Improved Mounting of Strain Sensors by Reactive Bonding

Abstract

Aim of this work is to improve the bond between a strain sensor and a device on which the strain shall be determined. As strain sensor, a CMOS-integrated chip featuring piezoresistive sensor elements was used which is capable of wireless energy and data transmission. The sensor chip was mounted on a standardized tensile test specimen of stainless steel by a bonding process using reactive multilayer systems (RMS). RMS provide a well-defined amount of heat within a very short reaction time of a few milliseconds and are placed in-between two bonding partners. RMS were combined with layers of solder which melt during the bonding process. Epoxy adhesive films were used as a reference bonding process. Under mechanical tensile loading, the sensor bonded with RMS shows a linear strain sensitivity in the whole range of tested forces whereas the adhesive-bonded sensor has slightly nonlinear behavior for low forces. Compared to the adhesive-bonded chips, the sensitivity of the reactively bonded chips is increased by a factor of about 2.5. This indicates a stronger mechanical coupling by reactive bonding as compared to adhesive bonding.

Keywords: joining; nanomaterials; reactive bonding; reactive multilayers; semiconductors; sensor mounting; strain sensor.

Fig. 1

Schematic drawing of a reactive multilayer system (RMS). Exothermic reaction was started at the left-hand side by an ignition-spark (or laser pulse). Self-propagating reaction zone travels from left- to right-hand side

Fig. 2

Picture of the RMS reaction front taken by a high-speed camera. The reactive multilayer was ignited on the left-hand side resulting in a reaction front traveling to the right. RMS were placed between two glass panes to keep the RMS flat. Reaction temperature is high enough to use a camera for visible light

Fig. 3

Top view drawing of sputter chamber from Fraunhofer IWS

Fig. 4

Layout and dimensions of the used sensor chip. The bond pads on the left side are used for contacting by wires

Fig. 5

Positions of the 32 PMOS strain-sensitive sensors on the chip

Fig. 6

(a) (left) and (b) (right): Chip assembly by wire bonding (left) and by flip chip bonding (right, both schematically)

Fig. 7

Precise alignment of the chip to substrate and RMS by means of a flip chip bonder (left and right), ignition of the RMS by two electrodes (right)

Fig. 8

Bonding assembly indicating materials and dimensions which were used for simulation of the heat flow during reactive bonding process

Fig. 9

Simulated temperatures during the reactive bonding process. Solder temperatures close to the steel specimen and close to the chip are displayed as well as the chip temperature itself and the liquidus temperature of the Sn solder

Fig. 10

Strain distribution (sensor offset) without chip assembly

Fig. 11

Change in strain distribution after chip assembly compared to the signals without assembly

Fig. 12

Standard tensile testing probe (stainless steel, 12.5 mm width, 1.5 mm thick) with mounted chip assembly

Fig. 13

Change in strain distribution after adhesive bonding

Fig.14

Change in strain distribution after reactive bonding

Fig. 15

Change in strain distribution at 1200 N tensile force for adhesive-bonded chip v43. The sensor which has been used for sensitivity measurement is marked by a circle

Fig. 16

Change in strain distribution at 1200 N tensile force for reactive-bonded chip v72

Fig. 17

Comparison of sensitivities of bonded chips v43 (adhesive) and v72 (reactive) for the marked sensor in Fig. 15 and 16. The improved sensitivity of the RMS-bonded sensor is clearly visible by the higher output signal