Comparison of Anodic and Au-Au Thermocompression Si-Wafer Bonding Methods for High-Pressure Microcooling Devices

Abstract

Silicon-based microchannel technology offers unmatched performance in the cooling of silicon pixel detectors in high-energy physics. Although Si-Si direct bonding, used for the fabrication of cooling plates, also meets the stringent requirements of this application (its high-pressure resistance of ~200 bar, in particular), its use is reported to be a challenging and expensive process. In this study, we evaluated two alternative bonding methods, aiming toward a more cost-effective fabrication process: Si-Glass-Si anodic bonding (AB) with a thin-film glass, and Au-Au thermocompression (TC). The bonding strengths of the two methods were evaluated with destructive pressure burst tests (0-690 bar) on test structures, each made of a 1 × 2 cm² silicon die etched with a tank and an inlet channel and sealed with a plain silicon die using either the AB or TC bonding. The pressure resistance of the structures was measured to be higher for the TC-sealed samples (max. 690 bar) than for the AB samples (max. 530 bar), but less homogeneous. The failure analysis indicated that the AB structure resistance was limited by the adhesion force of the deposited layers. Nevertheless, both the TC and AB methods provided sufficient bond quality to hold the high pressure required for application in high-energy physics pixel detector cooling.

Keywords: anodic bonding; bonding technology; burst pressure test; microcooling; microfluidic device; thermocompression bonding.

Figure 1

Schematic showing the typical layout of a silicon pixel detector used to localize the crossing point of an impinging charged particle.

Figure 2

Design of the pressure test structures: (a) plan view of the channel base; (b) side view of an individual chip after bonding; (c) wafer-level design of the channel structures with tank widths (mm) die naming convention (A1, B1, …, D8).

Figure 3

Fabrication of the test structure: (a) simplified process sequence; (b) SEM image of the inlet hole etching (DRIE 2) with the exposed Al stop layer; (c) SEM image of inlet hole and channel after Al removal; (d) Si channel base (2 × 1 cm²) following wafer dicing.

Figure 4

Anodic bonding: (a) schematic of the bonding setup; (b) voltage and current measured during the bonding process.

Figure 5

Thermocompression Au-Au bonding: (a) schematic of the bonding setup; (b) measured temperature of the upper (red) and lower (black) platens and applied force (green) during the bonding process.

Figure 6

Scanning acoustic microscopy (SAM) measurements of Au-Au bonded wafers: (a) home-built SAM system, zoom of the water tank showing the emitter/receiver and sample holder; (b) zoom of the sample holder; (c) typical SAM image of a well-bonded test structure.

Figure 7

Schematic of the high-pressure test bench for destructive burst pressure characterisation of bonded die test structures.

Figure 8

Typical rise of pressure inside a chip recorded with the high-pressure bench.

Figure 9

Characterization of the Si cover wafer with SiO2/BF33 intermediate layers for bonding: (a) wafer bow measurement showing the stress relaxation during annealing; (b) thickness uniformity of BF33 layer; (c,d) 3D and 2D wafer profiles after annealing, respectively.

Figure 10

Characterization of the anodically bonded wafer stack: (a) photo of the bonded wafer stack (channel side, visible inlet holes); (b) 3D profile showing the wavy surface.

Figure 11

Failure pressure of the AB test samples as a function of the channel tank width (the points have been slightly shifted horizontally around the true tank width value to avoid overlapping). Two categories of failure are distinguished: failures where a large piece is detached, showing clear unsealing over a large area (crimson), or failures where only a thin crack on the surface or no defect at all is observed (khaki). To guide the eye, a decreasing exponential function was empirically fitted to the data points.

Figure 12

Stop-motion pictures showing the DI water leaking from the bonding interface of the tested die (one image every 1/20 s). Several leaking sources are observed simultaneously.

Figure 13

Analysis of a broken AB sample (C2, w = 1.25 mm) after high-pressure test: (a) general view; (b) impact zone (IZ) both on cover and channel parts; (c) profile of the IZ on cover.

Figure 14

EDS analysis of the broken bonding interface of the C2 sample: (a) zoom on the EDS area inside the IZ (optical microscope); (b) map of Na and Si elements; (c) map of O, Na, and Si elements.

Figure 15

SAM images of AB dies having leaked at the bonding interface: (a) non-broken die after high-pressure test; (b) SAM image of chip C3 (w = 1 mm) that failed at a relatively low pressure (79 bar); (c) SAM image of chip D6 (w = 0.35 mm) having failed at high pressure (408 bar).

Figure 16

TC bonding results: (a) picture of the bonded wafer stack (channel side uppermost); (b) 3D surface profile (channel side); (c) SAM imaging of the wafer stack (the bottom part is hidden by the Teflon holder).

Figure 17

Maximal pressure reached in dies bonded by TC as a function of the channel tank width (the points have been slightly shifted horizontally around the true tank width value to avoid overlapping). Four categories are distinguished: dies reaching the indicated pressure without breaking (light blue), dies showing clear signs of breakage in the silicon only (dark blue), dies showing evidence for unsealing (red), and dies for which no diagnostic was possible (light grey).

Figure 18

Micrographs of broken samples showing clear breakage of the silicon on top of the tank base: (a) failure pressure: 316 bar (wafer position C2, w = 1.25 mm); (b) failure pressure: 293 bar (wafer position C1, w = 1.5 mm).

Figure 19

Micrographs of TC-bonded die pair that failed at unexpectedly low pressure. The tank width is 750 µm in both cases: (a) die showing clear signs of unbonding (wafer position B4, failure pressure: 93 bar); (b) die with an ambiguous failure type as the broken part is not fully detached (wafer position C4, failure pressure: 130 bar).

Figure 20

SAM images of TC bonded dies after high-pressure test: (a) die (wafer position C3; w = 1 mm) with a crack and having leaked at relatively low pressure (54 bar); (b) die (wafer position C4; w = 0.75 mm) with a crack and having failed at 130 bar; (c) non-broken die (wafer position D6; w = 0.35 mm); (d) non-broken die (wafer position D7; w = 0.2 mm).