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Research Papers

Size-Compatible, Polymer-Based Air-Gap Formation Processes, and Polymer Residue Analysis for Wafer-Level MEMS Packaging Applications

[+] Author and Article Information
Erdal Uzunlar

School of Chemical and Biomolecular Engineering,
Georgia Institute of Technology,
Atlanta, GA 30332-0100

Paul A. Kohl

School of Chemical and Biomolecular Engineering,
Georgia Institute of Technology,
Atlanta, GA 30332-0100
e-mail: kohl@gatech.edu

1Corresponding author.

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received December 19, 2014; final manuscript received June 22, 2015; published online July 23, 2015. Assoc. Editor: Satish Chaparala.

J. Electron. Packag 137(4), 041001 (Jul 23, 2015) (13 pages) Paper No: EP-14-1114; doi: 10.1115/1.4030952 History: Received December 19, 2014

This study aims at investigating a polymer-based air-gap creation method for the packaging of microelectromechanical systems (MEMS), and exploring the chemical composition of the polymer residue on the final package. Polymer-based air-gap formation utilizes thermal decomposition of a sacrificial polymer, poly(propylene carbonate) (PPC), encapsulated within an overcoat polymer. BCB (Cyclotene 4026-46) was used as the overcoat material because decomposition products of sacrificial polymer are able to permeate through it, leaving an embedded air-gap structure around the MEMS device. Size-compatibility and cleanliness of MEMS devices are important attributes of the polymer-based air-gap MEMS packaging approach. This study provides a framework for size-compatible and clean air-gap formation by selecting the type of PPC, optimizing thermal treatment steps, identifying air-gap formation options, assessing air-gap formation performance, and analyzing the chemical composition of the residue. The air-gap formation processes using photosensitive PPC films had at least twice the residue compared to processes using nonphotosensitive PPC films. The major contribution to the residue in photosensitive PPC films was from the photoacid generator (PAG), which was used to catalyze the thermal decomposition of the PPC. BCB is compatible with PPC, and provides mechanical stability during creation of the air-gaps. The polymer-based air-gaps provide a monolithic, low-cost, integrated circuit compatible MEMS packaging option.

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Figures

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Fig. 1

Schematics of the polymer-based air-gap packaging performed on (a) a surface-micromachined MEMS device and (b) a bulk-micromachined MEMS device, both with planar feedthroughs

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Fig. 2

Polymer-based air-gap formation process flows for (a) RIE process and (b) direct photopatterning process

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Fig. 3

TGA results of neat and purified PPC: (a) Novomer 160 K and (b) QPAC 40 141 K

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Fig. 4

The chemical structure of PPC with carbon and hydrogen atoms labeled for referencing to NMR signals. The left part shows polycarbonate part, and the right part shows the polyether part. The polycarbonate parts predominate in the PPC structure. The uppercase letters label the carbon atoms in the polycarbonate part. The lowercase letters label the hydrogen atoms in both polycarbonate and polyether parts.

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Fig. 5

13C-NMR spectra for neat QPAC 40 141 K (top), neat Novomer 160 K (middle), and purified Novomer 160 K PPC (bottom). The left column includes full spectra, and the right column includes the close-up spectra at the carbonate region.

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Fig. 6

1H-NMR spectra for neat QPAC 40 141 K (top), neat Novomer 160 K (middle), and purified Novomer PPC 160 K (bottom)

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Fig. 7

Kinetic comparison between BCB curing and PPC decomposition processes at (a) 170 °C, (b) 190 °C, and (c) 210 °C

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Fig. 8

The Dektak profiles of the air-gap structures obtained using (a) 60% BCB cure, (b) 70% BCB cure, and (c) 80% BCB cure at 180–190 °C. The PPC decomposition was done at 240 °C for 4 hrs. The Dektak profiles are leveled with respect to the BCB. The solid line corresponds to the profile obtained before the BCB cap is removed, and the dotted line corresponds to the profile obtained after the BCB cap is removed.

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Fig. 9

Top-view optical image of wrinkles observed on BCB cap in 80% BCB cure air-gap sample. The BCB curing was done at temperatures between 180 and 190 °C, the PPC decomposition was done at 240 °C for 4 hrs. The Dektak profile of the shown BCB cap is given in Fig. 8(c), where the wrinkles are small height changes.

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Fig. 10

Top-view optical images and Dektak line-scan profiles of a 2.5 × 3.2 mm air-gap feature obtained from the middle of a medium thick nonphotosensitive PPC sample subjected to two-step thermal treatment of 1.3 hrs at 190 °C and 11 hrs at 240 °C. All data were acquired after PPC decomposition. The optical image and Dektak profile on top were acquired before removal of BCB cap, and the optical image and Dektak profile at the bottom were obtained after removal of BCB cap. Dektak profile on top was leveled with respect to overcoat BCB layer, and Dektak profile at the bottom was leveled with respect to Si surface. The inset schematics on each Dektak profile show the cross section of the features from which the Dektak profiles were obtained. Around 5 μm of BCB thickness was lost over the air-gap due to curing of the overcoat BCB layer and pattern-transfer BCB layer (which was casted from a diluted BCB solution), and overetching of the pattern-transfer BCB layer in RIE as explained in the Experimental Procedure section (∼3 μm loss of initially 12.3 μm thick overcoat BCB layer measured before thermal treatment as shown in BCB processing data sheet [21], ∼2 μm loss of initially 2.5 μm thick pattern-transfer BCB layer measured before RIE due to combined overetching and thermal treatment). Considering the losses in BCB thickness over the air-gap and process variation in BCB and PPC thicknesses, the air-gap thickness can be inferred as around 18–19 μm from the Dektak profiles above, which is in agreement with the thickness of PPC used in medium PPC sample.

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Fig. 11

An SEM image of the silicon surface of a medium PPC sample after the BCB cap was removed (the sample was tilted by 55 degrees in SEM tool for image acquisition)

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Fig. 12

Cross-sectional SEM images of an air-gap structure obtained on a thin PPC sample: (a) complete air-gap structure (b) close-up image of the small rectangle in (a). The residues sitting on BCB were due to BCB parts broken due to cross sectioning.

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Fig. 13

An XPS depth profile obtained from the air-gap region of a medium thick nonphotosensitive PPC sample with initial PPC thickness of 19.3 μm subjected to a two-step thermal treatment of 1.3 hrs at 190 °C and 13 hrs at 240 °C

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Fig. 14

The configurations of two-layer PPC film for direct photopatterning process, where (a) photosensitive PPC layer is at the bottom and (b) photosensitive PPC layer is on top

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Fig. 15

An XPS depth profile obtained from the air-gap region of a two-layer PPC sample with initial photosensitive (PAG-loaded) PPC at the bottom (2.8 μm), and nonphotosensitive PPC on top (23.5 μm) subjected to a three-step thermal treatment of 30 hrs at 150 °C, 30 hrs at 180 °C, and 11 hrs at 240 °C

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Fig. 16

High-resolution XPS scan, peak fit and peak deconvolution of C1s peaks obtained from (a) nonphotosensitive PPC residue and (b) photosensitive PPC residue

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Fig. 17

A schematic showing the nanoindentation experiment done on (a) an air-gap structure and (b) on BCB-only region, both on the same sample

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