Michael E. Mills

Development of a Low‐Dielectric‐Constant Polymer for the Fabrication of Integrated Circuit Interconnect

For faster, smaller, and higher performance integrated circuits, a low dielectric constant insulator is required to replace silicon dioxide. Here the properties of a new dielectric—SiLK resin, a solution of a low-molecular-weight aromatic thermosetting polymer—are reviewed and examples of its application in the fabrication of interconnect structures, such as the one shown in the Figure, are given.

Benzocyclobutene (DVS-BCB) polymer as an interlayer dielectric (ILD) material

Abstract Polymer films of DVS-BCB (CYCLOTENETM 5021) exhibit a combination of material and processing properties which make them an attractive low k interlayer dielectric (ILD) material for integration into IC manufacturing processes. Key DVS-BCB film properties include isotropic dielectric constant (2.65 @ 1 MHz); equilibrium moisture absorption ( 350°C; low alpha-particle emission rates. The DVS-BCB films are easily produced using simple resist spin track equipment and are subsequently cured without generation of corrosive or volatile by-products under anaerobic conditions (less than 100 ppm oxygen). Films are completely cured for typical processing conditions at 250°C for 1 h or as rapidly as 30 s at 325°C. DVS-BCB has been demonstrated to fill 0.20 μm gaps at 5:1 aspect ratios. Fully cured films exhibit greater than 95% degree-of-planarization for isolated feature widths of 20 μm and less; 80% for widths between 20 μm and 100 μm. Currently, the acceptance of DS-BBC is challenged by todays thermal requirements of greater than 400°C for CD-W via/plug and post device/contact anneals. The development of moderate temperature metal deposition processes and reduced final device anneal temperatures would enable implementation of DS-BBC as an LID material with the realization of its low dielectric constant, excellent gap-fill and other attractive features.

Sequential Process Modeling for Determining Process-Induced Thermal Stress in Advanced Cu/Low-k Interconnects

The thermomechanical reliability of Cu/low-k interconnects, which is directly related to yield problems and premature device failures, has been a major issue. The development of a manufacturing process, which can satisfy the most stringent reliability standards, requires detailed information on the thermomechanical behavior of Cu/low-k interconnects. The thermomechanical behavior of Cu/low-k interconnects is complicated by the fact that processinduced thermal stresses are developed during the manufacturing process. A conventional finite element analysis (FEA) approach has some difficulties to model Cu/low-k interconnects that keep changing during process steps. Therefore, a sequential process modeling technique has been developed to simulate the interconnect behavior to substantially any level of detail and understand the complex thermomechanical behavior of Cu/low-k interconnects while being manufactured. In this paper, we briefly describe a sequential process modeling technique and demonstrate how we use the modeling technique to solve a Cu/SiC delamination problem in a Cu/SiLK* semiconductor dielectric dual damascene test structure.

Comparison of solar cell device thermal degradation and low-irradiance performance

The thermal degradation, low-irradiance performance, IV and EQE characteristics of CuInGaSe2 (CIGS), c-Si, and GaAs photovoltaic devices are presented. Typically thin-film polycrystalline materials are hypothesized to be advantaged over monocrystalline materials due to their lower thermal degradation coefficient and improved low-irradiance performance. In this work the performance of these different solar cell materials was evaluated with the intent being to determine if these hypothesized performance distinctions exist. Such differences could indicate that solar cells exhibit optimum performance in specific climates.

Development of a hybrid CVD/SOD integration sequence for reliable, high performance interconnect systems

The use of hybrid integration schemes is investigated using a combination of a SiOC film at the via level and a porous SiLK Y film at the trench level. Sequential finite element analysis is used to determine the mechanics and, subsequently, a hybrid damascene interconnect is built to demonstrate the approach.

A study of thermal, voltage, and photoinduced effects on the external quantum efficiency of CuInGaSe 2 (CIGS) photovoltaic devices

The purpose of this study was to assess the performance of CuInGaSe2 (CIGS) solar cells ranging from 6.9-11.2% efficiency to identify means of improving device performance. One of the main drivers for this work was to take an independent look at elevated operating temperatures associated with BIPV products like Dows POWERHOUSE™ shingle system for performance differences as a function of system operating temperature independent of increasing efficiency to help future product definition. For three of the four samples we observed a clear lack of dark-light current-voltage (JV) superposition and correspondingly via external quantum efficiency (EQE) measurements an electrical field dependent carrier collection response in the blue regime. By inference this performance indicates the presence of impurities in the device, likely in the CdS film that could present an opportunity to further improve the CIGS device performance.

Methodology for delivering reliable CIGS based building integrated photovoltaic (BIPV) products

A key challenge currently limiting the wide spread acceptance of Cu(In,Ga)Se2 (CIGS) thin-film photovoltaic technologies in building integrated photovoltaic (BIPV) systems is the demonstration of product reliability in accelerated testing to support rapid product improvement cycles and new product introduction. To augment multi-year & geographically diverse real world performance a priori, one must adopt a creative approach to ensure rapid product introduction of new, highly reliable solar PV systems. Here, we present a synopsis of Dow Solars reliability philosophy that utilizes multi-stress testing, a combination of accelerated and real world conditions, to provide predictive life stress relationships for CIGS based BIPV system level product performance. In addition, the methodology we present here includes a proactive philosophy of identifying and isolating individual reliability failure mechanisms in PV technologies. This philosophy enables significantly shorter development cycles and the obtainment of meaningful product performance feedback. The approach, which is balanced between accelerated testing and field testing data, may be utilized to establish lifetime performance of any PV technology.

Assembly processes for thin film CIGS solar cells: Approaches for improving interconnect repeatability and costs

We have developed new assembly equipment for manufacturing interconnected CIGS solar cell strings that delivers improved control of mechanical tolerances and improved form factor flexibility. The assembly approach is based upon the use of pallets that ensure alignment of solar cells and ribbons during the assembly and curing process. An overview of the assembly equipment is provided. Also discussed are the results of an improved multi-wire interconnect technology that provides a 10% reduction in series resistance relative to standard strings with interconnects provided via electrically conductive adhesive.

Thermal degradation and light capture performance of CuInGaSe 2 (CIGS) and c-Si photovoltaic devices

The thermal degradation and light capture performance of CuInGaSe2 (CIGS) and c-Si photovoltaic devices were explored. Typically thin-film polycrystalline materials such as CIGS are hypothesized to be advantaged over single crystal Si solar cells as having lower thermal performance and improved light capture response. To this end, we present our evaluation of the thermal and light capture performance of three different CIGS devices, having different absorber layer stoichiometry and bandgap (Eg), and a c-Si device. Our results indicate that at the cell level these CIGS and c-Si photovoltaic devices have similar thermal degradation and light capture performance. The thermal performance of the c-Si and three CIGS materials explored were on average 0.40, 0.47, 0.51, and 0.48 % efficiency/°C, respectively. At low light levels (∼150 W/m2) the voltages generated by each device were within 90% of those generated at full light intensity (1000 W/m2). The current levels generated by each device trended linearly with the irradiance, and the Rs for each device increased with the irradiance although no intentional changes were made to the electrical contacts. These results indicate that both CIGS-based and c-Si devices will perform equally well in comparable real-world conditions. With regards to the performance relative to the CIGS bandgap, tuning the CIGS material stoichiometry, i.e. making it closer to CGS, should broaden the materials spectral response and provide beneficial improvements to the thermal degradation rate and the light capture performance of CIGS devices.