Patterning Troubles

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    Patterning Troubles for

    Standard CellsDavid Pietromonaco

    ARM R&D

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    Contents

    Why double patterning?

    Introduction Types of double patterning

    What is stitching?

    Trouble for Standard cells

    LOTS of trouble for standard cells

    Average density vs. Peak density

    More trouble for standard cells

    It gets even worse for standard cells, too

    A few words about contacts

    Conclusions

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    Why Double Patterning?

    Its been looming in the room for some time due to the

    physics Nobody wanted to acknowledge it, really

    Much harder for everybody foundry, designers, tools

    Scaling has reached levels such that double patterning cant

    be ignored, anymore.

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    Double Patterning Introduction

    Scaling to the bitter end of optical lithography

    ~200nm illumination wavelength the current standard

    making ~20nm features with that isnt really possible without tricks

    Limit is about 80nm pitch for parallel lines, 100nm for vias.

    By the way, 20nm is as small as the smallest viruses

    EUV isnt really optical and it isnt ready.

    E-beam lithography isnt ready, either.

    Double patterning necessary to keep scaling going until someother way to image patterns becomes available.

    Several types of double patterning fall into 3 main categories

    Double patterning you dont know about no obvious restrictions

    No frequency doubling, either, only fractional improvement

    Non self aligned double patterning for frequency doubling

    Self aligned types of double patterning for frequency doubling

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    Double Patterning Types (1)

    The ones you dont know about

    Foundries have been using asymmetrical illumination and doublepatterning occasionally and quietly.

    You can think of this as breaking up the horizontal lines from thevertical lines and shooting each set with light optimized for each*

    One exposure to squeeze horizontal

    Additional exposure to squeeze vertical

    Rules are nearly the same as what youve

    been used to all along.

    Dont get true frequency doubling this way

    Helped get us to 80nm bidirectional line pitches!

    Need something with more enhancement power to get morescaling than this

    *Its not quite that simple of course, but it is basically the same net effect

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    Double Patterning Types (2)

    Non Self Aligned Double Patterning (LELE, Pitch Split, etc.)

    The simplest form of frequency doubling double patterning

    Simply create the layer in two steps.

    Each time create a different part of it

    You can double frequency by

    alternating the exposure of objects This is currently the most mature

    method.

    For design tools and processing

    Easiest to work with Stitching can be allowed

    Different sized objects can be allowed

    Well discuss this the most.

    Step1

    PlusStep

    2

    Final

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    Wait, what is stitching?

    Stitching simply refers to merging objects from the two

    processing steps. So, from our previous example:

    Step1

    PlusStep

    2

    Final

    Stitch

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    Double Patterning Types (3)

    Self aligned double patterning

    The most difficult types because the patterning correlates moreclosely to the spaces between what you are trying to create

    This is true for design tools, humans and processing

    Two main types

    Both use the sidewalls of printed objects as the foundation forcreating the desired objects

    Since each printed object has two sidewalls in each direction,frequency doubling occurs.

    Sadly, one type cannot be stitched, the other type cannot

    have different width lines The unstitchable kind is the simpler one and gaining some attention.

    For layout rules, it is basically the same as non self aligneddouble patterning but stitching is not allowed.

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    Pictures! We like Pictures!

    (Try not to think too hard how this works on complicated layout)

    DesiredLayout

    SacrificialMandrel

    Sidewall

    Block Mask

    DesiredLayout

    Is Metal Is Dielectric

    Fixed width metal onlyVariable spacing

    Stitching possible

    Variable width metalFixed spacing

    Stitching not possibleMost common SASL type

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    Trouble for Standard Cells

    Two patterns can be used to put any two objects close together

    Subsequent objects must be spaced at the same-mask spacing Which is much, much bigger

    Classic example: horizontal wires running next to vertical ports

    Bad

    OK

    Bad

    OK

    OK OK

    OK OK

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    More Trouble for Standard Cells

    If there are two horizontal metal tracks, it can be even worse

    Any use of stitching means the second track has to also moveout to the same-layer spacing

    Stitching is pretty common, too

    in folded circuits

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    Even More Trouble for Standard Cells

    Power rails further complicate things

    Via spacing, metal tips, etccan all create even more problems ????

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    Average Density Vs. Peak Density

    What just happened?

    Our worst case cell size just exploded by adding 2 tracksworth of same-mask spacing to each side (only the top wasshown)

    Eg. 9 track cell just became 13

    Defeats the whole purpose of double patterning then!

    But not all cases are the worst cases

    Unfortunately, the cases that have only two objects involvedhistorically had some other way to solve them so no gain

    IE. If you had a problem tip-side space and nothing else nearby, youcould change the tip to an L shape and get the side-side spacing.

    Average density is actually closer to the same-mask density,not the different-mask density

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    Yet Even More Troubles for Cells

    Patterning difficulties dont end, there.

    No small U shapes

    No opposing L sandwiches

    So several popular structures cant be made

    No ColoringSolution

    Have to turnvertical overlaps

    into horizontal ones

    But that blocksneighboring sites

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    The Problems Keep On Coming

    Some processes may not allow stitching

    May be preparing for SASL

    Or just think explicitly drawing two mask layers is too confusing

    Either way, it is terribly difficult to solve

    And cant just add pitches vertically to fix IT NEVER SOLVES

    This can neverbe solved

    without stitching

    Have to usesome other layer

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    Do the Troubles for Cells Ever Stop?!?

    If all that were the total of problems, it might not be so bad

    Actually, it would be pretty bad. But it gets even worse, though.

    Minimum metal areas arent decreasing as rapidly as the

    scaling

    Double patterning doesnt help this at all

    Ports are relatively huge

    Can hardly fit a M1 landing pad to jump straight to M2.

    You wouldnt be jumping to M2 if you had room on M1

    And minimum M2 blocks more ports than it used to, making multipleports in a row a big problem, too.

    Dont even get me started on contact scaling

    Or lack thereof

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    A Few Words About Contact Scaling

    Contact density slipping even farther behind line density

    So far behind, they arent even small enough to contact diffusion

    Entering the local interconnect age for the same reasonswere entering the double patterning age

    Sadly, local interconnect doesnt really help standard cells

    very much. Safe cells have transmission gates in their feedback inverters

    Theres generally no connection from diffusion to neighboring poly

    Helps memory cells more

    Generally only useful to offset various connections a little bit

    But you still have contacts to connect to the local interconnect

    And the pitches for those contacts are still pretty large

    So it only got more complicated, not really better

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    Conclusions

    The double patterned elephant is here to say in the room

    It cant be avoided

    It makes layout much more challenging

    Cell density estimates need to be realistic

    Cant achieve full double patterning scaling due to the interaction

    limitations of double patterning

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