The use of lasers in microelectronics production for trimming, link cutting, ablating, drilling and general “micromachining” continues to grow. Several new technologies, such as alternative wavelength processing and shaped, uniform laser spots have produced better processing quality, higher reliability, and greater yields. This paper will review the latest technologies of laser micro-machining in microelectronics. Laser drilling for printed circuit board will be dressed in another paper.
Laser drilling has emerged in the last five years as the most widely accepted method of creating microvias in high-density electronic interconnect and chip packaging devices. Most commercially available laser drilling tools are currently based on one of two laser types: far-IR CO2 lasers and UV solid-state lasers at 355 nm. While CO2 lasers are recognized for their high average power and drilling throughput, UV lasers are known for high precision material removal and their ability to drill the smallest vias, with diameters down to about 25 –30 micron now achievable in production. This paper presents an overview of techniques for drilling microvias with the lasers.
The use of lasers in microelectronics is production for trimming, ablating, drilling and general micromachining continues to grow. As one example, traditional laser trimming techniques for passive and active microelectronic circuits have been used for nearly thirty years to improve yields and/or device performance. The majority of these processes have been accomplished using the fundamental wavelengths of the Nd:YAG laser source. However, recent technological advances in microelectronics laser processing, mainly for hybrid integrated circuits (HIC), dynamic random access memories (DRAM) and printed wiring boards (PWB) have resulted in new process techniques. Several new technologies, such as alternative wavelength processing and shaped, uniform laser spots have produced better processing quality, higher reliabiltiy, and greater yields. This paper will review the past, present and future of laser micromachining in microelectronics.
Memory repair through the use of laser processing of redundant elements is an industry standard procedure for memory chip manufacturing. But, shrinking memory feature sizes and the industry's tendency to use metals as link materials rather than polysilicon imposes new challenges for laser processing. So far, the majority of the research on memory link laser processing has concentrated on: The vertical structure of a link (such as the multiple layers of passivation, link, field oxidation and silicon substrate); the laser beam absorption; and, the different temperature distribution within the structure as the result of laser beam heating. Until now, the emphasis in laser link processing optimization has been aimed at creating uniform temperature distribution while severing the link before exploding the passivation layer. Our study has shown that the link width plays an important roll in the processing as well. Analysis of the mechanical stress beneath the passivation layer using finite element modeling has been carried out. Different link width and passivation layer thicknesses vary the stress dramatically. The results of this simulation will be presented and their implication on link processing optimization will be discussed. To optimize the laser processing further, we have proven that absorption contrast of laser energy between the link material and the silicon substrate beneath the link must be maximized. Based upon the fact that while the absorption of most metal materials in the 1.3- to 2-micron range remains the same as that at 1 micron, it drops dramatically for silicon. By using laser wavelengths within the 1.3- to 2-micron range, a much wider laser processing window can be realized. Comparison analysis of link processing by different laser wavelengths will be discussed.
Due to imprecisions in the deposition process, linear semiconductors require post-production circuit adjustment. Lasers are used to adjust (or trim) thin film resistors to precise electrical performance specifications. The effect of coherent laser beam interference in multilayer structure on thin film resistor trimming has been well documented. As the layer structure becomes more and more complex, the difficulty in determining the interference effect on the trimming results becomes greater. To assist the circuit designer and laser trimmer user, a simplified multilayer thin-film resistor interference model and a universal software program have been developed. Several verifications have been made of customer designs using the model and software programs. While the model gives the user a clear picture of the physics of the interference effect, the software provides a precise recommendation for layer structuring that results in maximum laser absorption into the resistor.