From smart phones to intelligent power grids, Integrated circuits manufactured on semiconductor wafers are essential to our modern way of life. For the semiconductor industry, technological advancements are necessary to survive in the competitive economic environment of the Tech Industry. The goal for the “2D-LC Project” is to contribute to advancements in the industrious by developing a high sensitivity multi-axis load cell or 2D-LC (2 Dimensional Load Cell). With the co-operation of our sponsor company, International Test Solutions (ITS), Transverse Testing have identified an opportunity for the application of this device in advanced testing techniques to measure the life cycle of semiconductor test probes. This webpage focuses on the design process of developing the 2D-LC. For further discussion on application, and how the collected data will influence the semiconductor industry, please read Transverse Testing’s abstract selected for publication in IEEE’s South West Test Workshop 2014.
The above video shows the contact resistance (GREEN) and the downward force (RED) measured when a cantilevered semiconductor test probe touches down on an aluminum wafer. To measure contact resistance, the wafer is connected to a voltage source. As the probe touches down on the pad current passes through the probe. During each probe contact, aluminum debris accumulates on the probe tip; as a result more current is drawn on the following scrub. In the video the probe makes contact with the surface twice. On the second “scrub”, the probe has too much aluminum contamination causing atmospheric breakdown and burning the surface of the wafer.
The Concept Design proposes a vertically mounted cantilever beam, or flexure. Four strain gauges are configured concentrically about the longitudinal axis of the beam to measure strain due to forces acting in a horizontal plane. The flexure, strain gauges and harnessing are encased by an insulating material which will help reduce noise and protect sensitive components. The device is intended to function as a fixture, connecting to the compression load cell currently employed by ITS, for test probe analysis. Figure 2 shows the first concept ID of the 2D-LC.
The Proof of Concept involved testing two models of the flexure, each with singular premeditated simplifications. The first model tested was a 10: 1 scale model. The objective was to test the efficacy of the stress concentration zones which are intended to intensify strain at strain gauge locations. This test also served to prove that the design was capable of locating the direction of the force. The second test employed a model of a cantilever beam with a square cross section. The length of the beam was dimensioned to the length calculated for the 2D-LC. Mating features for interfacing the 2D-LC with the compression load cell and test probe were also included in this model. The results from testing this model were intended to prove functionality of the device while mated to the existing compression load cell. As a proprietary measure the test results will not be discussed in detail, though the findings did provide the information needed to begin prototyping.
The above video is of an iteration from a test performed for the proof of concept. The test is designed to induce maximum strain on the model at a controlled location by adjusting the angle of the probe scrubbing surface. Notice, as the probe touches down on an angled surface, a significant amount of deflection occurs. The graph on the right indicates force with respect to time for both the X and Y axes in a plane normal to the longitudinal axis of the beam. For this test the plate has been oriented so the probe will experience greater deflection along the Y-axis. Predictably, the change in amplitude for the Y-axis curve is larger than that of the X-axis curve. In the picture below, the second model discussed, used for testing the device interfaces, is shown beside a quarter for scale.
Fabrication and Assembly of the 2D-LC can be described as a process of three sequential steps; machining the flexure, installing strain gauges and terminals, and harnessing.
All machine work was done on the UNR campus, in the Mechanical Engineering Machine Shop. Figures 4 through 6 below show the flexure at various stages on the lathe.
Accurately locating the strain gauges on the flexure was critical to achieving reliable results. Using the Laboratory Test UNIT (LTU) at ITS, and a custom mounting bracket to position the flexure, a repeatable process was developed for mating the strain gauges on the flexure. Prior to applying adhesive, the beam surface was conditioned to achieve maximum surface bond between the flexure and strain gauge backing. Figures 7 through 9 show some of the steps and apparatus needed to properly install the strain gauges.
The 2D-LC interfaces with an NI PXIe-1078 chassis, which provides the source voltage and configures the strain gauges and reference resistors into a Wheatstone bridge circuit. Harnessing for the interface was the final step in the Fabrication & Assembly process and employed a ribbon cable soldered to the strain gauge leads.
