The thick film paste manufacturers are expected to produce conductors which are lead and cadmium free, yet have excellent fired film properties and the same performance and properties as the cadmium and lead containing formulations. The fired film surface of these conductors must be defect free (i.e. imperfections, pills, agglomerates) after multiple firing steps and must perform on dielectric as well as substrates from different suppliers. Typically, the thick film gold conductors are used in high reliability applications such as medical devices, military applications, and high frequency circuits, which require robust performance at high and low temperatures, in chemically aggressive environments, or extremely humid conditions. As circuits decrease in size and become more complex, the thick film gold properties become increasingly critical. The challenge is to develop an alternative gold conductor formulation, which can print and resolve fine features (down to 4 mil lines and spaces) as well as have the ability to be etched for higher density circuit designs (down to 1–2 mil lines and spaces). Gold conductors are typically used in conjunction with other high temperature thick films so good performance after multiple firings was also a targeted requirement. Heraeus has been proactive for the past decade in the development of thick film products that are both RoHS (lead and cadmium free) as well as REACH compliant. This paper discusses the experiments that were performed in order to understand the contribution of gold powder, organic and inorganic system to improve the fired film performance. These formulations were compared against existing gold conductors including the high performance gold conductor options as well as other available standard gold conductor options. Thin wire bonding trials including both gold and aluminum wire are used to compare influences of raw materials which includes high volume wire bonding reliability including failure modes and aged wire bond adhesion at elevated temperature exposures (300°C) for extended periods of time. In order to analyze fired film morphology and link this up to wire bond performance, SEM images of the conductor surface and cross sections were conducted. These studies resulted in a newly developed thick film gold conductor paste for use in a wide variety of applications. We present wire-bonding data with gold and aluminum wire and reliability results on both 96% Al2O3 ceramic substrates as well as on top of standard dielectrics.

In this paper, experimental results for electromigration void morphology and failure time distribution are presented as a function of test condition, grain size distribution and conductor linewidth. Unpassivated Al-Cu(1.5%)-Si(1.5%) conductors with linewidths in the range lum ≤W ≤l0μm were subjected to accelerated temperature stresses of T=150° C and 250°C and current stresses in the range j= 0.5-4X106 A-cm-2, for each temperature stress. Unlike previous reports, erosion-like voids dominated at all current densities and temperatures for both W/D50≤1 and W/D50 > 1. The void distribution within a line was strongly dependent on test conditions and linewidth. The critical current density was extracted from electromigration lifetimes measured as a function of current density. The results show that the linewidth dependence of electromigration lifetimes depend strongly on the Blech relation.

Thick film customers who require fine line resolution for their circuitry typically utilize wet chemical etching as a means to reduce conductor's lines and spaces when fine line definition cannot be reliably attained with screen printing alone. Wet chemical etching typically has the means to reduce conductor line widths from a printed definition of 3 mil (75 μm) to as low as 1 mil (25 μm) lines and spaces. The process of performing this chemical etching is time consuming and costly when factoring in the necessary process limitations. With the issues presented by wet chemical etching, thick film customers are presented with a high process cost, yield loss due to the imaging process, and costly wastewater/environmental treatment regulations. Therefore, laser etching will be presented as an alternative method to wet chemical etching for various thick film conductor products. For many years, specialized gold formulations have been etched using typical wet chemical etching processes. Standard and less costly conductor alloys that typically would not be suitable for wet chemical etching will be explored, possibly opening the doors for a wide variety of different applications which would benefit from utilizing this laser etching method. By being able to utilize different conductor alloys (Ag, Cu, etc.), laser etching offers alternative solutions for some of these applications with the added benefit of improved cost and increased throughput. As an example, wet chemical processing of silver conductors has proven to be very challenging in some cases due to the metal form-factor and specialized glasses required. By having the option of laser ablating the silver, a potentially advantageous and cost-effective option would now be possible. Taking into account that laser etching of thick film conductors on ceramic is a relatively new method, this paper will concentrate on some of the opportunities/advantages it can offer. It will illustrate the boundaries of laser etching and how it compares to wet chemical etching while determining/comparing the impact on several properties including adhesion, signal propagation, line definition, and other important defining characteristics of the fired film in the final application.

