520/ Photolithography (II) Andreas G. Andreou

Transcription

1 520/ Photolithography (II) Andreas G. Andreou Lecture notes from Positive Photoresists and Photolithography by R. Darling A.G. Andreou

2 Lecture Summary Positive Photoresists Photoresist Spinning Mask Aligning and Exposure A.G. Andreou

3 Advantages of Positive Photoresists Most commonly used in the IC industry. Superior to negative photoresists because: They do not swell during development. They are capable of finer resolution. They are reasonably resistant to plasma processing operations.

4 Phenolic Resins - 1 Phenolic resins are condensation polymers of aromatic alcohols and formaldehyde. Bakelite was the first thermosetting plastic. Phenolic resins are readily cross-linked by thermal activation into rigid forms. Most phenolic resins are readily dissolved by aqueous alkaline solutions, e.g. NaOH, KOH, NH 4 OH.

5 Phenolic Resins - 2 OH phenol + O C H H formaldehyde OH H 2 OH H 2 OH C C H 2 C bakelite + O H H water

6 Phenolic Resins - 3 OH CH 3 para-cresol + O H C H formaldehyde OH H 2 OH H 2 OH C C H 2 C CH 3 CH 3 CH 3 novolac + H O H water

7 Important Properties of the Base Phenolic Resin average molecular weight typically in the range of 1000 to 3000 g/mole (8 to 25 repeating units in the polymer chain) dispersity of the molecular weights isomeric composition of the cresols ortho-cresol meta-cresol para-cresol relative position of the methylene linkages (--CH 2 --)

8 OH CH 3 Cresol Isomers OH OH ortho-cresol meta-cresol CH 3 CH 3 para-cresol OH CH 3 OH H 2 C OH H 2 C CH 3 H 2 C CH 3 ortho-cresol resin meta-cresol resin para-cresol resin

9 Cresol Isomer Properties Single isomers and smaller molecular weights are desirable Manufacture of positive photoresist relies heavily upon obtaining only a single isomer of the resin, usually paracresol. Each monomer is [C 8 H 8 O] ( g/mole) Isomer Methylene Link Molecular Weight Dissolution Rate Plastic Flow Temp. ortho-cresol g/mole 2.7 A/sec 85 C meta-cresol g/mole 0.7 A/sec 73 C para-cresol g/mole 3.0 A/sec 119 C

10 Photoreaction in a Positive Photoresist O N 2 + H 2 O hν diazonaphthaquinone (DQ) COOH indene carboxylic acid (ICA) + N 2

11 Dissolution of Phenolic Resins - 1 Because of the OH groups, phenolic resins are hydrophylic and are readily dissolved by aqueous alkaline solutions. Diazonaphthaquinone (DQ) is a hydrophobic and nonionizable compound. When phenolic resins are impregnated with DQ, they become hydrophobic and their dissolution is greatly inhibited. After exposure, DQ is converted into indene carboxylic acid (ICA) which is hydrophylic and very ionizable. This allows the developer to wet and penetrate the novolac resin. Phenolic resins which contain ICA instead of DQ are readily dissolved by aqueous alkaline developers.

12 Dissolution of Phenolic Resins Dissolution rate, nm/sec in 0.15 M NaOH exposed novolac + DQ unexposed novolac + DQ DQ concentration, weight percent

13 DQ Primary Photoreaction O photolysis O N 2 hν + N 2 diazonaphthaquinone (DQ) a carbene ring contraction O C hydration + H 2O COOH a ketene indene carboxylic acid (ICA) See Otto Suess, 1944 and 1947 papers in Annalen.

14 Physical Requirements on the Photoactive Component Need an overlap of the absorption spectrum with the emission spectrum of the exposure source, e.g. a Hg lamp. Need bleachability at the exposure wavelength so that the photoreaction is able to reach the resist-substrate interface. Need compatibility with the base resin (novolac) so that the two form a single, miscible phase. Need thermal stability so that the photoactive dissolution inhibitor does not break down at prebake temperatures. Photoactive dissolution inhibitors are often modified to alter their spectral absorption, thermal stability, and miscibility characteristics.