This device is designed to investigate the effects of aluminum layer thickness, uneven surfaces, and surface roughness on a semiconductor test probe to determine the applied stress profile. Previous studies show that surface thickness has different effects on probe testing and needs to be considered in stress analysis. Bench-top tests of various materials were performed at ITS Test analysis lab using both vertical and cantilever probes. Advanced micro-stepping capabilities facilitate touchdowns across test surfaces in order to systematically investigate loading characterization. Shear and bending stresses in test probes were recorded using a high magnification, high speed video synchronized to surface interactions and probe force load.
In an effort to establish the 2D-LC as a technology capable of contributing to the advancement of the semiconductor industry, Transverse Testing is pursuing opportunities to publish test results. Consequently, our submission to the South West Test Workshop was selected. The abstract has been selected for the 2014 SWTW workshop, and the project will be presented at the 2014 conference. This is the first time an undergraduate research paper has been accepted to the conference. This is not only great for Transverse Testing, but gives the UNR Mechanical Engineering program national recognition. The full abstract can be viewed from the link below.
Click on the banner to visit the IEEE SW Test Workshop home page.
Soheil Khavandi (Team Leader)
Soheil Khavandi has been the Research Assistant at International Test solutions working on wafer testing for several years. He executes laboratory projects in support of key customer applications, manufacturing, product development, product testing, and performance. He has co-authored a few publications for the wafer level test and package test. Soheil will graduate with a Bachelor of Science in Mechanical Engineering from the University of Nevada Reno in May of 2014 and he is very interested in pursuing a career in the semiconductor industry and wafer level testing or any other industries. Soheil is also a world class fencer and silver medalist at 2008 and 2007 Asian fencing championship, and a silver medalist at 2007 fencing world cup.
Aaron Lomas, a native of Vancouver, BC graduated from Cowichan Senior Secondary school in 1998. After high school he left home to join the U.S. Navy serving six year at various commands in the rate of Electronic Warfare. Upon receiving his honorable discharge, he attended San Diego Mesa College for two years where he majored in business and captained the school’s collegiate soccer team. In 2009 Aaron moved back to Canada, to take on a role in his family’s business. Aaron moved to Reno in 2011, returning to school to study mechanical engineering at The University of Nevada. Aaron is married to his wife, Amanda and has a three year old daughter.
Robert Hartley is a senior mechanical engineering student at the University of Nevada Reno. Robert was born in Baton Rouge Louisiana, but has spent most of his life living in rural Nevada. Robert currently works for Costco as a technical tire sales representative with extensive training from Michelin and Bridgestone. Robert also has had the opportunity to supervise the tire department. During his free time, Robert has enjoyed mountain biking, hiking, camping, and spending time with his wife, Gemma and their two children.
Jordan James is a fourth-year engineering student at the University of Nevada Reno. He is 22 years old and was born and raised in Reno, Nevada. Jordan is graduating in May 2014 with a Bachelor of Science in Mechanical Engineering and a minor in Mathematics. He is an Engineer in Training with extensive sales experience.
Parker Fellows has been a Fire Protection Engineering Intern with the Los Alamos National Lab for three years. At the Laboratory he performed fire hazard analysis research and combustive loading simulations in radiological and nuclear facilities. During his time at the Lab he has also drafted documents for the Department of Energy along with testing criterion for fire application. Parker studied Fire Science in Reno and is working on his Bachelor of Science in Mechanical Engineering from the University Of Nevada Reno.
Transverse Testing would like to thank Dr. Jerry Broz and International Test Solutions, LLC. for their guidance and support throughout all phases of the project, as well as providing facilities for assembly and testing. We would also like to thank Tony Berendsen for his help and advice in machining and aluminum processing. Finally, thank you Dr. Geiger for all of your support, insight, and input to the project.
Transverse Testing was granted the Owen Stedham Award for exemplifying high levels of ingenuity, acumen, and professionalism as the best Senior Capstone Team from the ME Department at the University of Nevada, Reno.