of catastrophic breakage of the mask membrane, 4) reduces contamination of the absorber pattern by debris generated by the resist or the stepper environment during x-ray exposure, and 5) reduces photoelectrons generated by the absorber during x-ray exposure from reaching the resist. Furthermore, pellicles may offer inspection advantages by providing a flat surface without absorber topography. Several approaches for the fabrication of suitable pellicles using organic and inorganic films will be presented in this paper. Pellicles have been successfully tested in the SVGL and SUSS steppers in the IBM Advanced Lithography Facility in the East Fishkill, NY. Results on pellicles performance together with radiation damage measurements to estimate a pellicles lifetime are presented in this paper.

Improved excimer laser pattern generator for photomask fabrication

This paper describes a laser pattern generator constructed by modifying a GCA 3600F tool for the production of research photomasks. The mercury arc lamp illumination source was replaced with an excimer laser. Extensive modifications made to the subsystems and software are described. The result is a tool with a 25X average throughput improvement, a resolution of 1.0 micrometers , and an 8X increase in overlay precision. The laser pattern generator retains the ease of operation of the original system and exhibits improved reliability. The overall cost of the implemented improvements is a small percentage of a state-of-the-art laser or electron beam mask generation tool. We report on the results of generating 2X, 5X and 10X reticles as well as 1X photomasks from the past three years.

Development of electroless nickel-iron plating process for microelectronic applications

Electroless nickel-phosphor (Ni-P) plating has been extensively used as a diffusion barrier layer in printed circuit board applications. However, the interfacial reactions between electroless Ni-P and lead (Pb)-free Sn solders during high temperature reflow are known to dissolve Ni-P excessively into Sn leaving behind a brittle Ni3P compound at the interface. This often causes various reliability issues in the solder joints. To mitigate this problem, an application of NiFe alloy is proposed with the concentration of iron (Fe) greater than 20% in the Ni barrier. The NiFe alloy produces a more robust diffusion barrier in the microelectronic applications. Development of a stable electroless NiFe plating process is therefore essential for its successful implementation. In this work, we developed an electroless NiFe alloy plating process with a high deposition rate and superior bath stability. The deposition rate of three micron per hour was achieved and five weeks of bath aging did not deteriorate the bath performance. Stable iron content around 30% in NiFe deposit was achieved in this timeframe. Factors that impact deposition rate and alloy composition were identified. The valence of the iron ions in the electroless NiFe bath was found to have a significant impact on the bath stability. The interfacial reaction between Pb-free SnAgCu solder and electroless NiFe(P) is discussed and compared with the electroless Ni-P process. Energy dispersed X-ray spectroscopy (EDS) and scanning electron microscopy (SEM) were utilized to characterize electroless NiFe deposits and to analyze the interfacial reactions between Pb-free solder and electroless Ni-P or NiFe after multiple reflows.

Mask technology for excimer laser projection ablation

Excimer laser projection ablation is a dry, precise patterning process in which an intense beam of ultraviolet light from an excimer laser is used to directly pattern a material. This technique has been used in industrial applications for patterning both organic and inorganic materials. In the manufacturing of microelectronics devices, laser ablation is used extensively to pattern insulating layers in the multi- level thin film packages. Excimer laser projection ablation is very similar to optical projection lithography, both using a photomask or reticle which contains a master pattern. The mask used in a 1X projection laser ablation tool, however, must withstand significantly higher energy densities than conventional photolithographic masks. A number of mask technologies have been developed specifically for 1X excimer laser projection ablation. These masks include dielectric layers on quartz masks, thick films of aluminum on quartz masks, binary phase shifted grating masks and holographic masks. This paper presents a review of these mask types. Critical issues such as fabrication processes, advantages and disadvantages, cost and availability of each mask are discussed.

Data-processing improvements for photomask pattern generators

The performance of optical pattern generators as well as other lithographic tools was often limited by the performance of the data processing equipment and software used to control these tools. In fact, many performance attributes of modern pattern generators is due to the power and flexibility of the modern computers and software. For example, imperfections in the stage travel due to the mechanical components are often mapped out and corrected for in the control software. When key components are upgraded, these older tools regain useful life, at relatively low cost, for research and development and certain production applications. This paper describes modifications which have been made to a GCA 3600F pattern generator resulting in improved performance in key categories such as reliability and throughput. The modifications have been done to the operating system, data storage subsystem, job creation and monitoring system. Also, a generator has been constructed to simplify production of photomasks consisting primarily of regular arrayed patterns.

Low-cost mask for excimer laser projection ablation

Excimer laser projection ablation is a dry patterning process in which an intense beam of ultraviolet light from an excimer laser is used to directly pattern a material. This technique has been used extensively in the microelectronics industry for patterning both organic and inorganic materials. Excimer laser projection ablation requires the use of a mask which is similar to a conventional 1X photomask. The laser ablation mask must withstand significantly higher energy densities than conventional photolithographic masks. A dielectric mask structure which consists of a quartz substrate coated with a stack of dielectric thin films has been developed for this process. Although the dielectric mask has been used successfully in a manufacturing environment, it suffers from the disadvantages of a complex fabrication process and high cost. Alternatives to the dielectric mask have been explored and a new mask has been developed which consists of an aluminum film on a quartz substrate. This mask meets the requirements for the laser ablation process and has the advantage of a low cost fabrication process which is similar to conventional chrome on quartz photomasks. The mask development, specifications, fabrication and results are discussed.