High-power lasers (greater than 5 kW) have been in existence for about two decades. However, their use by industry is extremely limited. Numerous researchers have demonstrated laser welding to a thickness of nearly 1 inch. But almost all laser welding today is for a thickness of 1/8 inch or less. The welding discussed here is for material thickness greater than 1/4 inch in a shop setting for which small number job lots are common.
As experience has unfolded, Laser Fare has been presented with a variety of aerospace hardware components requiring anywhere from one to thousands of holes. Further experience has indicated that'aircraft engine manufacturers, in constantly striving to decrease fuel consumption yet improve performance, have been faced with operating certain components at higher temperatures than ever before. In an effort to prevent the engine from turning into a glob of molten metal, it has become evident that a flow of cooling air is essential on some components and a more uniform mixture of combustion gases apparently is required in others.
The laser, with its intense beam of focused pure light, precisely controlled and moved by computer, is being increasingly used for processing aircraft gas turbine engine components. Pratt & Whitney uses both solid state and gas lasers for a variety of applications including welding, cutting, hole drilling, hardfacing, and marking of parts for identification. Laser processing provides increased productivity and reduced cost compared to conventional methods. Specific uses of lasers in gas turbine fabrication are described and compared against the prior method of manufacture. Tooling approaches and metallurgical effects associated with various laser processes are also discussed.
Eigh power laser material processing is regarded in Western Europe as one of the nost important technologies for the near future. For that reason strong research End development efforts are undertaken in many countries of the common market End in the smaller European countries as Scandinavia, Switzerland and in tustria. All that work is done to enhance material processing performance, to improve processing quality and efficiency and to extend the range of ETplications of laser material processing. These efforts include developments of lasers with higher output, smaller volume and improved beam quality, as e.g. high frequency excited high power as lasers. These lasers can be pulsed and are thus well suited for laser material processing purposes. Considering the large importance of beam quality for processing performance and quality, strong attention is paid to high power laser beam diagnostics and control. Much work has been devoted to a more efficient coupling g of the radiation to the workpiece under use of the controlled formation of nicroplasmas. Also material processing with shorter wavelength is investigated. In order to optimize laser material processing, new theoretical models including dynamic descriptions have been developed for laser cutting, laser welding and laser surface treatment. New machining processes are under development, as for instance processing with pulsed radiation or with augmention, for instance with plasma devices. In order to extend the range of applications three dimensional processing either with special focusing heads, with moving mirrors or by combination with robots have been developed. In the following three examples for advanced European activities are given:
A dynamic description of laser cutting based on a previously developed steady state model has been elaborated. With that dynamic model the periodic striation pattern that is formed on the cut surfaces can be satisfactory explained. i2loreover, the above model can be used to analyse cutting with pulsed radiation and elucidates some important technical aspects of pulsed cutting.
A high wear point exists at the notch between adjacent blades forming the outer shroud of a jet engine turbine stage. This notch is commonly hardfaced to reduce wear and improve turbine blade endurance. Until recently, the blades were manually hardfaced by the gas tungsten arc process. A laser hardfacing process was developed for this application which has increased production rates and reduced rework requirements. The laser's precise energy control, inherent repeatability, and ability to be automated are the principal reasons for these process improvements. Laser hardfacing fundamentals and process development are described. Production equipment characteristics are reviewed and unique features of the process are identified. Finally, the results of several years of production hardfacing experience are discussed.
With the greater acceptance of laser technology as a viable alternative to traditional metals joining methods, the need has arisen to integrate lasers into efficient high production systems. This paper will describe one such system which is dedicated to the automated processing and laser welding of automotive transmission gear components. The system features two (2) 6 KW CO2 lasers, robotic part manipulation, vapor degreasers, air cylinder press stations, fully enclosed weld stations incorporating bottom delivery methods, and programmable computer control which allows complete monitoring throughout the entire production cycle. It is the intent of this paper to describe all segments of the system in detail as to design, manufacture, and integration. Concerning this specific application, an overview from initial inquiry through final installation of the manufactured system will be presented and will focus on the laser welding process and parameter development as it relates to the total systems concept and production goals. The paper concludes with a summary of system field performance to date.
