James Hammond Brannon

Excimer laser etching of polyimide

It is reported that thin films of polyimide are efficiently etched in air at pulsed excimer laser wavelengths of 248, 308, and 351 nm. Etch rate versus incident fluence data are found to obey a Beer–Lambert etching relation. Sharp laser fluence thresholds for significant etching are found to correlate with the wavelength‐dependent absorption coefficient. The absorbed energy density required to initiate significant etching is found, within experimental error, to be independent of the wavelengths examined. It is felt that this information demonstrates the predominantly thermal nature of the laser etching mechanism. Additionally, infrared spectroscopy and coupled gas chromatography/mass spectroscopy were used to identify several gases evolved during pulsed laser etching of polyimide in both air and vacuum.

Threshold behavior in polyimide photoablation: Single-shot rate measurements and surface-temperature modeling

The ablation rate of Kapton™-type polyimide has been measured as a function of incident fluence and excimer laser wavelength using a sensitive quartz-crystal microbalance (QCM). The experiments were performed such that the fluence and the ablated depth were known for each laser pulse, avoiding the need to average rate and fluence data over many pulses. By limiting the investigations to the low-fluence regimes near ablation threshold, high precision and detailed curve shapes were obtained. It was found that the ablation rate increases smoothly and exponentially with increasing fluence for 248, 308, and 351 nm wavelengths. This exponential behavior was modeled using an Arrheniustype thermal rate equation. In contrast, the 193 nm curve is linear in fluence, displays a sharp threshold, and is consistent with a possible photochemical ablation mechanism. Using a sophisticated surface temperature modeling code, the maximum laser induced surface temperature at the fluence at which ablation can first be detected is found to be the same, ∼ 850° C, for all four wavelengths. This “ablation” temperature is significantly higher than the approximately 500° C temperature at which Kapton™ starts to degrade under isothermal heating conditions.

Pulsed CO2 laser etching of polyimide

Etching of thin polyimide films in air was investigated using a line tunable, pulsed CO2 laser. The threshold fluence for etching at a wavelength of 944 cm−1 (10.6 μm) exceeds that at 1087 cm−1 (9.2 μm) by a factor of 4. This is consistent with the infrared absorption spectrum which shows polyimide to be significantly more absorbing at 1087 cm−1. As a result, etching at 1087 cm−1 produces a cleaner, more precisely defined region. Analysis of the vapors generated during laser etching shows the simple gases CO2, H2O, and CO to be present.

Ambient gas effects on debris formed during KrF laser ablation of polyimide

The surface debris that results from KrF excimer laser ablation of polyimide has been investigated as a function of the pressure, and atomic or molecular weight of several ambient gases: H2, He, Ne, air, Ar, Kr, and Xe. A linear relation between the measured debris radius and the inverse third root of the ambient pressure was found to exist, consistent with the predictions of blast wave theory. No measurable debris could be observed using helium or hydrogen gases up to 1 atm, similar to previous reports on helium. The derived value of the blast energy, equal to about 5% of the incident pulse energy, was used to estimate a nascent blast pressure of approximately 150 atm. By making the assumption that surface debris will form if the ablation fragments are confined in a ‘‘small’’ volume for a ‘‘sufficient’’ time, conclusions from blast wave theory suggest how to decrease the amount of generated debris.

Ultraviolet photoablation of a plasma-synthesized fluorocarbon polymer

Plasma polymerized tetrafluoroethylene (PPTFE) is shown to undergo efficient 248 nm excimer laser ablation. The principle difference between this material and the analogous polytetrafluoroethylene (PTFE), which results in only poor quality ablation, is PPTFEs much greater absorption coefficient (7×104 vs. ∼102 cm−1). A plot of the ablation depth per pulse versus incident fluence indicates that the threshold for significant ablation occurs near 50 mJ/cm2, and that approximately 0.7 μm/pulse can be removed at 800 mJ/cm2. Near threshold, the ablation rate curve can be fit by a single Arrhenius-type exponential. This suggests that the removal process is at least partially governed by a photothermal process, similar to well-known laser induced thermal desorption experiments. In the very low fluence regime between 10 and 30 mJ/cm2, small removal rates are measured in a process likely dominated by non-thermal ablation. The paper concludes with a discussion of the high quality, micron-size features that can be directly patterned into PPTFE surfaces.

