Measuring PVD Deposition Rates via Profilometry

Three methods are presented for measuring film PVD (Sputter or Evaporation) deposition rates via Profilometry. These methods may be used on the same sample if necessary.

APPLICATIONS

 

Due to its very wide applicability, profilometry is one of the preferred methods to measure deposited thicknesses, especially when optical methods are not available. 

We assume here that the substrates are cleaned glass slides, but the methods may be extended to other substrates with appropriate modifications. 

 

There are three basic methods, to be selected according to the following film attributes:

• Adhesion to substrate

• Deposition temperature

• Sensitivity to volatile contaminants

 

The following table is used to decide which method is preferable:

Film attributes Scratch Kapton lift-off Resist Pen lift-off
Adhesion to substrate POOR EXCELLENT GOOD
Deposition temperature ANY  <150°C  <150°C 
Sensitivity to contaminant  ANY  LOW  LOW 
       

Note that the Resist (or SharpieTM) Pen lift-off method is usually superior to the Kapton lift-off method, except in rare instances where the film adhesion is so strong that the resist does not lift-off. In any case, when in doubt, several methods may be used on the same sample.

SCRATCH METHOD

When the film adhesion to its substrate is poor, scratching the layer is the simplest and most effective method. An example is a Gold (Au) layer deposited directly on a glass slide. 

Procedure:

  1. Choose a sharp object made of a material that will not scratch the glass, a plastic knife for example. 
  2. Use the plastic knife to scratch the Au surface over a straight line of at least 2 to 3 mm of length. The scratch width should ideally be 1 mm or less. 
  3. Scan via profilometry across the scratch width. Compute the average of 3 to 5 scans taken at slightly different locations.

KAPTON LIFT-OFF METHOD

The Kapton lift-off method has been known to many users for a long time. Kapton is a polyimide from DuPont and an excellent dielectric, with high voltage breakdown strength, high vacuum compatibility and is resilient to temperatures up to 400 C.  

Unfortunately the Kapton must be coated with a silicone adhesive to stick to the glass surface. This adhesive is NOT compatible with high vacuum or high temperatures. This limits in practice the method to temperatures < 150 C.

Another problem with the Kapton film is its thickness: it is typically ~ 1 mil thick; with a ~ 1.5 mil adhesive; the total film stack is ~ 2.5 mils or ~ 64 microns thick. That thickness is thought to interfere with the sputtered deposition process by partially masking the oblique sputtering of the Denton. Being a good dielectric, the Kapton may also charge more than the substrate, and develop a positive charge that repels incoming positive ions. Whatever the explanation, the result is a tapered edge profile that is more difficult to measure, especially for very thin films on a curved substrate.

ITO being a transparent film, it is easy to see the thickness tapering corresponding to color fringes near the Kapton edge, before as well as after the lift-off. This interpretation is confirmed by the film profilometry and Nanospec measurements.

Procedure:

Stick a Kapton disc to the glass substrate prior to deposition. Ideally a 1 x 3 mm cut out from the disk should be used to facilitate measurement. After deposition, mechanically remove the kapton, using tweezers if necessary, but being careful of not scratching the glass where the profilometer will scan. Plastic tweezers are preferable for this. Scan via profilometry across the scratch width. Compute the average of 3 to 5 scans taken at slightly different locations. Try to avoid areas where the film edge is curled up from the substrate due to being teared away from it, as this may induce measurements errors.

RESIST PEN LIFT-OFF METHOD

The Resist pen lift-off method can replace the Kapton lift-off method in most instances. 

Procedure:

The Resist pen can be substituted by a Sharpie(TM) pen, or equivalent. The pen tip should be as fine as possible, 1 mm or less. The color red is preferred because of its enhanced visibility.  

The pen is used to draw a line or a cross on the substrate prior to deposition. 

The “ink” solvent is very similar to acetone, and evaporates within seconds, leaving a polymer residue < 1 micron thick. That residue is coated by the film during deposition. After deposition, the mark is removed via an acetone bath with US agitation for ~ 2 minutes (use the small dedicated Aluminium container for that purpose) ; if not enough, gently rub a lab wipe soaked with acetone over the trace. Then the slide is rinsed with an IPA spray, and then blown dry with an N2 gun. 

ITO being a transparent film, it is easy to see that the profile is abrupt near the trace edge, before as well as after the lift-off. This interpretation is confirmed by the film profilometry.