The life of a sputter target is typically quantified in terms of units of power and time, like kilowatt/hours. For a target being sputtered at 500 watts for a total duty cycle of 100 hours that’s 50 kilowatt/hours. Target life is also a function of the sputter yield of the material or how many target atoms are ejected for each gas ion (typically argon) that strikes the surface. For example when an argon ion with a mass of 39.948, hits a light material, like carbon, with an atomic mass of 12.01, about 3 carbon atoms are ejected. For materials like platinum with an atomic mass of 195.09, nearly 5 argon ions have to strike the target to get one platinum atom out (Ref 1).
Other factors that affect sputter yield include the bias voltage used to accelerate the argon ion to the target surface and the incident angle of the collision. In addition, there are big differences in sputter yield for metals verses oxides. Typically oxides will last many more kilowatt/hours than metals like aluminum.
One of our resident thin film deposition experts, Rob Belan, recommends that a target be replaced when the trench depth of the race track is ¾ of the total target thickness. For a ¼″ thick target there will be 0.062″ of material remaining at the bottom of the trench. He adds that if you are particularly careful you may be able to sputter the trench to a thickness of 0.031″, but beyond that you are risking a complete burn through.
A handy online calculator for sputter yield can be found at TU Wein’s Institute fur Angewandte Physik. I’m not sure how accurate it is, but it does list out several single element metals and their yields:
https://www.iap.tuwien.ac.at/www/surface/sputteryield
For a list of sputter yields using other ions, such as xenon and neon, there is an expansive data base on the web site of the National Physical Laboratory. Their data base also includes sputter yields at various powers (Ref 2).
So to determine when it is time to change out a sputter target you will need to have a depth gauge, either digital or dial. Check the depth of the trench in the race track after every deposition until you get a feel for the number of kilowatt/hours it takes to thin the target out to 25% of its original thickness (Ref 3). For those looking for an in-situ, real time method for measuring target thickness during sputtering, check out the 2007 publication from Alex Leybovich of TOSOH SMD who used ultrasonic time of flight measurements to monitor the health of sputter targets and target bonding during thin film depositions (Ref 4).
References:
1. Argonne National Laboratory, “Noble Gas Sputtering Calculations using TRIM,” https://www.osti.gov/biblio/435114
2. National Physical Laboratory of the UK, http://www.npl.co.uk/science-technology/surface-and-nanoanalysis/services/sputter-yield-values
3. MSC Direct, https://www.mscdirect.com/browse/Measuring-Inspecting/Dimensional-Measuring-Tools/Depth-Gages/?navid=12107671&cid=ppc-bing-New%20-%20Measuring%20%26%20Inspecting%20-%20Product%20-%20PPC%20-%20Exact_I4PUCfCI_depth%20gauge_be_73667346999721_c_&mkwid=I4PUCfCI|dc&pcrid=73667346999721&utm_source=bing&utm_medium=cpc&utm_campaign=New%20-%20Measuring%20%26%20Inspecting%20-%20Product%20-%20PPC%20-%20Exact&utm_term=depth%20gauge&utm_content=Depth%20Gages
4. In-situ real time sputtering source health monitoring using ultrasonics, Alex Leybovich, TOSOH SMD, Grove City, OH, 2007, https://www.sciencedirect.com/science/article/pii/S0257897206008905?via%3Dihub
Category: Deposition Equipment
Sub-Category: Sputtering Targets
Related Topics: Sputtering, Process