15 Spectral Absorption of Novolac, DQ, and ICA 10,000 novolac 365 Hg arc lamp lines DQ (unexposed photoinhibitor) 100 ICA (exposed photoinhibitor) Wavelength, λ, nm

16 Primary Components of a Positive Photoresist Non-photosensitive base phenolic resin usually novolac Photosensitive dissolution inhibitor usually a DQ-derived compound Coating solvent n-butyl acetate xylene 2-ethoxyethyl acetate very carcinogenic, TLV = 5 ppm now removed from most positive photoresists

17 Secondary Components of a Positive Photoresist Antioxidants Radical scavengers Amines to absorb O 2 and ketenes Wetting agents Dyes to alter the spectral absorption characteristics Adhesion promoters Coating aids

18 Sensitometric Curve for a Positive Photoresist Developed Resist Thickness normalized to working point for the resist ,000 Exposure Dose, D, mj/cm 2

19 Novolac Dissolution - 1 A minimum concentration of [OH - ] is required to produce a net forward rate: CH 2 H 3 C OH + OH - CH 2 H 3 C O - + H 2 O The dissolution rate is R = kc n, where C is the base concentration. For NaOH solutions, R = (1.3 x 10 5 ) [Na + ] 1 [OH - ] 3.7 Angstroms/second.

20 Novolac Dissolution - 2 Typical data for different developer solutions: Solution Dissolution Rate, Angstroms/second Unexposed Exposed 0.15 M NaOH M KOH M NaOH M Na2SiO M NaOH M Na3PO M NaOH M Na2CO

21 Positive Photoresist Exposure Latitude Kodak Micro Positive 820 Critical Dimension Shift, µm C prebake in convection oven C develop with agitation, 1:2 mix lines and islands spaces and windows Exposure Latitude working point: 185 mj/cm 2 BLOAT SHRINK Exposure Dose, D, mj/cm 2

22 Positive Photoresist Prebake Latitude Kodak Micro Positive 820 Critical Dimension Shift, µm 185 mj/cm 2 exposure C develop with agitation, 1:2 mix spaces and windows lines and islands Prebake Latitude working point: 95 C BLOAT SHRINK Prebake Temperature, degrees C

23 Photoresist Spin Coating Wafer is held on a spinner chuck by vacuum and resist is coated to uniform thickness by spin coating. Typically rpm for seconds. Resist thickness is set by: primarily resist viscosity secondarily spinner rotational speed Resist thickness is given by t = kp 2 /w 1/2, where k = spinner constant, typically p = resist solids content in percent w = spinner rotational speed in rpm/1000 Most resist thicknesses are 1-2 µm for commercial Si processes.

24 Photoresist Spin Coating resist dispenser excess resist flies off during rotation photoresist wafer to be coated vacuum chuck speed level out slow coat spin down time

25 Spinning Artifacts Striations ~ 30 nm variations in resist thickness due to nonuniform drying of solvent during spin coating ~ µm periodicity, radially out from center of wafer Edge Bead residual ridge in resist at edge of wafer can be up to times the nominal thickness of the resist radius on wafer edge greatly reduces the edge bead height non-circular wafers greatly increase the edge bead height edge bead removers are solvents that are spun on after resist coating and which partially dissolve away the edge bead Streaks radial patterns caused by hard particles whose diameter are greater than the resist thickness

26 Prebake (Soft Bake) - 1 Used to evaporate the coating solvent and to densify the resist after spin coating. Typical thermal cycles: C for 20 min. in a convection oven C for 45 sec. on a hot plate Commercially, microwave heating or IR lamps are also used in production lines. Hot plating the resist is usually faster, more controllable, and does not trap solvent like convection oven baking. photoresist coating solvent is driven off wafer hot plate chuck

27 Prebake (Soft Bake) - 2 A narrow time-temperature window is needed to achieve proper linewidth control. The thickness of the resist is usually decreased by 25 % during prebake for both positive and negative resists. Less prebake increases the development rate: dissolution rate, nm/sec temperature, C

