Julian Kähler

Pick-and-Place Silver Sintering Die Attach of Small-Area Chips

A method for high-temperature-stable die attaches based on sintering of micro and nano silver particles is described. A low-temperature (200°C) and low-pressure (3 N/mm2) process was established to ensure compatibility with conventional adhesive die attach and to avoid surface damage on the dice, respectively. A modified flip-chip bonder providing high placement accuracy (2 μm) is used for a precise pick-and-place die attach. A thermal finite element modeling simulation was performed to analyze the bonding process. Additionally, the influence of the surface properties on the adhesion of sintered silver layers was investigated. The area-selective sintering method allows combination with other standard processes for die attach. It is now possible to establish pressure-assisted silver sintering for the series production of hybrid electronic circuits, which is an option requested by the industry to expand the operation range of sensors and electronics in harsh environments (e.g., measurement while drilling).

Sintering of Copper Particles for Die Attach

First steps are taken toward a low-cost alternative to silver sintering as a highly reliable die attach technology for deep drilling applications and future power electronic modules. In this feasibility analysis, we evaluate sintering of copper particles for die attach. Particulate copper pastes are pretreated in H2 atmosphere (50 mbar) in order to gain oxide-free particles. Subsequently, particles are sintered at a pressure of 40 N/mm2 and a temperature of 350°C for 2 min. Porosity, Youngs modulus as well as electrical and thermal conductivities of sintered layers are analyzed. Moreover, shear tests at ambient temperature are performed for evaluating the adhesion of monometallic as well as Cu-Au bonds according to the American military standard for chip-substrate contacts (MIL-STD-883H, method 2019.8).

Die-attach for high-temperature applications using fineplacer-pressure-sintering (FPS)

A new joining technique called “Fineplacer-Pressure-Sintering” (FPS) for die-attach of small electronic components (e.g. LEDs and photodiodes) is described. Using a modified Flip Chip Bonder, bare dies could be bonded onto substrates with high positioning accuracy. For the FPS process a 50 tons press, which is conventionally used for pressure sintering, is no longer required. Very high average shear strengths (63 MPa) were achieved on molybdenum substrates (metallization: Ni/Au). With the help of silver powder of micro-to-nanometre grain size the electrical and mechanical properties of the compound layer could be further increased. The bond strength of metalized GaN-LEDs on Al2O3 substrates with a Ti/Pd/Au metallization is twice as high as with standard micro-powder and the process temperature could be reduced to 200° C. Finally the applicability of FPS was demonstrated by an optoelectronic module consisting of two commercial InGaN -LEDs and GaP -photodiodes on a metalized Al2O3 substrate. Successful function was found with prototype modules at temperatures up to 250° C.

Packaging of MEMS and MOEMS for harsh environments

A method for die-attach based on sintering of micro- and nano-silver-particles, which is stable in harsh environments, was described. A modified flip-chip bonder providing high placement accuracy was used for precise pick and place die-attach. Components of sensors designed for data logging during deep drilling, i.e., a MEMS vibration sensor and a MOEMS pressure sensor, were assembled and tested at temperatures up to 250°C. Shear tests of bonded devices were performed before and after temperature load. Bonded silicon-on-insulator Wheatstone bridges and GaP-PD were tested by temperature cycling (50 cycles from 100°C up to 250°C).

Design and fabrication of piezoresistive p-SOI Wheatstone bridges for high-temperature applications

For future measurements while depth drilling, commercial sensors are required for a temperature range from -40 up to 300 °C. Conventional piezoresistive silicon sensors cannot be used at higher temperatures due to an exponential increase of leakage currents which results in a drop of the bridge voltage. A well-known procedure to expand the temperature range of silicon sensors and to reduce leakage currents is to employ Silicon-On-Insulator (SOI) instead of standard wafer material. Diffused resistors can be operated up to 200 °C, but show the same problems beyond due to leakage of the p-njunction. Our approach is to use p-SOI where resistors as well as interconnects are defined by etching down to the oxide layer. Leakage is suppressed and the temperature dependence of the bridges is very low (TCR = (2.6 ± 0.1) μV/K@1 mA up to 400 °C). The design and process flow will be presented in detail. The characteristics of Wheatstone bridges made of silicon, n- SOI, and p-SOI will be shown for temperatures up to 300 °C. Besides, thermal FEM-simulations will be described revealing the effect of stress between silicon and the silicon-oxide layer during temperature cycling.