Continued miniaturization of conductor geometry below 0.5gtm has by a concomitant decrease in Al deposition temperature, typically below 300'C. The degree of reliability exhibited by these films is strongly dependent on the grain size distribution and metallurgical configuration. This investigation focuses on the impact of post-deposition processing on changes in conductor microstructure and electromigration for films deposited at low temperature. AICu(I%)Si(1%) was deposited at 300'C on PECVD phosphosilicate glass. The impact of post-deposition thermal budget on the as deposited grain size and distribution, preferred orientation and stress/strain states were analyzed using scanning electron microscopy, X-Ray diffraction and bending beam technique, respectively. These characteristics were also measured on unpassivated films subjected to the same thermal budget as Si0 2 passivated films so that the geometric confinement and AI-Si0 2 surface interaction could be quantified. Electromigration characteristics were measured for linewidths in the range W=1.Ogtm to W=10tm, for passivated and unpassivated films. A direct correlation between passivation and grain size was observed for both failure modes. This paper will also discuss the relationship between film stress and preferred orientation, the observed failure modes and their linewidth dependence. INTRODUCTIONAs semiconductor technology has taken a quantum leap in improvement over the last decade, semiconductor devices have been incorporated into almost every aspect of our daily life. Any unexpected and unpredictable failure will not only simply be an inconvenience, but in many cases will cost human life. Therefore, understanding the early stage failure mechanisms and improving the device reliability, has become the most important segment of this industry. The reliability of metal interconnects is the most critical in the reliability arena, because the failure is catastrophic, i.e. devices either work or do not work. Material scientists and engineers are and will always be playing an extremely important and inevitable role in the interconnect reliability. Any set of processing parameters [1] will produce a specific microstructure such as chemical compositions [2][3][4][5][6][7], grain size and distributions [3,5,7-10], grain boundary structure [11,12], second phase precipitates [3,4,13], preferred orientations [3,7,9], stress states [3,5,7,[14][15][16][17][18][19][20][21][22], passivation [20][21][22][23][24], geometry [25][26][27][28] ...... etc. The device lifetime, failure initiation and growth mechanisms are the results of specific microstructures. Therefore, material scientists and engineers not only have to understand the kinetics of how to create specific microstructures, by precisely controlling the appropriate processing parameters, but also have to know how to characterize the microstructure. Sometimes, invention and implementation of appropriate state-of-the-art characterization tools is necessary for the technology break through.In this work, we will stu...

We have investigated the effects of Cu and Si dopants on electromigration mass transport in Al interconnects for VLSI technology. Four Al alloys with different Cu and Si dopant concentrations (AI-1.5%Cu, AI-1.5%Cu-I.5%Si, AI-0.5%Cu-I.0%Si, and AI-I.O%Cu-I.0%Si) were sputter deposited onto Si substrates covered with 0.55 pRm of Si02 (tetraethylorthosilicate). All metal films were deposited using an MRC903 sputter deposition system to a thickness of 6800Å. Deposition parameters were held constant for each metallurgy, with a base pressure of 7x 10−2 torr, deposition pressure of 102 torr, forward power of 7 Kwatts, and substrate bias of 125 volts. Test structures designed according to National Institute of Standards design guidelines were fabricated in each metallurgy using conventional photolithography and reactive ion etching methods for line widths of 1.0, 1.8, 3.0, 5.0 and 10.0 μm. Accelerated test conditions of T=200°C and DC current density of 2x106 A/cm−2 were used. The results show that electromigration resistance increases with increasing Cu content and decreases as Si content increases. These results are explained in terms of precipitate, grain size distribution, orientation and stress by Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD). Our results provide a general guideline relating Cu and Si dopant concentrations, film microstructure and the intrinsic reliability of the metallization system.