Welcome.
My name is Carrie Donley and I'm the director of
the Chapel Hill Analytical and Nanofabrication Lab or CHANL at UNC.
In this video, we will cover the basics of a fabrication technique called hot embossing.
Hot embossing is a relatively simple fabrication technology that involves the use
of heat and force to press a master mold into a polymer substrate.
In this way, the inverse of the structures on the master
are transferred into the surface of the opposing polymer substrate.
Although the embossing process has been used for centuries to
pattern macroscale structures on objects such as coins and paper,
more recently, it has been used to replicate structures at the micro and nanoscale.
For instance, hot embossing has been used for the production of CDs and DVDs,
which have features as small as a few hundred nanometers.
Hot embossing is also heavily used in
the life sciences field to make microfluidic devices,
which are basically miniaturized chemistry labs.
The primary component of hot embossing is the master mold.
The master mold only needs to be fabricated once
but it can be used many times to replicate the same structure.
While the fabrication of the master mold can be expensive and time-consuming,
the replicated part takes only a few minutes to make and cost significantly less money.
This makes hot embossing especially suitable for mass production.
There are a variety of ways to fabricate a mold and the end-user must
decide what mold properties are important such as lifetime of the mold,
size of the features, and cost.
One technique that is commonly used to fabricate molds is called micro milling.
Micro milling uses a miniature drill bit to carve out structures to make a mold.
Micro milling is great for making strong master molds that can be reused many times.
However, it cannot be used to make
very small nanoscale structures because the smallest drill bit is too large.
The smallest structures this technique can make are limited by
the geometry of the milled structure and the size of the drilling bit,
which is about 25 microns.
In order to make nanoscale structures with hot embossing,
more advanced technologies are required for
fabricating the master mold. So, what do we mold?
Polymers. A type of polymer that people are familiar with is plastic.
Plastics are used just about everywhere because they are inexpensive and easy to shape.
Also, because there are so many varieties of polymers,
they have a wide range of material characteristics.
All of these properties make polymers very
useful for making micro and nanoscale structures.
Poly(methyl methacrylate) is shown here and is widely used because it
is transparent and can be molded at lower temperatures than other substances.
As the polymer is heated to be used for hot embossing,
it changes from a hard brittle substance to become soft and flexible.
As the polymer is heated further, it melts.
The temperature at which the polymer becomes soft is
called the glass transition temperature or Tg.
As the polymer is heated above Tg,
it becomes softer and can flow into the mold during the embossing process.
Different polymers have different Tgs which need to be
considered when determining what material will be used for hot embossing.
Now, let's discuss the hot embossing process in a little more detail.
The hot embossing process follows four main steps.
Heating, embossing, cooling, and demolding.
In the first step, the master mold and polymer substrate are heated
independently in a vacuum chamber to
a temperature just above the glass transition temperature of the polymer.
The system is typically placed under vacuum to
remove gases trapped in cavities between the mold
and the substrate and to remove
water vapor driven out of the polymer during the heating process.
In the second step,
the mold and substrate are pressed together with a controlled force.
At this stage, force and temperature are kept constant over a defined amount of time.
Force, temperature, and time are critical parameters for ensuring that
the mold cavities and structures are completely filled by the polymer substrate.
Once the embossing step is completed,
the system is cooled below the Tg of
the polymer where the polymer substrate returns to a rigid state.
In the final step,
the mold and polymer substrate are carefully separated from one another.
This de-embossing or demolding step is usually the most critical step due
to the large shear and friction forces that act on the polymer structures by the mold.
Structures with vertical sidewalls and high aspect ratios are
especially prone to deformation and require precise control of temperature,
force, and position during demolding.
When the embossed part is complete,
the user may inspect the quality of the structures using
an optical or electron microscope to
determine if any embossing parameters require adjusting.
Thank you for joining me for this video on hot embossing.