The performance limits of the oxygen-assisted laser cutting of mild steel have been
extended by the incorporation of an electric arc in the laser cutting process. It is shown
that cutting performance (as determined by cutting speed) increases with arc power and
approximately 100$ increase in cutting performance can be obtained without any significant
deterioration of cut quality. At high arc powers (greater than about 4kw), although cutting
speed increases as the arc power increases, the cut quality (as determined by the kerf width
and heat affected zone measurements) decreases remarkably. The laser beam and the electric
arc were used on opposite sides of the same workpiece and the deterioration in cut quality
was restricted to the arc's side. Although the laser-arc interaction phenomena are not yet
fully understood, the results from the experiments reported in this paper indicate a cooperative
mechanism between the laser and the arc. The electric arc provides a cheap
additional energy source to the laser energy absorbed by the workpiece while arc stability
is maintained at high arc speeds when the arc on its own would be unstable. This increase
in arc stability is caused by a laser-generated hot spot on the same side of the workpiece
as the arc. The laser-generated hot spot provides a low resistance path for the arc to root
Recent advances in the use of lasers for integrated circuit processing have developed effective techniques for cleaning, deposition, and etching of materials for both steered and large area projection to the degree that lasers are serious contenders to play an important role for in situ integrated circuit manufacturing.
Maskless, automated, five minute interconnection of 1000 gate CMOS array die has been accomplished using discretionary laser-induced chemical vapor deposition. Eight 125-stage ring oscillators written on single CMOS gate array die were shown to have the device-limited performance of similar patterns manufactured by photolithographically patterned aluminum-silicon alloy. These results suggest the feasibility of using this method, Laser Pantography (LP), for rapid implementation of prototype and limited volume semicustom VLSI circuits immediately after their design is completed.
A laser direct writing method based on the pyrolysis of a thin solid state organometallic film for repairing transparent defects in pholomasks has been developed. The film is applied using standard photoresist spin-coating techniques and a focused Ar laser beam is used to locally decompose the metallo organic coating. This technique is capable of generating submicron opaque metallic features under ambient atmosphere conditions. Experimental results as well as limitinp factors are discussed.
Theoretical and practical considerations for optimized laser microsurgery on semiconductor devices are presented and discussed. Various commercially available lasers suitable for microsurgery are described, although the present work centers around the use of a xenon laser. Along with a thorough discussion of the xenon lasers operating characteristics, a number of novel xenon laser-based failure analysis techniques developed in the wafer scale integration program at Trilogy Systems Corp. are reported. Over the past 10 to 15 years, laser technology has been used in semiconductor manufacturing technology to solve an increasing number of problems to which it is uniquely suited. Lasers have been successfully used in the areas of photomask repair, resistor trimming, cutting of redundant circuit links, laser formed connections, and failure analysis (1-4). The property of laser technology which makes it so useful is the ability to apply a relatively uniform dose of photonic energy (0.01 - 1.5 mJ) in a very short time (1nS - 1.5µS) to the microscopic areas of interest in integrated circuits (1 Am2 - 100 Am2) using more or less standard optical components. In this paper, several laser based techniques which have proven exceptionally useful in the production of fully functional wafer scale integrated circuit prototypes at Trilogy Systems Corp. are reported as well as the results of a study of the operating characteristics of the Florod L.F.A. xenon laser system used in this study.
There are four general types of laser-assisted processes which have been employed for dry etching organics, semiconductors, and metals used in microelectronics . The first is laser ablation or laser-induced evaporation, which is physical rather than chemical in nature [2-7]. The second type of process uses the laser to heat the material surface, thereby increasing the rate of a thermally activated chemical reaction [8-10]. The third is gas-phase photochemical etching in which the laser is used to generate the reactive gas-phase species responsible for etching a semiconductor or metal [11-16]. The fourth type is surface photochemical etching in which the laser is used to create the photogenerated carriers responsible for etching; highly selective etching can be achieved by capitalizing on the surface electronic properties which influence carrier generation or subsequent behavior [17-20]. These four laser-assisted dry etching methods will be reviewed, with special emphasis on the advantages and disadvantages of the three chemical processes for etching semiconductor materials.
