The Discontinuity

Discontinuity

• With each new Technology node, previously manageable challenges in physical implementation emerge as extremely disruptive discontinuities
• At 180nm, timing closure was a disruptive challenge, which led to new physical synthesis technology
• At 130nm, Signal Integrity (SI) closure was the main discontinuity
• The new generation of challenges started at 65nm, are in full force at 45nm
• The challenges will get worse as ICs venture into more advanced Technology nodes like 22/14nm
• Designers are working at these Technologies to fully understand the new discontinuities
• Special design enhancements are introduced under the title Design-forManufacturability (DFM) and Design-for-Yield (DFY) to overcome these Discontinuities

Discontinuity: Classification

DFM/DFY

• Design for Manufacturability (DFM)/ Design for Yield (DFY)
  • Techniques to ensure the design can manufacture successfully with high yield
  • To ensures survival of the design, during the complex fabrication process
  • Lithography, etch, Chemical Mechanical Polishing (CMP), and mask systematic manufacturing variations surpass random variations as the prime limiters to catastrophic and parametric yield loss
• Yield
  • Percentage of manufactured products that meet all performance and functionality Specifications
  • The number of die that work as a percentage of the total number of die on the silicon wafer
                 Yield = Good Chips/ Total Chips
                Measured Yield = Good Parts + Test Escapes − False Rejects/ All Parts
    
  • Memory fails more than logic, so repairable memory can improve Yield
  • DFY predicts chip yield at two points of the manufacturing flow wafer probe and during final test of the packaged chip and identifies what defects result in yield loss

Yield Classification

Why DFM/DFY ?

• Current Lithographic techniques (193nm Laser) cannot print deep-submicron technology patterns without distortion
• Higher design complexity and shrinking device geometries
• More devices per unit area on a chip (device density)

• Impact of variations, if not addressed in the design, will cause manufacturing issues, such as poor yields, long yield ramp-up times and poor reliability
• The chips may completely miss the market window or may hit the market window but not economically viable
• The chips may still function, but not at the required/expected speed
• The chips appear to be reliable after volume production, but may suffer catastrophic failures in the field earlier than their expected life-cycle

DFM/DFY Solutions

DFM: Recommendations

• The wire distribution spreads wires that are on the same metal layer as well as across different metal layers
• The benefits gained from lower routing density are in improved manufacturing yield, reduced crosstalk noise, crosstalk delay and random particle defects

• Dummy metal fill
• Timing aware metal fill
• Unbalanced metal density across a chip may cause yield loss, so fill the empty spaces in the design with metal wires to meet the metal density rules required by most fabrication processes
• Improved surface planarity helps decrease manufacturing variations that contribute to timing variability

• Hot Spot/ Critical Area is the region at the center of a random defect which will cause circuit failure (yield loss)
• By analyzing the critical areas, defect-limited yield can be estimated based on the probability of the failures of vias and point defects on routing
• The larger the defect size, the larger the Critical Area
• Critical area reduction improves yield
  

• Chemical-Mechanical Polishing (CMP) is a technique for surface smoothing and material removal process to get globally planar wafer surface
• Simultaneous polishing of copper, dielectric and barrier
• Combination of chemical and mechanical interactions
  • The chemical effect by pH regulators, oxidizers or stabilizers
  • The mechanical action by submicron sized abrasive particles contained in the slurry flow between the polishing pad and the wafer surface
• Dishing
  • Difference between the height of the copper in the trench and the height of the dielectric surrounding the copper trench
  • Copper dishing is higher for wider copper line or the spacing
  • It can thin the wire or pad, causing higher-resistance wires or lower reliability bond pads
• Erosion
  • Difference between the dielectric thickness before CMP and after CMP
  • Dielectric erosion is higher for higher density
  • Erosion can result in a sub-planar dip on the wafer surface, causing short-circuits between adjacent wires on next layer
• On-Chip Variation (OCV) from the interconnect thickness variation due to CMP becomes relatively larger and needs to be taken into consideration in the postlayout RC extraction and timing flow
• Solution to CMP is CMP hotspot detection and fixing

• Various degrees of Copper Dishing and Dielectric Erosion occur at different densities and metal line widths
• In advanced nodes minimal material removal with atomically flat and clean surface finish has to be achieved
• CMP is influenced by line width and pattern density
• The dishing and erosion increase slowly as a function of increasing density and go into saturation when the density is more than 0.7
• Oxide erosion and copper dishing can be controlled by area filling and metal slotting
  

