4 Introduction (Processes) OxidationDiffusionIon ImplantationDepositionEtchingLithographyDepositionRemovalPatterningModification of electrical propertiesIntroduction (Processes)
5 LithographyLithography is the process of transferring patterns drawn on a mask to a thin layer of radiation sensitive material (resist) covering the surface of the material to be etched (oxide or metal).The main equipment is optical ultraviolet unit (UV wavelength 200 to 400 nm).In this case the mask is called photomask (PM), the resist is called photoresist (PR) and the process is called photolithography.
6 LithographyPatterning process consists of mask design, mask fabrication and wafer printing.
7 LithographyIn case of X-ray (wavelength 0.2 to 5 nm) the resist is called X-ray resist and the process is called X-ray lithography.In electron-beam (EB) lithography (wavelength ≈ 0.1 nm) using EB resist.The shorter the radiation wavelength the smaller the feature size that can be produced.Since the EB radiates very small spots, no mask is needed and direct exposure of the EB resist is carried out (serial exposure of the resist or direct writing on the resist).
8 LithographyWhen masks are used (photo- and X-ray- lithography), we irradiate the whole resist surface using parallel exposure.EB lithography is used when the product volume is small because masks are very expensive to fabricate.Masks are themselves fabricated by EB lithography for shaping chromium metallic layer deposited on quartz transparent supports.Masks are only used to reproduce very large number of wafers exceeding
9 As processing equipment and technologies become more advanced, transistor size shrinks In the 70’s, 8Now, 0.02This is due to advancement in lithography.Each layer of dopant or material has its own layout (15 – 20 mask).Lithography
10 PhotolithographyPhotolithography is the heart of integrated circuit processing. It is the method used to transfer the individual circuit design knowledge onto the silicon wafer.Probably one half of the wafer fabrication costs go into obtaining proper photolithography.The basic elements consist of:An Align and Expose ToolMasks containing design informationPhoto-sensitive Resist
11 Basics of Photolithography Silicon Wafer with Silicon DioxideSiO2To be etchedSilicon Wafer
12 Basics of Photolithography Apply photo resistPhotoresistSiO2Silicon Wafer
13 Basics of Photolithography Apply photomaskPhotomaskSiO2Silicon Wafer
14 Basics of Photolithography Apply Ultraviolet LightSilicon WaferSiO2PhotomaskDecreasing feature size require the use of shorter λ
15 Basics of Photolithography Develop photoresistSilicon WaferSiO2
16 Basics of Photolithography Etch Silicon DioxideSilicon WaferSiO2
17 Basics of Photolithography Strip PhotoresistSilicon WaferSiO2
18 Photoresist ProcessThis flow chart shows the typical process used for VLSI Lithography.
19 Photoresist Process 1. Substrate Cleaning and Preparation The surface is as clean as possible immediately after a high temp operation.Acid cleaning may be necessary if wafers sit for a long time.
20 Photoresist Process 2. Wafer Coating The goal of coating is to apply a uniform, defect free film of photoresist over The entire wafer. Spin Coating is used for VLSIaccelerate the wafer - fastspin at constant speed to get uniform thicknessBefore spin After spinPR Wafer PRChuck
21 Photoresist Process 3. Pre-Bake (Soft-Bake) The Soft-Baked is used to: Drive off solvent from resist. (reduced to ~5%)Improve adhesion and anneal stresses in resistTypical process Temp is °COvensConvection oven - Very stable over time. ~ 30 minIR oven - Most popular min.Hot plate - very effective for single wafers.
22 Photoresist Process 4. Expose To produce the desired photochemical effects in the shortest time possible.Time is important because:Wafers are individually processedAlign machines are expensiveHowever, short times usually mean lower resolution.Exposure to UV light chemically changes the resistSilicon WaferSiO2Photomask
23 Photoresist Process 5. Develop Silicon WaferSiO2Development removes the unwanted photoresist.Positive Resist - Area exposed is removedNegative Resist - Area not exposed is removed
24 Photoresist Process 6. Post-Bake 100-120°C 30 minutes Hardens Resist Prior to EtchTemperature is criticalToo high a Temp (>120°C) causes resist to flowExtra high Temp (>180°C) causes resist to lift.
25 Photoresist Process 7. Inspection The object of this inspection is to verify the photoresist is acceptable prior to etch.A complete rework can now be performed if the photoresist is not acceptable.
26 Photoresist Process 8. Etch Silicon WaferSiO2Dry Plasma Etch is commonly used in commercial manufacturing lines.We use Wet Etch with acids because of the cost and flexibility of the process.
27 Photoresist Process 9. Strip It is a complete removal of the Photoresist.Dry Plasma Strip in O2 is commonly used in commercial manufacturing lines.We use Wet Chemical Strip in special solvents because of the cost and flexibility of the process.Silicon WaferSiO2
30 Photomask The pattern to be etched on the wafer surface is drawn on the pohtomaskPhotomasks are made from chromiumMany masks are needed in recent CMOS technologies. The # of masks depends on the process complexity
31 Clean RoomsPhotolithography must be carried out in a clean room otherwise dust particles causing fabrication errors.The total number of dust particle are controlled with temp and humidity.Clean rooms standards:Class 100 < 100 pp ft3 (0.5µ)Class 10 < 10 pp ft3 (0.5µ)Class 1 < 1 pp ft3 (0.5µ)
32 ThanksMany thanks to Prof. Hany Fikry and Prof Wael Fikry for their useful materials that help me to prepare this presentation.