ECE 6450 - Dr. Alan DoolittleGeorgia TechLecture 7Lithography and Pattern TransferReading: Chapter 7ECE 6450 - Dr. Alan DoolittleGeorgia TechUsed for Pattern transfer into oxides, metals, semiconductors.3 types of Photoresists (PR):1.) Positive: PR pattern is same as mask. On exposure to light, light degrades the polymers (described in more detail later) resulting in the photoresist being more soluble in developers. The PR can be removed in inexpensive solvents such as acetone.2.) Negative: PR pattern is the inverse of the mask. On exposure to light, light polymerizes the rubbers in the photoresist to strengthen it’s resistance to dissolution in the developer. The resist has to be removed in special stripping chemicals. These resists tend to be extremely moisture sensitive.3.) Combination: Same photoresist can be used for both negative and positive pattern transfer. Can be removed in inexpensive solvents.Lithography and PhotoresistsMask PatternPattern transferred to the Photoresist on the waferPositive PRLightNegative PRLightECE 6450 - Dr. Alan DoolittleGeorgia TechPhotoresists are used in a process typical of this process: Dehydration Bake, Apply Adhesion Promoter, Apply Resist, Soft bake, Exposure with Mask, Post Exposure Bake, Develop, Optional Processing. For example:1.) Dehydration in an oven at ~120 degrees C for as long as 30 minutes2.) Spin coat (verbally explain) adhesion promoter such as hexamethyldisilane (HMDS)3.) Spin coat resist4.) Soft bake to partially solidify PR (85-95 degrees C for 1 to 30 minutes depending on the resist)5.) Expose to few hundred mJoules/cm2of high energy light6.) (Optional) Hard bake, removes more solvent (~110-150 C)7.) Develop: weak regions of PR dissolved8.) Additional Hard bake or chemical treatment to harden PR for aggressive processes such as Ion implantation or Plasma etchingLithography and PhotoresistsMore details at the Gt microelectronics teaching lab web page: http://www.ece.gatech.edu/research/labs/vc/ECE 6450 - Dr. Alan DoolittleGeorgia Tech1.) Etching Processes: open windows in oxides for diffusion, masks for ion implantation, etching, metal contact to the semiconductor, or interconnect.2.) Lift off Processes: Metalization (more common in III-V).Uses of Lithography:WaferMetal, Oxide, etc…PhotoresistWaferMetal, Oxide, etc…Wafer WaferSpin PR LithographyEtch Layer using PR as Mask Remove PRWaferPhotoresistWafer WaferMetalWaferMetalSpin PR LithographyEvaporate MetalLift Off excess metal with PRECE 6450 - Dr. Alan DoolittleGeorgia Tech1.) Resolution: How small of features can you make. (Current production state of the art is ~0.065 um)2.) Registration: Can you repeatability align one layer to another. (~1/3 of resolution or 0.06 um)3.) Throughput: Can these be done in a cost effective time. (50-100 wafers an hour, down to 1 chip per hour).At this point, CMOS example will be given in class using supplemental lecture 7b:Issues with PhotolithographyECE 6450 - Dr. Alan DoolittleGeorgia Tech1.) Contact: Resist is in contact with the mask: 1:1 magnificationAdvantages: Inexpensive equipment ($~50,000-150,000), moderately high resolution (~0.5 um or better but limited by resist thickness- 0.1 um demonstrated)Disadvantages: Contact with the mask degrades the mask (pinholes and scratches are created on the metal-oxide layers of the mask, particles or dirt are directly imaged in the wafer, Wafer bowing or local loss of planarizationresults in non-uniform resolution due to mask-wafer gap variations., and no magnification2.) Proximity: Resist is almost, but not in contact with the mask: 1:1 magnificationAdvantages: Inexpensive equipment, low resolution (~1-2 um or slightly better)Disadvantages: Diffraction effects limit accuracy of pattern transfer. Less repeatable than contact methods, no magnification3.) Projection: Mask image is projected a distance from the mask and de-magnified to a smaller image: 1:4 -1:10 magnificationAdvantages: Can be very high resolution (~0.065 um or slightly better), No mask contact results in almost no mask wear (high production compatible), mask defects or particles on mask are reduced in size on the wafer.Disadvantages: Extremely expensive and complicated equipment, diffraction effects limit accuracy of pattern transfer. Photolithography SystemsECE 6450 - Dr. Alan DoolittleGeorgia Tech1.) Resolution:Resolution is “diffraction limited”. As patterns approach the same order of magnitude as the wavelength of light, one must be concerned with the wavelike nature of light.222rgW +>>λIssues with PhotolithographySquare Mask in the Near Field (Mask close to Wafer)The mask can be placed in close proximity or directly in contactwith the wafer (contact or proximity printing). We define this case, known as the near field or Fresnel diffraction limit, by the expression:DgWW =∆rgWDWW+∆WDefinitions used for Resolution EquationsECE 6450 - Dr. Alan DoolittleGeorgia Tech(Contd…) Square Mask in the Near Field (Mask close to Wafer)Effect of increasing mask-wafer gap spacingAssuming:λλ2Wg <<Then the minimum feature size that can be resolved is:gkWλ≈minwhere k is a constant, normally close to 1, that depends on the photoresist and the development proceduresExample: For a k=1, and λ=0.365 (I-line)1 um0.6 um5 um1.35 um10 um1.9 um20 um2.7 umg (gap)WminECE 6450 - Dr. Alan DoolittleGeorgia TechSquare Mask in the Far Field (Mask far away from the wafer) The mask can be placed far away from the wafer (projection printing used in stepping and scanning systems). We define this case, known as the far field or Fraunhofer diffraction limit, by the expression:dThe intensity on the wafer (arealimage) is then as shown.Resolution, R, (Diameter of the central maximum) is:dfRλ22.1=d = Diameter of focusing opticsf = Focal length of focusing opticsFrom geometry, d= 2[n(f sin(α)] where n is the index of refraction (normally 1 for air), and αis the angle to the edge of the focusing optics222rgW +<<λf αECE 6450 - Dr. Alan DoolittleGeorgia Tech(cont’d…) Square Mask in the Far Field (Mask far away from the wafer) NAkWλ≈min()()()NAnfnfdfRλαλαλλ61.0sin61.0sin222.122.1====Where NA is the numerical aperture of the focusing optics. The Numerical Aperture describes the focusing strength of the projection system:However, all our derivation is based on a “point source” which is not ever possible, thus, we can generalize using a constant k (normally ~0.75) the result as:Briefly