• Redundant Vias use two, or more, Vias to connect the upper and lower routing layers together
• Replacing single Vias with redundant (or double) Vias on signal nets improves reliability and reduce yield loss, due to via failures
• Critical Area Analysis (CAA) identifies the requirement of Redundant Vias

• RET are methods used to modify photo-masks to compensate for limitations in the lithographic processes used to manufacture the chips
• Have significantly increased the cost and complexity of sub-micron nanometer photomasks
• The photomask layout is no longer an exact replica of the design layout
• As a result, reliably verifying RET synthesis accuracy, structural integrity, and conformance to mask fabrication rules are crucial for the manufacture of nanometer regime VLSI designs

• Problem: Some DRC clean layouts do not print on silicon
• Solution: Must-have litho hotspot detection and fixing of design

• Well Proximity Effect (WPE)
• Poly Spacing Effect (PSE)
• Length of Diffusion (LOD)
• OD to OD Spacing Effect (OSE)
• Layout Patterning Check (LPC )
• OD/Poly Density

Resolution Enhancement Techniques

• Optical Proximity Correction (OPC)
• Scattering Bars (SB)
• Double Patterning (DP) or Multiple Patterning
• Phase Shift Masking (PSM)
• Off-axis Illumination (OAI)
  
  

Optical Proximity Correction (OPC)

• OPC is a Photo-lithography Enhancement technique commonly used to compensate the mask pattern for image errors due to diffraction or process effects (by reducing the value of the k1 factor in CD equation)
• OPC is an effective way to deal with geometry distortion from design to chip; however, it does come at a price
• First, there is the cost of the EDA tools you need to implement the OPC corrections
• Second, you have an exponential increase in volume of the data representing the chip's layout, along with a huge increase in the time it takes to process this data and prepare it for photo-mask generation
  
              
  

Scattering Bars (SB)

• Sub resolution assist features that improves the depth of focus of isolated features
• Scattering Bars are added only for the most outer line of the dense pattern        
  

Multiple Patterning

• Involves decomposing the design across multiple masks to allow the printing of tighter pitches
• 38-nm features with 193-nm light water immersion lithography is the limitation with the current lithographic process
• Multiple Patterning is a technique used in the lithographic process that can create the features less than 38nm at advanced process nodes
• Multiple patterning basically changing the value of K1 in the Critical Dimension equation
• Double Patterning
  • Double patterning counters the effects of diffraction in optical lithography
  • Diffraction effects makes it difficult to produce accurately defined deep sub-micron patterns using existing lighting sources and conventional masks
  • Diffraction effects makes sharp corners and edges become blur, and some small features on the mask won’t appear on the wafer at all
  • Double patterning is expensive because it uses two masks to define a layer that was defined with one at previous process nodes

Phase Shift Masking (PSM) (not considered in PD)

• Phase-shift masks are photo-masks that take advantage of the interference generated by phase differences to improve image resolution in photolithography
• Controlling the phase enables constructive or destructive interference at desired locations in the image plane, thus sharpening or dulling the contrast as desired
• These are photo-masks with structures that manipulate not only the amplitude of the transmitted waves but also their phase
• Etching quartz from certain areas of the mask (alt-PSM) or replacing Chrome with phase shifting Molybdenum Silicide layer (attenuated embedded PSM) to improve CD control and increase resolution
• There exist alternating and attenuated phase shift masks
• Types of masks
  • Conventional (binary) mask, Alternating phase-shift mask, Attenuated phase-shift mask
    

Off-Axis Illumination (OAI) (not considered in PD)

• Off-axis illumination is one of the practical techniques to enhance resolution of a given optical system with bigger advantage of improvements in depth of focus
• The specific illumination geometry is designed to enhance the contrast in the wafer plane of the photo-mask features whose dimensions are most Critical
• With OAI, resolution of a given system can be improved without going for shorter wavelength or higher numerical aperture (NA)
• This technique basically has no on-axis illumination component as oppose to partial coherence
• The shape and size of the source plays an important role when different conditions of mask features such as density and orientation are considered
• To obtain the highest resolution, illumination of the photo-mask is not performed by a discshaped source
• The angular distribution of the illumination beam may have a complex structure, such as an annulus, a set of off-axis circles, or even a continuously varying profile