Pulsed laser stripping of polyurethane‐coated wires: A comparison of KrF and CO2 lasers

Compared to mechanical, thermal, electric, or chemical means of removing plastic wire insulation, laser removal offers precision and speed without the necessity of contacting the material. Utilizing the technique of excimer laser ablation of organic polymers, it is shown that excimer laser removal of polyurethane‐type wire insulation proceeds with much higher precision and cleanliness than does pulsed CO2 laser wirestripping. A reason for this difference is polyurethane’s much higher absorptivity of ultraviolet compared to infrared radiation. Scanning electron microscope photographs of both stripped wires and ablated regions on polyurethane‐coated flat disks clearly show the superior quality of the excimer method. The paper concludes with a discussion of the photophysical parameters and mechanisms responsible for the large difference between excimer and CO2 stripping.

Laser Texturing Of Glass Disk Substrates

Infrared laser pulses of microsecond duration derived from a stabilized carbon dioxide laser are used to produce a textured zone on a glass disk substrate. Each laser pulse can produce a microscopically smooth dome-shaped bump that protrudes from the initial disk surface. Typical bump heights of interest are 20-30 nm with bump diameters /spl sim/10 /spl mu/m. The laser-textured zone, composed of /spl sim/10/sup 5/ bumps, provides a dedicated region for contact start/stop. The zone exhibits excellent tribological properties (stiction and durability).

Chemical Etching of Silicon by CO2-Laser-Induced Dissociation of NF3

The pulsed infrared laser dissociation of NF3 is reported for the first time, and is used to investigate silicon etching. The role played by collision-enhanced multiple-photon absorption and dissociation is considered, with data on the nonlinear decrease of the absorption cross-section with increasing pulse energy and increasing pressure presented. Using an experimental arrangement in which the laser beam is focussed parallel to the surface, the dissociation process induces spontaneous etching of silicon. Fluorinecontaining radicals diffuse from the focal volume to the surface where a heterogeneous chemical reaction occurs. Etching was monitored by use of a quartz-crystal microbalance upon which a thin film of amorphous silicon was deposited. For a surface with no previous exposure to the photolysis products, dissociation causes the formation of a surface layer prior to the onset of etching. X-ray photoelectron spectroscopy demonstrates this to be a fluorosilyl layer possessing a significant concentration of SiF3 and SiF4. In contrast, a surface already thickly fluorinated does not form a thicker layer once laser pulsing commences again. In this case, etching starts immediately with the first pulse. The etch yield dependencies on several parameters were obtained using silicon samples possessing a thick fluorosilyl surface layer. These parameters are NF3 pressure, laser wavenumber, pulse energy, buffer gas pressure, and perpendicular distance from focal volume to surface. Modeling of the etch yield variation with perpendicular distance shows the time-integrated flux of radicals impinging on the surface to be inversely proportional to the distance. Attempts at etching SiO2 under identical conditions were unsuccessful despite the evidence that thin native oxide films are removed during silicon etching.

KrF LASER ABLATION OF POLYURETHANE

Polyurethane can be effectively and cleanly ablated with 248 nm excimer-laser radiation. For fluences above 200 mJ/cm2, very little post-ablation debris is observed — a fact indicative of a polymer that decomposes readily into volatile, small molecular-weight compounds. Ablation-rate data have been obtained both by stylus profilometry and the quartz-crystal microbalance (QCM) technique, and the results of both methods are in good agreement. The more sensitive QCM technique first detects material removal near 20 mJ/cm2, which is likely due to outgassing, surface chemistry, or low quantum-yield processes. At 37 mJ/cm2, an ablation “threshold” with a sharp increase of the ablation rate is observed and marks the onset of efficient, explosive ablation. The densely sampled rate curve provided by the QCM permits the conclusion that an Arrhenius-like exponential does not give a satisfactory fit to the data. This demonstrates that the ablation process is not solely governed by thermal processes. Applying a Beers-law analysis of rate versus the natural logarithm of the fluence yields excellent agreement with the data up to 300 mJ/cm2. The absorption coefficent derived from this analysis agrees well (within 4%) with the value obtained from the low-intensity absorption spectrum.

Excimer laser ablation of polyimide: a 14-year IBM perspective

IBM introduced the first commercial high-end mainframe computer system incorporating laser ablation technology in 1991. This milestone was the culmination of nearly a decade of scientific, engineering, and manufacturing effort. Extensive research and development on 308 nm laser ablation of polyimide lead to the first IBM prototype ablation tool in 1987 for the production of via-holes in thin film packaging structures. This prototype, similar to step and repeat photolithography systems, evolved into full-scale manufacturing tools which utilize sophisticated beam shaping, beam homogenizing, and projection optics. But the maturity of this technology belies the fact that the scientific understanding of the laser ablation process is still far from complete. This paper briefly reviews the engineering and scientific accomplishments, both within and external to IBM, that lead to the commercial utilization of the laser ablation process. Current technical tissues are discussed, in addition to alternative IBM applications of polyimide ablation. The paper concludes by discussing the relative merits of excimer vs. solid-state lasers, and how each may impact future manufacturing technology.