Photolithography. Class: Figure Various ways in which dust particles can interfere with photomask patterns.

Transcription

1 Photolithography Figure Various ways in which dust particles can interfere with photomask patterns. 19/11/2003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XIII 16 Figure Particle-size distribution curve for English (---) and metric ( ) classes of clean rooms. 4 Class: Metric: Number of particles > 0.5 µm per m 3 English: Number of particles > 0.5 µm per ft 3 19/11/2003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XIII 17

2 Resolution: minimum feature dimension to be transferred Regisration: reproducibility Throughput: number of wafers that can be exposed per hour Figure Schematic of optical shadow printing techniques1: (a) contact printing, (b) proximity printing. Resolution due to diffraction 2-5 µm Critical dimension Gap=10-50 µm Best resolution; Dust Critical Dimension : CD = λ gap 19/11/2003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XIII 18 Figure Image partitioning techniques for projection printing: (a) annual-field wafer scan, (b) 1:1 step-and-repeat, (c) M:1 reduction step-andrepeat, and (d) M:1 reduction step-andscan. 19/11/2003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XIII 19

3 Figure Typical high-pressure mercuryarc lamp spectrum. 19/11/2003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XIII 20 Mask CAD electron-beam lithographic system on fused silica substrate covered with a Cr layer Figure An integrated-circuit photomask. 1 19/11/2003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XIII 21

4 Photoresist The photoresist is a radiation sensitive compound. POSITIVE: the exposed regions become more soluble and thus more easily removed in the development process. The images are the same of those on the mask. NEGATIVE: the exposed regions become less soluble, and the patterns formed in the negative resist are the reverse of the mask patterns. 19/11/2003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XIII 22 Positive photoresist The photosensitive compound is insoluble in the developer solution. Negative photoresist Polymers+photosensitive compound After exposure, the photosensitive compound absorbs radiation in the exposed pattern areas, change its chemical structure and becomes soluble in the developer solution. After development, the exposed areas are removed. EXPOSURE DEVELOPMENT After exposure, the photosensitive compound absorbs optical energy initiate the polymer linking reaction.the cross linked polymer has a higher molecular weight and becomes insoluble in the developer solution. After development, the unexposed areas are removed. 19/11/2003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XIII 23

5 Figure Exposure-response curve and cross section of the resist image after development. 1 (a) Positive photoresist; (b) negative photoresist. 19/11/2003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XIII 24 Pattern transfer Photoresists are not sensitive to λ>500 nm. Adhesion layer=hexa-methylene-di-siloxane Spin speed= rpm ; thickness = µm depending on viscosity. Soft bake ( C for 1-2 min to remove the solvent. Light exposure Development by flooding the wafer with the developer solution Acid ambient which etch the exposed insulation layer but does not attack the resist Resist is stripped (solvent or plasma oxidation) Figure Details of the optical lithographic pattern transfer process. 8 19/11/2003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XIII 25

6 Figure Liftoff process for pattern transfer. The film thickness (e.g. Al) must be smaller than that of the resist. Not suitable for ULSI 19/11/2003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XIII 26 Wet chemical etching Figure Basic mechanisms in wet chemical etching. 19/11/2003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XIII 27

7 Silicon and silicon dioxide etching Oxidation Dissolution of the oxide by a chemical reaction Nitric acid (HNO 3 ) oxidizes silicon to form SiO 2 layer: Si + 4HNO + 3 SiO2 + 2H2O 4NO2 Hydrofluoric acid (HF) dissolves the SiO 2 layer: SiO2 + 6HF H2SiF6 + 2H2O Water or acetic acid (CH 3 COOH) are used as diluent. 19/11/2003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XIII 28 KOH in water and isopropyl alcohol. Etch rate for the (111) plane is much slower than the (100) and (100) plane. Figure Orientation-dependent etching. (a) Through window patterns on <100>-oriented silicon; (b) through window patterns on <100>-oriented silicon. 19/11/2003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XIII 29

8 Dry etching = plasma assisted etching The major disadvantage of wet chemical etching for pattern transfer is the undercutting of the layer underneath the mask (the wet chemical etching of the insulating layer is isotropic) resulting in a loss of resolution in the etched pattern. The film thickness should be about 1/3 or less of the resolution required. Figure Comparison of wet chemical etching and dry etching for pattern transfer /11/2003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XIII 30 Figure Basic steps in a dryetching processing /11/2003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XIII 31

9 Figure The relative reflectance of the etching surface of a composite silicide/poly-si layer. The end point of the etch is indicated by the cessation of the reflective oscillation. 19/11/2003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XIII 32 Figure Comparison of ion energy and operating pressure ranges for different types of plasma reactors. 19/11/2003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XIII 33

10 Figure Schematic of a triod reactive ion etch reactor. The ion energy is separately controlled by a bias voltage on the bottom electrode. rf, radio frequency. 19/11/2003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XIII 34 Figure Schematic of an electron cyclotron resonance reactor /11/2003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XIII 35

11 Figure Schematic of a transformer-coupled plasma reactor. 19/11/2003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XIII 36 Figure Cluster reactive ion etch tool for multilayer metal (TiW/AlCu/TiW) interconnect etching. 2 19/11/2003 Ettore Vittone- Fisica dei Semiconduttori - Lectio XIII 37