We have developed ultraviolet laser-induced, radical-etching processes that can provide practical etch rates and selectivities for most of the important substrate combinations used in silicon microelectronic devices. These processes have been demonstrated, in simple proximity and projection exposure experiments, to produce etch features on surfaces with dimensions of a few tenths of a micrometer. The technique could significantly simplify the fabrication of submicrometer linewidth devices by eliminating photoresist patterning and acid or plasma etching processing steps.
The feasibility of detecting voids in triple extruded power cable insulation by measure-ment of Mie scatter power of incident farinfrared (FIR) laser radiation is explored. Measurements of scatter power of a 100-250 um range of void sizes in polyethylene insulation show that void scatter power and experimental SNR decrease as wavelength increases from 119 to 447 um. Extrapolations of experimental data to larger void sizes show that detection of voids larger than 250 on in polyethylene insulation at 447 pm wavelength would have acceptable SNR using direct detection methods with no noise reduction techniques. Triple extruded insulation was modeled by taking into account the attenuation of an insulation shield at 447 um with 14 percent transmission. Modeling results show simulated detection of voids in triple extruded insulation would provide acceptable SNR for detection of voids larger than 250 um using higher laser irradiance levels than those required for nonshielded, or tandem extruded, insulation.
A laser interferometer where intensity is measured as function of incidence angle is described. Number of fringes and/or their location in interferogram defines plastic film thickness. With simplest method of signal processing, thickness accuracy is within 2% for films over 50 micrometers. The fringe interval method improves accuracy to within 0.1% for films over 10 micrometers. Special least-square methods may improve accuracy further or even allow a simultaneous index of refraction and thickness measurement. The method is intended for industrial on-line monitoring of plastic film thickness.
Coherent anti-Stokes Raman spectroscopy (CARS) is a nonintrusive laser diagnostic tech-nique for spatially- and temporally-precise measurements of temperature and species in practical combustion systems. Over the last several years, the development of CARS has progressed, at our laboratory and others, from simple flames of increasing complexity to practical application in internal combustion engines, gas turbine combustors, furnaces and a myriad of other real world devices. In this paper, the theory and application of CARS will be reviewed. Its practical applications will be surveyed with particular emphasis on our work in afterburning jet engine exhausts and development of a mobile CARS instrument for field applications.
The cellular flame structure of a suddenly propagating flame into a quiescent rich propane air premixed combustible gas charged inside a Vycor glass flame tube, whose inner diameter is 28.5 mm and vertical height is 285 mm, i.e.,ten times longer than its inner diameter, ignited at opened end of the flame tube by the usual automotive Capacitor-Discharge-Ignitor was investigated by using 3 low power He-Ne laser diagnostics simultaneously probed into the propagating flames. Initial state of the combustible gas was under the conditions of room temperature and atmospheric pressure.
The challenges to metrology provided by the extreme precision required by disc memory technology is described. Heterodyne interferometry is shown to be able to meet the metrology requirements and to provide a more direct interpretation of the results than other methods.
An improved method for determining the thin film thickness in terms of the variation of reflectivity in Attenuated Total Reflection (ATR) is proposed. In which, the correlation of the difference of thin film thickness (1d) and reflectivity (R) is derived on the basis of Fresnel's formula. In addition, a scanning unit is specially designed to scan the laser beam and detector synchronously. Experimental results indicate that the measurement accuracy is not affected by small fluctuation of the incident angle and the reflectivity change. Moreover, possible applications of this technique in developing the displacement sensor with high sensitivity are also suggested.
By analysing the diffraction pattern obtained by side illumination of an optical fiber with a parallel beam we obtain information about the fiber geometry. In this paper we discuss shortly the solution of the boundary value problem and the calculation of the diffraction pattern of graded-index fibers. We describe the optics and the signal processing electronics of a set-up that is used to measure the outer diameter and the core diameter of graded-index fibers during the drawing process.