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Friday, July 17, 2015

Effects of Etching Process (part2)

Effects Of Etching Process
(Few Terminology and Effect in the Thickness variation)

Index Chapter1 Chapter2 Chapter3 Chapter4
Introduction Parasitic Interconnect Corner (RC Corner) Manufacturing Effects and Their Modeling Process Variation


3.1 3.2a 3.2b 3.3a 3.3b 3.3c
Introduction Effect Of Etching Process-1 Effect Of Etching Process-2 Introduction to CMP (Chemical Mechanical Planarization)

Importance Of CMP process Dishing and Erosion (CMP effects)

In the previous post we have discussed the etching process and its main effect. If you remember then you can yourself figure out that those effects contribute in the width variation. Similarly there is another effect of Etching which contribute in the thickness of the layer. In this post I will discuss about that.

To understand this I will discuss few of the terminology related to etching. Few are going to help me in my story building (Means while I will explain the concept – how etching effects thickness), while few are for you info. 
Terminology used in Etching Process:
  • Etch Rate
  • Selectivity
  • Anisotropy
  • Uniformity
  • Etch Profile
  • Loading effect
  • Over etch
  • Residue

Let me try to provide one line definition…

Etch rate is a measure of how fast material is removed in the etch process.
Etch rate = (Thickness before etch - Thickness after etch)/Etch time
Etch rate depends on:
  • Gas flow / gas composition
  • Pressure
  • Source power
  • Bias voltage
  • Wafer temperature

Selectivity is a ratio of the etch rates between the different materials, especially the material that needs to be etched with respect to the material that we do not want to remove.

S=ER1/ER2
Where: S->; Infinity if ER1 >> ER2
ER is dielectric Constant.


Uniformity:

The etching should be uniform within the wafer, from wafer-to wafer, from tool-to-tool. The uniformity is measured with wafer thickness by measuring the thickness at certain points before and after the etch process, and calculating the etch rates at these points.

(Max Etch rate – Min Etch rate)/(Max Etch Rate + Min Etch rate)


Etch Profile:

One of the most important characteristics of etch is the etch profile, which affects the next deposition processes. The profile is grouped into isotropic, anisotropic etch, and in between. Regarding these we have already discussed in the previous post.


Loading effect:

When the etch rate is dependent upon the amount of etchable surface exposed to the etchant, the phenomenon is called a Loading effect.


There are 2 type of loading effects (Note: below figures are copied from the Internet – not original version)
  • Macro Loading effect
    • In the Constant supply of Reactants, Etch rate goes down with increase the Surface Area

  • Micro Loading effect
    • For contact and via hole etch processes, the smaller hole has a lower etch rate than the larger holes because it is more difficult for the etchants to pass through the smaller hole, and etch by products are harder to diffuse out. Micro loading effect can be decreased by the low pressure process because the mean free path is longer.


Note: “Loading Effect” effects the thickness of the layer. Above few figure demonstrate this very well.

Over Etch:

When we do the etching, sometime (or I should say most of the time) unwanted material etched during the process. Like in the below example, we don’t want to etch Substrate but while etching FILM, some part of the substrate also etched. This is known as “Over Etch”.
(Below Figure is copied from the Internet)


Note: “Over Etch” effects the thickness of the layer. Above figure try to show the same.



Residue:

Residue is reverse of the “over etch”. It usually happen as the corners. In short we want to remove the Film but because of corners/shapes, etch process don’t able to remove the Film completely.
(Below Figure is copied from the Internet)



When I have discussed a lot about the ETCH, let me give you some introduction of 2 different etching process here also (next article will be more focused on Manufacturing variation).
  • Positive Resist Etching Process
    • Positive photoresists develop at the exposed areas, while the unexposed resist remains on the substrate after development.
  • Negative Resist Etching Process.
    Negative photoresists behave opposite: The areas exposed crosslink during a subsequent baking step (post exposure bake, PEB) and remain after development.

    Note: “Residue” effects the thickness of the layer. Above explanation try to prove this.

    Last 3 (Loading effect, Residue, Over etch) are root cause of the thickness variation of layers.

    So before I close the ETCH process, let me summarize the concepts from manufacturing variation point of view.


    Because of different etch process (which we have discussed in this post and last post), the actual shapes may be any of the above. (Blue is what we need ideally and brown is what we will get actually/practically). If you have gone through above post seriously– you can easily figure out the reason of these (else message me).


    Let’s discuss few other fabrication related concepts in next few articles before we talk about the modeling of different type of variations.


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