28 Convection ovens: Prebake (Soft Bake) - 3 Solvent at surface of resist is evaporated first, which can cause resist to develop impermeable skin, trapping the remaining solvent inside Heating must go slow to avoid solvent burst effects Conduction (hot plate): Need an extremely smooth surface for good thermal contact and heating uniformity Temperature rise starts at bottom of wafer and works upward, more thoroughly evaporating the coating solvent Generally much faster and more suitable for automation

29 Overview of Align/Expose/Develop Steps (x,y,θ) alignment of mask to substrate uniform UV exposure illumination chrome on glass photomask photoresist (PR) latent image created in photoresist after exposure substrate wafer wet chemical development NEGATIVE PHOTORESIST Photoresist is photopolymerized where exposed and rendered insoluble to the developer solution. POSITIVE PHOTORESIST Exposure decomposes a development inhibitor and developer solution only dissolves photoresist in the exposed areas.

30 Photomasks Master patterns which are transferred to wafers Types: photographic emulsion on soda lime glass (cheapest) Fe 2 O 3 on soda lime glass Cr on soda lime glass Cr on quartz glass (most expensive, needed for deep UV litho) Dimensions: 4 x 4 x for 3-inch wafers 5 x 5 x for 4-inch wafers Polarity: light-field = mostly clear, drawn feature = opaque dark-field = mostly opaque, drawn feature = clear

31 Mask to Wafer Alignment degrees of freedom between mask and wafer: (x,y,θ) Use alignment marks on mask and wafer to register patterns prior to exposure. Modern process lines (steppers) use automatic pattern recognition and alignment systems. Usually takes 1-5 seconds to align and expose on a modern stepper. Human operators usually take seconds with well-designed alignment marks. alignment mark on wafer, created from prior processing step alignment mark on mask, open window in chrome through which mark on wafer can be seen

32 Mask to Wafer Alignment - 2 Normally requires at least two alignment mark sets on opposite sides of wafer or stepped region. Use a split-field microscope to make alignment easier: L R L R

33 Visual alignment: Mask to Wafer Alignment - 3 Process of getting wafer coarsely centered under mask All that is needed for the first mask of the set, since no patterns on the wafer exist yet Accomplished by special windows on a dark field mask

34 Oriel Alignment Fixture mask retainer wafer or substrate mask foam rubber mask gasket mask holder vacuum chuck height adjustment ring 1 wafer stage 2 3 main chamber gasket (x,y,θ) adjustment micrometers

35 Postbake (Hard Bake) - 1 Used to stabilize and harden the developed photoresist prior to processing steps that the resist will mask. Main parameter is the plastic flow or glass transition temperature. Postbake removes any remaining traces of the coating solvent or developer. This eliminates the solvent burst effects in vacuum processing. Postbake introduces some stress into the photoresist. Some shrinkage of the photoresist may occur. Longer or hotter postbake makes resist removal much more difficult.

36 Postbake (Hard Bake) - 2 Firm postbake is needed for acid etching, e.g. BOE. Postbake is not needed for processes in which a soft resist is desired, e.g. metal liftoff patterning. Photoresist will undergo plastic flow with sufficient time and/or temperature: Resist reflow can be used for tailoring sidewall angles. 100 C 110 C 120 C 130 C 140 C

37 Photoresist Removal (Stripping) Want to remove the photoresist and any of its residues. Simple solvents are generally sufficient for non-postbaked photoresists: Positive photoresists: acetone trichloroethylene (TCE) phenol-based strippers (Indus-Ri-Chem J-100) Negative photoresists: methyl ethyl ketone (MEK), CH 3 COC 2 H 5 methyl isobutyl ketone (MIBK), CH 3 COC 4 H 9 Plasma etching with O 2 (ashing) is also effective for removing organic polymer debris. Also: Shipley 1165 stripper (contains n-methyl-2-pyrrolidone), which is effective on hard, postbaked resist.