Etching process characterization - Princeton University
Transcript of Etching process characterization - Princeton University
Etching process characterization
1. Initial mask check
a. Good understanding of etch process starts with understanding the initial mask profile,
be it photoresist or hard mask. Important parameters of the mask are thickness and
sidewall angle. If possible, perform SEM inspections of cross sections to characterize
sidewall angle at different feature sizes that are applicable to your etch step. For large
features and non‐critical etches, only mask thickness may be necessary to measure.
b. Check to see if descum is required for your resist mask. In general, descum will make the
etch step much more consistent from run to run by removing organic residue left by
development step. In some cases, this residue remains regardless of exposure and
development times, and the descum step is a required step for the etch to work at all.
An example of AZ resist on SiO2 is below:
c. To measure mask thickness, you can use cross sectional SEMs and supplemental
software. In addition, for transparent films, such as photoresist and dielectrics, you can
use either a reflectormeter or ellipsometer tool as applicable. The initial thickness of the
mask is important because of an etch process result called selectivity, defined as the
etch rate of your material to the etch rate of the mask. If the selectivity for your
selected process is poor, you will not be able to etch very deep into your material, or
you may need a much thicker mask. At the other spectrum, selectivity for some
processes, such as deep silicon etch, can be extremely high to a properly selected mask,
in excess of 100 to 1.
Top left: initial resist sidewall
with scum next to the edge. Top
right: SiO2 etch without descum.
Bottom left: optimized etch that
includes descum with near
perfect sidewall. Process info in
SiO2 case study document.
2. Tool selection
a. The etch tools are highly specialized, each with its own set of process gasses, materials
and sample size restrictions. It is important to choose a proper sample for your process
and to ensure that your device meets the material restrictions. Putting wrong materials
in the etch can damage your own process, as well as contaminate the tool for everyone
afterwards. Restoring the chamber to original condition can bring the entire etch bay
down for a whole day to clean the contaminant.
b. Use the chart below as a guide for your tool selection. It is never a bad idea to check
with staff to make sure your samples and processes are compatible.
c. Most etch tools readily accept 4” wafers and can be reconfigures for either 2” or 6”.
However, irregular shapes, such as small pieces and square substrates, require a carrier
to hold them inside the chamber. Selecting the carrier is as important as selecting the
process chemistry. During the etch, the gasses will interacting with the carrier and
create byproducts that will affect your sample. In addition, thermal conductivity has a
big impact on selectivity of the mask and on etch rate. Although some tools have spare
inert carrier provided, it is always a good idea to have your own carrier for your process
to prevent cross contamination.
3. Sample mounting
a. If you are not using round wafers, and instead need to mount a sample onto the carrier,
the cleanroom provides several bonding materials. For smaller pieces and short etches
that do not pose risk of overheating, thermal release tape is the cleanest and easiest
approach. For larger sample, it becomes increasing more likely with sample size to trap
bubbles between the tape and the substrate or carrier, which will cause overheating of
sample surface. In this case, vacuum grease or CoolGrease are preferred. For
applications that require pristine cleaning of the mounting side of the sample (double
sided devices), crystal bond can be used. The table below outlines properties of each of
the provided bonding materials.
b. Regardless of the bonding material, there are a few general good practices to observe.
Try to minimize the amount of bonding material that is exposed to the plasma: while the
material may be vacuum compatible, its interaction with plasma can add contaminants
and particulates into the chamber that will end up on the surface of your sample. The
carrier itself should also be clean, especially the backside that is in contact with the
chuck.
4. Preparing the chamber: Clean and Condition
a. In a multi‐user facility one can never be sure of the status of the tool and the etch
chamber when first coming up the tool. Although there are rules regarding cleaning the
chamber after use for most of the etch tools, it is a good idea to check to make sure the
previous user has performed proper cleaning steps. You can also run your own cleaning
step using standard cleaning recipes using empty chamber or with a provided carrier for
cleaning.
b. The next step is to condition the chamber. This means running a dummy sample or
carrier with our actual recipe for a short amount of time, typically no more than 5
minutes. This step will prepare mass flow controllers and will ensure the cylinders are
open. In addition, this step will coat the sidewalls of the chamber with material that
your sample will see during the etch. Without conditioning, the sample will see different
chamber conditions during the beginning of the etch (clean chamber) than during the
end of the etch (conditioned chamber). Running multiple samples one after another has
similar effect. Ideally, you want your sample to see the same chamber condition
thoughout the process and the conditioning step prepares the chamber for the first part
of first etch. The conditioning is optional, but is highly recommended to ensure
repeatable results.
c. Another clean step is usually required after all of the etching to leave the tool in a clean,
predictable state for the next user. The graph below summarizes all of the steps.
5. Metrology after the etch
a. After performing the etch process, there are several measurements to take to
characterize the process properly. Among them are etch rate, selectivity, uniformity,
and sidewall angle. Taking cross sectional SEMs will help you extract data on thicknesses
of both the etched material and the final thickness of the mask. The ratio of those etch
rates determine selectivity. You can also measure sidewall angle, if that is important,
and, if the pattern was designed for it, extract data on the dependence of etch rate on
feature type and size. Small isolated holes will etch differently than large open areas.
This is called aspect ratio dependent etching (ARDE). Example is below for deep silicon
etch.
Clean
(optional)
•Run the "clean" recipe with enpty chamber or the appropriate carrier. Time and process varies depending on the tool
Condition
(optional)
•Run your recipe with provided carrier (or your own wafer) for 2 to 5 min depending on application
•This will prepare the chamber conditions for your device
Process
•Run your process
•If using chips, make sure to select proper carrier wafer
Clean•Run another "clean" recipe. This is usually a required step.
SAMCO 800 Deep Si etch. Nope
the difference in etch depth
between dense let side and open
right side.
b. To measure uniformity across a larger sample or wafer, take measurements at five
point: center and four edges in a cross pattern. Use the formula below as a quick
measure of uniformity:
(MAX ‐ MIN)/(2*MAX)*100%
Uniformity may not be applicable to small chips.
c. There is an important note on selectivity that mostly relevant to photoresist and organic
masks. Etch recipes sometimes include a descum steps in the first part or the recipe.
This can remove a non‐trivial amount of resist, and can give a notion that the selectivity
is dependent on etch time, with longer time producing higher selectivity. Make sure you
understand the impact of descum on the mask thickness and perform mask
measurements at the appropriate times. In some cases, it is important to measure the
resist removal rate of just the first descum steps to know what the starting resist
thickness is at the beginning of the actual etch step in the recipe.
6. Next steps
a. Depending on the results, the etch recipe can be modified to suit your needs. In some
cases the mask may need to be changed instead of the etch step itself. There are many
relationships between process values and final result, but most of the interactions are
not linear. The staff is here to support your project and help you be successful.