3.Photolithography and resist systems

Transcription

1 3.Photolithography and resist systems Exposure Mercury arc lamp Shadow printing projection printing Photomask Substrates Resist systems DNQ-Novolak-based Epoxy-based Polyimide based 1 Exposure Mercury arc spectrum (total power emission: 100 to 2000 W) 405 nm 365 nm 436 nm Broad emission (depending on exposure tool) Single emission lines g-line 436 nm h-line 405 nm i-line 365 nm each line contains less than 2 % of the total power of the lamp 2

2 Exposure Maskaligner MA6 [SÜSS] No. 2: Hg-lamp No. 3-12: mirrors, lenses No. 13: mask No. 14: wafer 3 Exposure and Alignment Hint for first level: Align to Flat! Top Side Alignment Back Side Alignment [SÜSS] 4

3 Resolution of Shadow Printing Minimum printable linewidth b min *: b min = k λ d prox + t Re sist 2 Contact and proximity printing λ exposure wavelength d gap between mask and photoresist t photoresist thickness k variable factor, depending on process Example: λ = 436nm, t Resist = 2µm; k=1 Proximity: gapd prox = 20µm; b min 3 µm Contact: gapd prox = 0 b min 0,7µm [SZE03] * assumed: reproduction of a grating with equal width lines and spaces 5 Resolution of Shadow Printing Reduce λ b min = k λ d prox Reduce d prox tre + 2 sist Reduce t Resist Reduce k Ways to improve resolution Achievable resolution (dependent on process conditions) [Süss] 6

4 Resolution of Projection Printing Numerical aperture NA NA = n sin θ = D / 2f f: focal length D: lens diameter n: refractive index for the medium Theoretical resolution W of a lens[1] W = k 1 λ/na λ: exposure wavelength k: empirically determined, process dependent varies from 0.3 to 1.1 k = 0.75 for single layer resists k = 1.1 for resists over reflective surfaces like Aluminium [More88] Depth of focus DOF DOF = k 2 λ/2(na) 2 [1] Rayleigh limit for two opaque objects 7 Resolution of Projection Printing Reduce λ W = k λ/na IncreaseNA NA = n sin θ = D / 2f Reduce k Increase refractive index for the medium* Ways to improve resolution Numerical Aperture (NA) Theoretical resolution (W) for k = 0.75 Depth of focus (DOF) λ = 400 nm λ = 200 nm λ = 400 nm λ = 200 nm μm 0.8 μm 5.0 μm 2.5 μm Achievable resolution (dependent on process conditions) μm 0.5 μm 2.0 μm 1.0 μm μm 0.4 μm 1.5 μm 0.8 μm μm 0.3 μm 0.8 μm 0.4 μm *NA 0,85 immersion lithography 8

5 Photomask Material transparency Absorption layer Structure resolutions sodalime, quarz-glas; polymer chromium; ink laser, e-beam; photoplotter Mask Polarity Shadow mask 5 x 5 inches square, sodalime/chromium Quarz-glas low coefficient of thermal expansion transparent in the deep UV [Wolf86] 9 Substrates Silicon Wafer Material Format Flatness Roughness Adhesion 300 mm 10

6 Resist Systems Important Resist Types for Photolithography DNQ-Novolak-based Epoxy-based Polyimide-based Positive tone Negative tone Image Reversal Negative tone Negative tone Positive tone Some trade names: ma-p, AR-P, AZ Some trade names: ma-n, AR-N, TI Some trade names: AZ..E, TI..XR Some trade names: SU-8, GM Some trade names: Durimid Some trade names: HD Resist Systems DNQ-Novolak-based positive tone Epoxy-based negative tone Polyimide-based negative tone resin PAC* solvent Novolak DNQ-sulfonates Organic solvent (PGMEA) Epoxy resin Organic salt Organic solvent (e.g. γ-bytyrolactone and propylene carbonate) Photoactive precursor (polyamic acids/esters) organic solvents main effect of exposure change of molecular structure of DNQ and post exposure bake: crosslinking of epoxy resin imidisation crosslinked intermediate developer aqueous alkaline solvent organic solvent (e.g. PGMEA = safer solvent) organic or aqueous solvent (denpending on resist) hardbake optional optional (= curing) completing imidisation polyimide stripping concentrated developer; acetone PG remover, RIE plasma ashing, laser ablation,... plasma ashing... * PAC = photoactive compound 12

7 Resist Systems Some Applications DNQ-Novolak-based Epoxy-based Polyimide-based Masking for processes Wet etching Dry etching Electroplating Lift off process Micromechanical devices Masking for processes Electroplating (UV-)LIGA Lift off process Stress buffer Passivation layer Dielectric interlayer (multichip module) Coatings for optic fiber and quartz capillary coatings Masking for processes Dry etch mask for silicon nitride passivation layer 13 DNQ-Novolak-based resists Resist Chemistry Resist: Interaction beween novolak and DNQ: hydrogen bonds Resin: phenol novolak PAC: DNQ-Sulfonate DNQ photolysis: Dissolution rate scheme: fhgfhfghfhfhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh hhhhhklklllllllllllllllllllllllllllllllllllllllllllllllllllll [Damm93] 14

8 Mesomerie der Carboxylgruppe: als Tafelbild 15 DNQ-Novolak-based resists fhgfhfghfhfhhhhhhhhhhhhhhhhhhhhhh hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh hhhhhhhhhhhhhhhhhhhhhhhhklklllllllllll lllllllllllllllllllllllllllllllllllllllll Dissolution rates of unexposed and fully exposed mixtures of DNQ and novolak Absorption spectra of DNQ-based photosensitizers before and after bleaching [Damm93] 16

9 DNQ-Novolak-based resists [micro04] Clariant AZ and MicroChemicals TI resists 17 DNQ-Novolak-based resists: positive tone Two layer process [Roth99] 18

10 Erklärung der Ursachen eines typischen Resistprofils für DNQ-Novolak-Resiste: als Tafelbild 19 DNQ-Novolak-based resists: image reversal Goal of image reversal: enhance the process latitude Most important principles: indirect (base-catalysed) direct (acid-catalysed) HO-A-OH: bifunctional, acid-captivated crosslinker Dissolution rate scheme Principle of direct image reversal 20

11 Unterschied belichtete und unbelichtete Bereiche im säurekatalysierten Umkehrresist: Belichter Resist Unbelichter Resist Vernetzende Substanz Temperatur Katalysator (Säure) 21 Epoxy-based resist Resist Chemistry photosensitiver epoxy resin Photochemical reaction: crosslinking Frage: Säure HA ist ein kleines Molekül. Was bedeutet das für die Vernetzung? [Lee95, Micro01] 22

12 Epoxy-based resist Inclined/rotated exposure of SU-8 [Manh03] 23 Polyimide-based resists Chemical Principle and Processing Scheme (Starting from polyamic acid methacrylate ester) Nachteil? [Ahne95] 24