Sinter-attach of high-temperature sensors for deep-drilling monitoring

An optimized fabrication process for high-temperature piezoresistive vibration sensors made of p-doped double-layer SOI material is described. Thus, an easy and straightforward production of very precise sensor geometries is now feasible. Subsequently, it is demonstrated that sintering of silver particles can be successfully applied for die attach of these MEMS sensors. Sintering takes place at a very low pressure of 4 N/mm2 and a temperature of 250 °C for 2 min. Furthermore, the attached sensors are tested according to the requirements of deep drilling projects such as geothermal power generation from deep geologic layers. In doing so, the operation of the bonded sensors under realistic conditions for vibration monitoring is analyzed, i.e. simultaneous temperature (250 °C) and vibration (± 50 g) stress.

Sinter-attach of Peltier dice for cooling of deep-drilling electronics

We evaluate Ag particle sintering as a highly reliable die attach technology for the assembly of thermoelectric modules. Bismuth telluride (Bi2Te3)-based Peltier coolers are realized using Ag sintering and tested for high-temperature applications (e.g. Measurement-While-Drilling, MWD). Bonding takes place at a pressure of 6 N/mm2 and a temperature of 250°C for 2 min. Shear tests are performed for evaluating the adhesion according the American military standard for chip-substrate contacts (MIL-STD- 883H, method 2019.8). Sintered layers are analyzed for porosity, Youngs modulus, electrical conductivity, and thermal conductivity. It is shown that due to specific additives (i.e. micro-diamond particles) the coefficient of thermal expansion (CTE) of sintered Ag layers can be reduced to meet the requirements of Bi2Te3 as well as next-generation thermoelectric materials (e.g. skutterudites) for minimized thermally induced stress during drilling.

Sensors for data logging during deep drilling

Geothermal power is one of the most important sources of energy worldwide.1 However, to meet the increasing needs for alternative energy supplies, geothermal economic efficiency must be increased. In particular, the technical reliability of the drilling systems must be improved and bore costs reduced. To achieve these goals, modern drilling systems have been equipped with sensor technologies for measurements while drilling (MWD). These sensors have a variety of functions that include process monitoring, early identification of faults, and timely response to critical situations, for example, detecting vibrations caused by interactions between the chisel and rock formation. The severe environmental conditions expected in geothermal energy projects1, 2—such as temperatures up to 300C and bore depths up to 10,000m—make heavy demands on the materials and electronics used. Here, we describe our progress in developing sensors capable of operating at high temperatures for MWD. Piezoresistive silicon sensors are widely used for sensitive measurements in a variety of applications, from household appliances and the automotive industry to biomedical devices.3 In general, they consist of a mass and n-doped silicon cantilever with integrated p-type resistors connected to a full Wheatstone bridge. When an external load deflects the cantilever, the resulting stress changes the resistance and unbalances the Wheatstone bridge. This in turn produces a voltage that can be measured. Unfortunately, commercial piezoresistive sensors cannot be used in deep drilling because they only operate at temperatures less than 175C. This limitation at higher temperatures is caused by the supply current of the bridge flowing through both the p-doped resistors and n-doped substrate, rather than just the resistor. That is, an undesirable p/n junction forms, which causes current to ‘leak,’ thus reducing the voltage through the bridge and skewing measurements. A common remedy for this involves embedding Figure 1. (a) Scanning electron micrograph (SEM) of the top view of the cantilever sensor. (b) Tilted SEM of the p-doped siliconon-insulator (SOI) Wheatstone bridge consisting of four resistors .40 3 5 m3/.

Nanostructured silicon for thermoelectric

Thermoelectric modules convert thermal energy into electrical energy and vice versa. At present bismuth telluride is the most widely commercial used material for thermoelectric energy conversion. There are many applications where bismuth telluride modules are installed, mainly for refrigeration. However, bismuth telluride as material for energy generation in large scale has some disadvantages. Its availability is limited, it is hot stable at higher temperatures (>250°C) and manufacturing cost is relatively high. An alternative material for energy conversion in the future could be silicon. The technological processing of silicon is well advanced due to the rapid development of microelectronics in recent years. Silicon is largely available and environmentally friendly. The operating temperature of silicon thermoelectric generators can be much higher than of bismuth telluride. Today silicon is rarely used as a thermoelectric material because of its high thermal conductivity. In order to use silicon as an efficient thermoelectric material, it is necessary to reduce its thermal conductivity, while maintaining high electrical conductivity and high Seebeck coefficient. This can be done by nanostructuring into arrays of pillars. Fabrication of silicon pillars using ICP-cryogenic dry etching (Inductive Coupled Plasma) will be described. Their uniform height of the pillars allows simultaneous connecting of all pillars of an array. The pillars have diameters down to 180 nm and their height was selected between 1 micron and 10 microns. Measurement of electrical resistance of single silicon pillars will be presented which is done in a scanning electron microscope (SEM) equipped with nanomanipulators. Furthermore, measurement of thermal conductivity of single pillars with different diameters using the 3ω method will be shown.