3 DefinitionPhoto-imaging method by which geometric patterns are transferred from a mask to the substrate (wafer).Uses photosensitive polymer (called “photoresist”).Features transferred to substrate surface by shining light through glass plates (called “masks”).
7 The NeedElectronics fabrication requires a clean processing environment for lithography.Goal: minimize dust particles that can settle on substrates or masks and cause DEFECTS.Dust on a mask looks like an opaque feature; will get transferred to underlying layers; can lead to short circuits or open circuits.
8 Graphic IllustrationParticle 1 may result in formation of a pinhole in underlying layer.Particle 2 may cause a constriction of current flow in a metal runner.Particle 3 can lead to a short between the two conducting regions.
9 ClassNumerical designation taken from maximum allowable number of particles 0.5 mm and larger per ft3 (English system).For IC fabrication, a class 100 clean room is required (about four orders of magnitude lower than ordinary room air).For photolithography, class 10 or better is required.
11 Sample ProblemA 300 x 300 mm square substrate is exposed for 1 minute under laminar flow at 30 m/min. How many dust particles will land on this substrate in a Class 1000 clean room?SOLUTION:Class 1000 => 35,000 particles/m3 (from graph)Air flow volume over wafer/min = 30 m/min (0.3m x 0.3m) = 2.7 m3# of particles = 35,000 x 2.7 = 94,500!!!If each of these causes a defect, we are in serious trouble!
13 Performance MetricsResolution: minimum feature dimension that can be transferred with high fidelity to a resist film.Registration: how accurately patterns on successive masks can be aligned (or overlaid) with respect to previously defined patterns.Throughput: number of wafers that can be exposed/unit time for a given mask level.
14 Shadow PrintingMask and wafer in direct contact (contact printing); orMask and wafer in close proximity (proximity printing).
15 Contact Printing*Contact between the resist and mask provides a resolution of ~1 mm.Drawback: dust particles on the wafer can be imbedded into mask where mask makes contact with the wafer.Imbedded particles cause permanent damage to mask and result in defects with each succeeding exposure.* We use this in lab.
16 when l = wavelength and g = gap Proximity PrintingSmall gap (10 – 50 mm) between the wafer and the mask.Minimizes mask damage, but …Gap results in optical diffraction at feature edges that degrades resolution to 2–5 mm.Minimum linewidth (or critical dimension):when l = wavelength and g = gap
17 Projection Printing Wafer many centimeters from mask To increase resolution, only small portion of the mask is exposed at a time.Small image area is scanned or stepped over the wafer to cover the entire wafer surface.After exposure of one site, wafer is moved to next site and the process is repeated.Called step-and-repeat projection, with a demagnification ratio M:1
18 Step and Repeat Projection After exposuring one site, wafer moved to next site and the process repeats.Demagnification ratio M:1
19 Resolution Given by: where k1 is a process dependent factor and NA = numerical aperture, which iswhere is the index of refraction
20 where k2 is another process-dependent factor Depth of FocusExpressed as:where k2 is another process-dependent factor
22 Making MasksCAD system used to describe the circuit patterns electrically.Digital data produced by CAD system drives a pattern generator that transfers the patterns directly to electron-sensitized mask.Mask consists of a fused silica substrate covered with chrominum.Circuit pattern is first transferred to the electron-sensitized layer (electron resist), which is transferred into the underlying chrominum layer for the finished mask.
23 Use of Masks Patterns on a mask represent one level of an IC design. Composite layout is broken into mask levels that correspond to the manufacturing process sequence.15 – 20 different mask levels are typically required for a complete IC process.
24 Mask Composition Fused silica plate 15 15 cm, 0.6 cm thick Accommodates lens field sizes for 4:1 or 5:1 optical exposure tools
26 DefinitionPhotosensitive polymer compound that either gets more or less soluble when exposed to light.Photolithography labs have yellow light because photoresist is sensitive to wavelenghts > 500 nm.
27 Types Positive: gets more soluble after exposure Negative: gets less soluble after exposure.
28 DevelopmentMore exposure energy vs. Higher resolution
29 Contrast Ratiowhere: ET = sensitivity or “threshold” energy (where resist becomes completely soluble)E1 = energy to reach 100% resist thickness (50% for negative resist)Larger g => higher solubility of resist and sharper imagesET and E1 interchanged for negative resists
33 AlignmentMask for each layer must be aligned to previous layer patternsFor a minimum feature size ~ 1 mm => alignment tolerance should be +/- 0.2 mmTo align, wafer is held on vacuum chuck and moved around using an xyz stageAlignment marks: special patterns on mask used to facilitate accurate alignment.
35 Limitations of Optical Lithography Resolution becoming a challenge for deep-submicron IC process requirementsComplexity of mask production and mask inspectionHigh cost of masks
36 Electron Beam Lithography Involves direct exposure of the resist by a focused electron beam without a maskCurrently used to primarily produce photomasksResolution as low as 10 – 25 nm
37 Schematic Electron gun generates beam of electrons Condenser lenses focus the e-beamBeam-blanking plates turn beam on and off
38 Advantages Generation of submicron resist geometries Highly automated and precisely controlled operationGreater depth of focus than that available from optical lithographyDirect patterning on wafer without using a mask
39 ScanningRaster: beam scans sequentially over every possible location on the mask and turned off where no exposure is requiredVector: beam directed only to requested features, jumps from feature to feature
40 Disadvantages Low throughput Expensive resists Proximity effect: backscattering of electrons irradiates adjacent regions and limits minimum spacing between features