Lithography Layout, Mask, photo

Index Quick Review Layout Basics Mask Basics Lithography Key Steps Details Optics Production 1-Dec-18

Layout, Mask & Photo Layout Mask Photo (Lithography) Convert electrical design to physical design 2D ‘picture’ of the final chip (quantitative) Mask Creates a physical layout (similar to photo negative) Can be used to general many chips (similar to obtaining many copies from a negative) Photo (Lithography) Process of obtaining a ‘picture’ from a ‘negative’ (ie generating shapes on wafer from a mask) 1-Dec-18

Chip manufacturing: Snap shot Electrical Chip Design R C Physical “Layout” Design Blue Print- Photo “negative” Creating the chip Review Testing “Print” Quality Control 1-Dec-18

Layout: Basics Electrical circuit Physical circuit N P+ Oxide Diode Resistor Electrical circuit Physical circuit N P+ Oxide Substrate (p silicon) Oxide N P+ Substrate (p silicon) Oxide 1-Dec-18

Layout: Basics Layout Example Each layer is made one at a time. Hence each layer has a ‘photo negative’ or mask Therefore, each layer must be specified separately in the ‘layout’ file. It is a computer file (softcopy) M1 layout Example A sample part of a chip Layout provides the top view (2D) The ‘depth’ is controlled by the deposition process (and not by the layout) 1-Dec-18

Layout: Basics Z1 Z2 Layout will provide information on x and y Substrate (p silicon) Oxide N P+ Y1 Y3 Y2 Y4 X1 X2 X3 X4 Z1 Z2 Layout will provide information on x and y Process flow sheet will provide information on z 1-Dec-18

Layout: Basics Example A sample part of a chip M1 Via 12 M2 The file has different layers (numbered from 1 to...) A table which correlates the layers to numbers is also provided 1-Dec-18

Layout: Basics Via and contacts are shown as squares (or in rare cases rectangles) in the layout. Others are in rectangular geometry Cheater rules for SRAM However, the mask will have circular (or in rare cases elliptical) holes Completely filling rectangular features with high aspect ratio (with metal) is difficult Aspect ratio: Depth/width (or diameter) width Low aspect ratio High aspect ratio Dia height 1-Dec-18

Layout: Basics Hierarchy vs Flat file Levels, Layers GDS (stream format), GDSII, MEBES, CIF Review Softcopy (quantitative) of what is ideally required Top view (2D) only Hierarchy 1-Dec-18

Mask: Basics The layout file is given as ‘input’ the mask making machine E-beam writer (electron beam) Scans the “blank” mask Mask may be a plate of glass, coated with chromium. On top of chromium, a photosensitive coating is applied A mask is often referred to as ‘chrome’ E-Beam, ‘sensitizes’ the areas Develop the resist Remove chromium on the ‘exposed’ areas 1-Dec-18

Mask: Basics Relatively slow process Mask has to be very very clean. beam goes to each ‘pixel’ which has to be written ‘on’ switches “on” and then “off” moves to the next pixel which has to be written on Mask has to be very very clean. Any particle on the mask will cause incorrect ‘feature’ in all the chips ==> chips will to fail A mask is then coated with a clear film, to protect it. Eg. If a particle falls on the mask, then you can remove the clear film and put a new film. (Easy to ‘repair) 1-Dec-18

Mask: Basics A mask may be 100 mm x 100 mm (for example) The size is decided by the size of the lens in the lithographic tool A mask is usually 4 to 5 times larger than the actual feature. The features are ‘reduced/ zoomed out’ during the lithographic process Correspondingly, a mask is referred to as ‘4x mask’ or ‘5x mask’ Example: If you want a 100 nm wide line on the wafer layout file will show 100 nm wide line on the screen mask making machine will enlarge everything by 4 Actually, layout file will be converted to another file which will enlarge it by 4 and then the mask making machine will use it. The process is sometimes called MDP (Mask Data Preparation) Mask will have 400 nm wide opening and Lens in the litho process will ‘shrink’ the image to 100 nm. 1-Dec-18

Mask: Basics A mask may be 100 mm x 100 mm (for example) So, one ‘print’ will be about 25 mm by 25 mm, on the wafer. A chip may be only 5 mm by 4 mm So, one mask will have perhaps 20 chips, if the chip is small The gaps between the ‘real chips’ are used for alignment marks (TBD) test structures (TBD) 1-Dec-18

