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Dry etching tutorial

Modules

1. Plasma assisted etching
2. Solid phase etching

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  • Reactions for SiO2 etching
    • CF4 and O2 are used for etching SiO2
    • the main reactions are:
      • CF4 = CF3 + F (dissociation of CF4)
      • 2CF3 + O2 =  2CO + 6F (more formation of F by reaction with O2)
      • SiO2 + 4F =  SiF4 + O2 (F attacks SiO2, etching it)

 

  • Possible etching mechanisms of Si
    • CF4 is used as etchant gas
    • The Si surface is saturated with F in plasma, as it can be seen in the picture, by reacting directly with CFx
    • C-F + Si-Si = Si-F +17 kcal/mole (one type of reaction in which SiF is formed at the surface)
    • F is created in the plasma by CF4 dissociation and it participates in the reaction with Si, forming in the end SiF4
    • SiF4 is a volatile product, thus it can be removed by the pump 
    • this is just one of the possible mechanisms, other reactions are also possible

                 

 

  • SiO2/Si selectivity
    • sometimes it is preferable to etch only the silicon, and not the underlying SiO2
      • in this case a mixture of gases is selected to etch Si selectively
    • in F2 ambient, the ratio between the etching rates for the two materials is Si:SiO2 = 50:1
      • the reaction is F-F + Si-O = Si-F - 5 kcal/mole
    • Because F2 is not good for environment, it is prefered to use CF4 as etching gas, however CF3 etches more aggressively silicon oxide than F2, thus it is not very selective
      • in order to increase the selectivity in CF4 plasma, O2 is added
        • By adding O2, it reacts with C, and F concentration is increased, increasing also the Si/SiO2 selectivity
        • if too much O2 is added, it can react with Si and SiO2 can be deposited, and the selectivity decreases again

 

  • Anisotropy in high pressure plasma
    • To ensure anisotropy, deposition has to occur on the vertical walls
    • After deposition, the deposited layer on the horizontal surfaces can be removed by ion bombardment, as it can be seen in the lower picture
    • The bombardment is higher on horizontal surfaces, thus the layer is only removed here, on the vertical wall the layer remains
    •  The process takes place only if H2 is added
      • more H2, means less F and more C, thus the Si/SiO2 selectivity is lost (drawback)

                    

 

  • Competitive etching and deposition
    • In order to etch selectively and anisotropic, in the reactor is introduced:
      • in the first part H2 to create the protective layer - anisotropy
      • and then O2 to etch selectively

 

  • Si3N4 & Loading effect
    • To insure selectivity in relation to SiO2,  CF4 - O2 ambient can be chosen (similar as in Si case)
    • To insure selectivity in relation to Si, CF4 - H2 ambient can be chosen
    • Loading effect
      •  If the number/area of the wafers increase, the reactive species is depleted, decreasing the etch rate
      • the etch rate as a function of initial etching rate Ro (for one wafer) and etching area A can be written:

1. Plasma assisted etching
2. Solid phase etching

pages: previous | 1 2 3 [4] 5 6 7 | next
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