Applications > MEMS

SU-8 SERIES and KMPR Plasma Removal/Rework

Removal or reworking highly crosslinked SU-8 epoxy resist is difficult because it is chemically very stable. To rework very thick SU-8 layers economically without damaging other microstructures requires a highly selective process. The Muegge/R3T reactor uses a microwave source to generate a high density of radicals to obtain high removal rate and high throughput. Remote plasma greatly reduces thermal load. Pure chemical etching with free radicals generated from O2 and CF4 results in high selectivity removing thick SU-8 and KMPR films with no metal attack.

SU-8 and KMPR
R3T Plasma Remove / Rework Advantages

  • High etch rate of more than 200 µm/h on large areas (e.g. batch of 9 x 6" wafers) independent of thickness; >20 µm/min for small samples; temperature below 70C
  • Etch rates nearly independent of hard bake conditions; differences in etch rate <10 % between no HB vs. 200°C HB
  • Stripping of very thick resist (>1 mm) possible
  • Pure chemical etching: No damage by ions, heat impact only by reaction energy
  • No attack on metals like Ni, NiFe, Au, Cu etc.
  • Only very slight attack on Si and Si compounds as SiO2 or Si3N4 (selectivity >500:1)
  • No organic residues; inorganic residues removable by additional cleaning step
  • Simultaneous etching of substrates with different resist thicknesses possible
  • End point detection for each individual substrate

Chemical dry etching /
Chemical downstream etching

Dense plasma excitation inside the water cooled Remote Plasma Source (RPS) leads to high amount of radical generation which is necessary to get high etch rate and throughput (Fig 2). Remote plasma implies much less thermal load than plasma in the chamber (e.g. RIE). The main contribution to the thermal load is the reaction heat between resist and oxygen, all other contributions together (e.g. from microwave plasma) are less than 10%. Substrate temperature is kept below 70°C.

Pure gentle chemical etching with no attack onto the etching sample by ions; ions and electrons combine before they reach the substrates, therefore high (e.g. Si, SiN, SiO2) resp. infinite (e.g. Au, Cu, Ni, NiFe, NiP) selectivities are achievable.

Because of the low thermal load and the missing ion impact no densifying of the resist takes place. Therefore the strip rate doesn't slow down during strip time; it is independent on resist thickness even for resists thicker than 1 mm.

Inside the microwave plasma more than 95% dissociation takes place and whereas the ions and electrons recombine inside the plasma chamber of the microwave source the neutral long-living oxygen (O*) radicals stream down to the substrates and react with all organic materials which built up volatile etch by-products like CO2; these are pumped away together with the residual gases. A small percentage of fluorine radicals (F*) helps to crack the resist bonds and to increase the etch rate at low temperatures.

The stripping process is a pure isotropic etch process without any directionality (Fig 3). That means that even parts behind sloped walls or resist out of caverns can be removed.

The resist strip rate is accelerated along the metal side walls as it can be seen in Fig 5 with partial stripped resist. That means that the metal structures are cleared before the structures between, and that small structures with high aspect ratio have a higher strip rate than other structures.

Contrary to wet chemical removal of organic materials the removal by oxygen radicals is quite less dependent on the kind of organic material and pre-treatment. As an example the rates for SU-8, KMPR and PMMA are nearly the same. Also the difference in etch rates between resist with 90°C and 200°C post bake are only in the range of 10%.

End point detection
For each of up to nine different samples the strip progress can be tracked and the endpoint of each of these samples can be detected in this way (see Fig 4).

SU-8: Benefits/Attributes

  • High aspect ratio structures with vertical sidewalls
  • Photo-definable, ultra thick structures
  • Outstanding thermal and chemical stability, ideal for electroplating
  • Excellent electrical and physical properties for dielectrics
  • Highly cross-linked photo-epoxy system
  • Chemical and thermal stability
  • Excellent dielectric properties
  • Film thickness range 500nm >100µm
  • High contrast and aspect ratios
  • High transparency
  • Low temperature processing

Fig 1: Small gears after SU-8 resist removal
Muegge/R3T GmbH STP2020 technical brochure
Source: Mimotec SA. See also

Fig 2: Working chamber with remote plasma source on top
Source: Muegge/R3T

Fig 3: Cross section of a wafer with metal micro parts and SU-8 during stripping phase
J. Mathuni, SU-8 removal from metallic HAR microstructures, July, 2008 Stuttgart

Fig 4: Example for end point signals at three different samples, stripped together at the same time
Source: Muegge/R3T

Fig 5: Partial stripping of a SU-8 resist after Ni galvanic; 50% of the resist still present; resist etch rate near the Ni surface faster
courtesy of Mimotec SA

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