Mask: Basics If a mask gets dirty in a particular location, with in a ‘field’, a die will always fail may also happen due to lens issues Pizza masks, to verify design changes 1-Dec-18

Mask: Basics A mask may be dark-field or light-field Depends on the photo resist that will be used in the process If photo resist dissolves in the exposed regions, it is positive photo resist; else negative +ve resist; dark field mask -ve resist; light field mask Desired structure Mask Review 1-Dec-18

Lithography: Overview Shape Definition Key steps Details of steps Optics: Relevant information Production: Relevant information Review of Lithography 1-Dec-18

Lithography: Basic steps Coat the wafer with light sensitive material (photo-resist) Place the mask over the wafer and align with the wafer. Adjust the focus of the camera Expose the wafer to light (for a controlled time period) Exposed photoresist will have changed chemically Wash the wafer+photoresist in a solvent (Developing the resist) Removes only exposed photo resist Etch the ‘open’ areas Remove all the remaining resist Animation 1-Dec-18

Lithography: Schematic Note: Some schematics will show mask between lens and wafer. In reality, at least one lens will come between the mask and the wafer

Lithography: Basic Steps Coat the wafer with light sensitive material (photo-resist) Mask over the wafer, align, focus Exposure for a precise amount of time Move (Step) to the next ‘Field’ Stepper Actually, the wafer moves, not the litho system Repeat the above procedure 1-Dec-18

Lithography: Basic Steps After the whole wafer is exposed.... Repeat for other wafers in the lot Crude schematic below Develop the lot Etch the open areas Remove the remaining photo resist and the etch products Note: Etch product forms a thin film or ‘veil’. Removal of the veil is called ‘de-veil’ 1-Dec-18

Lithography: Details Photo Resist Coating Mostly positive resists used now More resistance to pinhole formation Easier removal (stripping) More expensive Harder to develop (solubility differential is not high) -ve resist; most area open +ve resist; most area blocked 1-Dec-18

Lithography: Details Photo Resist Coating Prepare the wafer Clean the wafers (to remove particles) Dry (150-200 C bake) Primer (HMDS- Hexa Methyl Di Silazane) Immersion (liquid) Spin coat (liquid) and high speed drying Vapor prime Photo resist coating (spin coating) 1 um thick (± 1% variation) 1-Dec-18

Lithography: Details Photo Resist Coating Dynamic coating Low rpm (500) dispense High rpm (5000) thinning (form uniform film) Final resist thickness depends on rpm and time and viscosity, surface tension, drying characteristic of solvent... Moving arm to enhance uniformity 1-Dec-18

Photo Resist properties Lithography: Details Photo Resist properties Contains photosensitive materials, sensitivity enhancers (called sensitizers), solvent Resolution Contrast Response to a given wavelength (spectral response) Solubility differential between unexposed vs exposed regions Etch/ Implant resistance 1-Dec-18

Lithography: Details Pre Expose Bake Soft Bake: To drive out the solvent Resist is still ‘soft’, but not liquid like Temperature: Baking options: Hot Plate (manual, moving) Convection Oven (may form crusts) Infra-red (wafer surface heated) Microwave (volume heating) Vacuum baking (radiation) 1-Dec-18

Wafer (with resist coating) Lithography: Details Exposure Litho Tool (current): Projection aligners Stepper vs Scanner Historical: Contact, proximity aligners Proximity: Mask almost in contact with wafer Mask may become dirty over time, but not as bad as contact 1x mask, minor optical distortions Contact: Mask in contact with wafer Mask may become dirty over time 1x mask, no optical issues Wafer (with resist coating) MASK 1-Dec-18

Lithography: Details Exposure Litho Tool (current): Projection aligners Stepper vs Scanner Stepper: What we saw before Scanner: Actually Step & Scan Need smaller lens for the same sized mask Or with same sized lens, can handle larger mask Stepper Step & Scan Lens Lens MASK 1-Dec-18

Schematic 1-Dec-18

Lithography: Details Exposure Light Sources Hg Lamp: Filter G,H, or I-line (365 nm) Excimer Lasers (DUV) ArF193 nm , RF 248 nm etc EUV, X-Ray, E Beam (in R&D) EUV: Reflective optics G I H 400 nm Intensity 600 nm 1-Dec-18

Lithography: Details Develop Post Exposure Bake Similar to pre-exposure bake Help minimize standing wave effects Developing Immersion Spray develop Plasma (dry) develop Rinse Hard bake: 200 C, 30 mins (for example) To protect from etch/implantation Resist flow Etch 1-Dec-18