Design and Modeling of a Microscale Selective Laser Sintering System

Abstract

The development of micro and nanoscale additive manufacturing methods in metals and ceramics is important for many applications in the aerospace, medical device, and electronics industries. Unfortunately, most commercially available metal additive manufacturing tools have feature-size resolutions of greater than 100 μm, which is too large to precisely control the microstructure of the parts they produce. A few research-grade metal additive manufacturing tools do exist, but their build rate is generally too slow for commercial applications. Therefore, this paper presents a new microscale selective laser sintering (μ-SLS) that can be used to improve the minimum feature-size resolution of metal additively manufactured parts by up to two orders of magnitude, while still maintaining the throughput of traditional additive manufacturing processes. In order to achieve this goal, several innovative design features like the use of (1) ultra-fast lasers, (2) a micro-mirror based optical system, (3) nanoscale powders, and (4) a precision spreader mechanism, have been implemented. The micro-SLS system is capable of achieving build rates of approximately 1 cm3/hr while achieving a feature-size resolution of approximately 1 μm. This paper will also present new molecular scale models that have been developed for the micro-SLS to quantify and certify the micro-SLS build process. Modeling of the micro-SLS process is challenging, because most macroscale models of the SLS process contain assumptions that are no longer valid when the size of the particles that are being sintered is smaller than the wavelength of the laser being used to sinter them. Therefore, in modeling the micro-SLS process we must account for the wave nature of light and can no longer rely on the ray tracing models commonly used to model the SLS process. Also, heat transfer in the micro-SLS process is dominated by near-field radiation due to the diffraction of the light off the nanoparticles in the powder bed and the ultrafast lasers that are used in the micro-SLS system. This means that the assumptions of heat transfer by conduction and far-field radiation in the macroscale SLS systems are no longer valid for the micro-SLS system. Finally, the agglomeration of nanoparticles in the powder bed must be accurately modeled in order to precisely predict the formation of defects in the final parts produced. Overall, the goal of this modeling effort is to be able to predict the quality of a part produced using any given processing conditions, in order to produce parts that are “born certified” and do not need to be tested post fabrication.

A novel microscale selective laser sintering (μ-SLS) process for the fabrication of microelectronic parts

“…Thus, a wafer handling chuck with heating capabilities is used which reduces the wattage requirement of the laser and leads to better quality parts with smaller geometrical aberrations and less warpage (see supplementary Fig. S2 for the design of this chuck) 52 .…”

A Comprehensive Study of the Sintering of Copper Nanoparticles Using Femtosecond, Nanosecond, and Continuous Wave Lasers

“…In this model, a few simplifying assumptions have been made. It is assumed that: (1) some properties of the NPs such as the specific heat capacity, heat of fusion, and heat of vaporization are the same as that of the bulk material; (2) threshold fluences (onset of sintering, melting, and ablation) are independent of exposure times as the time between each pulse is long enough so that particle bed returns to its initial state and past train of pulses do not affect the threshold fluences; (3) all of the laser energy during each pulse is expended in raising the temperature of the particles, i.e., there are no losses to the surroundings or neighboring particles; and (4) the NPs in the ink start sintering at 330 C which have been observed [9] are much lower than the melting point of the bulk Cu. This means that the model does not capture all of the physics going on in the sintering process but it does provide a good initial estimate of the potential processing window for good sintering.…”

“…3), and then, volume of the Cu nano-ink layer can be obtained as the product of the Sintering Temperature Determination. Though the melting temperature of bulk Cu is 1085 C, NPs have been observed to sinter [9,23] much below their bulk melting temperatures. Differential scanning calorimetry of the ink showed an exothermic peak at around 330 C, indicating the onset of the sintering process [9].…”

“…Though the melting temperature of bulk Cu is 1085 C, NPs have been observed to sinter [9,23] much below their bulk melting temperatures. Differential scanning calorimetry of the ink showed an exothermic peak at around 330 C, indicating the onset of the sintering process [9]. Scanning electron microscope (SEM) images that were taken just before and after the exothermic peak (see Fig.…”

“…Recently, the sintering of nanoparticles (NPs) has attracted much interest as an alternative to conventional integrated circuit fabrication techniques [2,5,6]. To bridge the gap between the process capabilities, a new AM technique called microscale selective laser sintering (l-SLS) has been developed [7][8][9].…”