Flat-plate solar array project. Volume 5: Process development

The goal of the Process Development Area, as part of the Flat-Plate Solar Array (FSA) Project, was to develop and demonstrate solar cell fabrication and module assembly process technologies required to meet the cost, lifetime, production capacity, and performance goals of the FSA Project. R&D efforts expended by Government, Industry, and Universities in developing processes capable of meeting the projects goals during volume production conditions are summarized. The cost goals allocated for processing were demonstrated by small volume quantities that were extrapolated by cost analysis to large volume production. To provide proper focus and coverage of the process development effort, four separate technology sections are discussed: surface preparation, junction formation, metallization, and module assembly

Intermetallic Bonding for High-Temperature Microelectronics and Microsystems: Solid-Liquid Interdiffusion Bonding

Solid-liquid interdiffusion (SLID) bonding for microelectronics and microsystems is a bonding technique relying on intermetallics. The high-melting temperature of intermetallics allows for system operation at far higher temperatures than what solder-bonded systems can do, while still using similar process temperatures as in common solder processes. Additional benefits of SLID bonding are possibilities of fine-pitch bonding, as well as thin-layer metallurgical bonding. Our group has worked on a number of SLID metal systems. We have optimized wafer-level Cu-Sn SLID bonding to become an industrially feasible process, and we have verified the reliability of Au-Sn SLID bonding in a thermally mismatched system, as well as determined the actual phases present in an Au-Sn SLID bond. We have demonstrated SLID bonding for very high temperatures (Ni-Sn, having intermetallics with melting points up to 1280°C), as well as SLID with low process temperatures (Au-In, processed at 180°C, and Au-In-Bi, processed at 90–115°C). We have verified experimentally the high-temperature stability for our systems, with quantified strength at temperatures up to 300°C for three of the systems: Cu-Sn, Au-Sn and Au-In

MEMS suljenta kuparin lämpöpuristusliitännällä

Copper thermocompression is a promising wafer-level packaging technique, as it allows the bonding of electric contacts simultaneously to hermetic encapsulation. In thermocompression bonding the bond is formed by diffusion of atoms from one bond interface to another. The diffusion is inhibited by barrier forming surface oxide, high surface roughness and low temperature. Aim of this study was to establish a wafer-level packaging process for MEMS (Mi-croElectroMechanical System) mirror and MEMS gyroscope. The cap wafer of the MEMS mirror has an antireflective coating that limits the thermal budget of the bonding process to 250°C. This temperature is below the eutectic temperature of most common eutectic bonding materials, such as Au-Sn (278°C), Au-Ge (361°C) and Au-Si (370°C). Thus a thermocompression bonding method needed to be developed. Copper was used as a bonding material due to its low cost, high self-diffusivity and resistance to oxidation in ambient air. The bond structures were fabricated using three different methods and the bonding was further enhanced by annealing. The bonded structures were characterized with scanning acoustic microscopy, scanning electron microscope and the bond strength was determined by shear testing. Exposing the bond structures to etchant during Cu seed layer removal was found to drastically increase the surface roughness of bond structures. This increase proved detrimental to bond strength and dicing yield and thus covering the bond surface during wet etching is recommended. The native oxidation on copper surfaces was completely removed with combination of ex situ acetic acid wet etch and in situ forming gas anneal. Successful thermocompression bonding process using sputtered copper films was established at a low temperature of 200°C, well below the thermal limitation set by the antireflective coating. The established wafer bonding process had high yield of 97% after dicing. The bond strength was evaluated by maximum shear strength and recorded at 75 MPa, which is well above the MIL-STD-883E standard (METHOD 2019.5) rejection limit of 6.08 MPa.Kuparin lämpöpuristusliitäntä on lupaava kiekkotason pakkausmenetelmä, sillä se mahdollistaa sekä sähköisten liitäntöjen, että hermeettisen suljennan toteuttamisen samanaikaisesti. Lämpöpuristusliitännässä sidos muodostuu atomien diffuusiosta liitospinnalta toiselle. Diffuusiota rajoittavat estokerroksen muodostava pinta oksidi, korkea pinnan karheus ja matala lämpötila. Diplomityön tavoitteena oli luoda kiekkotason pakkausmenetelmä mikroelektromekaaniselle (MEMS, MicroElectroMechanical System) peilille ja MEMS gyroskoopille. Peilin lasisen kansikiekon pinnalla oleva antiheijastava kalvo rajoitti liitännässä käytettävän lämpötilan korkeintaan 250°C:een, mikä on alempi lämpötila kuin useimpien kiekkoliitännässä käytettyjen materiaaliparien eutektinen piste. Esimerkkinä mainittakoon mm. Au-Sn (278°C), Au-Ge (361°C) ja Au-Si (370°C). Kuparin alhainen hinta, korkea ominaisdiffuusio ja hidas hapettuminen ilmakehässä puoltavat sen valintaa liitäntämateriaaliksi. Liitäntärakenteet valmistettiin kolmella menetelmällä ja liitännän vahvuutta parannettiin lämpökäsittelyllä. Liitetyt rakenteet karakterisoitiin pyyhkäisy elektronimikroskoopin, akustisen mikroskoopin ja liitoslujuus-mittauksen avulla. Liitospintojen altistamisen hapolle havaittiin lisäävän pinnankarkeutta ja olevan siten haitallista liitokselle ja laskevan saantoa. Liitospintojen suojaaminen siemenkerroksen syövytyksen aikana on suotavaa. Pintaoksidi pystytään poistamaan täysin suorittamalla oksidin märkäetsaus jääetikalla sekä lämpökäsittely N2/H2 atmosfäärissä. Sputteroidut kuparikalvot pystyttiin liittämään onnistuneesti yhteen 200°C lämpötilassa, mikä on alle anti-heijastavan pinnan asettaman lämpötilarajan. Tällä liitäntä menetelmällä saavutettiin kiekkoliitoksella yhteen liitettyjen sirujen sahauksessa korkea 97% saanto. Liitoslujuus määritettiin maksimi-leikkausvoiman avulla ja sen suuruudeksi mitattiin 75 MPa. Lujuus oli yli kymmenkertainen MIL-STD-883E standardin (METHOD 2019.5) asettamaan hylkäysrajaan 6.08 MPa nähden

Low temperature, low pressure CMOS compatible Cu -Cu thermo-compression bonding with Ti passivation for 3D IC integration

In this paper, we report the methodology of achieving low temperature, low pressure CMOS compatible Wafer-on-Wafer (WoW) Cu-Cu thermo-compression bonding using optimally chosen ultra-thin layer of Titanium (Ti) as a passivation layer. We systematically studied the effects of Ti thickness on bonding quality via its effects on surface roughness, oxidation prevention and inter diffusion of Cu. Through this study, we have found that a Ti thickness of 3 nm not only results in excellent bonding but also leads to a reduction in operating pressure to 2.5 bar and temperature to 175° C. The reduction in pressure is more than an order of magnitude lower relative to the current state-of-the-art. The lower operating pressure and temperature manifest themselves in a very good homogenous bond further highlighting the efficacy of our approach. Finally, our results have been corroborated by evidence from AFM study of the Cu/Ti surface prior to bonding. The bond strength of Cu-Cu as measured by Instron Microtester measurement system is found to be 190 MPa which compares very well with the reported literatures