MEMS-based Ni–B probe with enhanced mechanical properties for fine pitch testing
© The Author(s) 2017
Received: 4 November 2016
Accepted: 30 December 2016
Published: 7 January 2017
We fabricated and characterized microelectromechanical systems (MEMS)-based Ni–B probes with enhanced mechanical properties for fine pitch testing. The Ni–B micro-probes were compared with conventional Ni–Co micro-probes in terms of the mechanical performance and thermal effect. The elastic modulus and hardness of Ni–B were found to be 240.4 and 10.9 GPa, respectively, which surpass those of Ni–Co. The Ni–B micro-probes had a higher contact force than the Ni–Co micro-probes by an average of 41.38% owing to the higher elastic modulus. The Ni–B micro-probes had a lower average permanent deformation than the Ni–Co micro-probes after the same overdrive was applied for 1 h by 56.58 µm. The temperature was found to have a negligible effect on the Ni–B micro-probes. These results show that Ni–B micro-probes are useful for fine pitch testing and a potential candidate for replacing conventional Ni–Co micro-probes owing to their advanced mechanical and thermal characteristics.
KeywordsNi–B micro-probe Ni–Co micro-probe Contact force Deformation
With the rapid growth in semiconductor technology for integrated circuit (IC) manufacturing processes, microelectromechanical systems (MEMS)-based probes play an important role in testing ICs on a wafer [1, 2]. The wafer-level test is the first step in the device manufacturing process, where chips fabricated on a bare wafer are subjected to standardized tests by using the input/output (I/O) terminals of the chips in order to determine defective chips before the packaging process . During the wafer-level test, an individual chip is tested by using a probe card with micro-probes. This test makes it possible to reduce unnecessary packaging costs by avoiding the fabrication of defective devices at the initial stage. Because the test is conducted before the wafer is diced, it provides early feedback on the overall stages of the fabrication process so that corrections can be applied in an early stage of fabrication .
As the ICs on chips become more complex and the chip size becomes smaller, the number of test pads on the substrate increases, which has decreased the pitch among test pads. This trend requires narrower micro-probes, which in turn reduces the stiffness of the micro-probes. Micro-probes must exhibit high stiffness (or high elastic modulus if the geometry of the micro-probe is known) to be able to break the oxidation layer on aluminum I/O pads [4, 5]. In order to extend the service time, the mechanical properties of micro-probes should exhibit a high yield strength so that they do not undergo permanent deformation, high hardness to minimize the abrasion caused by a number of touchdowns, and high thermal stability so that they can maintain their mechanical performance in a high-temperature environment such as from localized Joule heating .
Under these conditions, Ni alloys, especially Ni–Co, are conventionally used for micro-probes because of their suitable properties and affordable fabrication processes, such as electroplating [2, 5–10]. However, since integration has increased continuously over the years, there is a growing need to find another material to meet the stringent requirements.
In this paper, we propose Ni–B as a potential candidate for micro-probes for fine pitch testing since it has been reported that alloying nickel with boron significantly enhances the hardness, elastic modulus, and creep resistance [11, 12]. A MEMS Ni–B probe was fabricated by the electrodeposition technique. In order to measure the elastic modulus and hardness, we estimated the mechanical properties of electroplated Ni–B and Ni–Co by using nanoindentation. In addition, we conducted comparative experiments from various points of view to verify the applicability of Ni–B micro-probes to conventionally available Ni–Co micro-probes. The results showed that Ni–B micro-probes exhibit the potential applicability for micro-probes for fine-pitch testing.
Design and fabrication
Here, 4-in silicon wafers were used as substrates, and a seed layer of Ti/Cu (50/500 nm) was deposited with electronic beam evaporation. The micro-probe patterns were fabricated by a photolithographic process using the photoresist THB-151N. The Ni–B probes were electroplated from a nickel sulfamate bath using dimethylamine borane (DMAB) as the B source. Then, the micro-probe patterns were removed by a stripper, and the Ni–B micro-probes were released by a 20% ammonium persulfate solution.
In order to determine the hardness and elastic modulus of the micro-probes, nanoindentation tests were performed with a nanoindenter (G200, Agilent) having a Berkovich tip. The maximum load was 5 mN, and the loading and unloading rates were 1 mN/s. The Oliver–Pharr method was used to calculate the elastic modulus and hardness from the indentation load–displacement data.
Experimental conditions of Ni–B and Ni–Co MEMS probes
Overdrive: 0–700 µm (25 µm)
Overdrive: 150, 300, 450 µm
Time: 60 min/step
Temperature: 23 ± 5, 95.5 ± 5 °C
Overdrive and time: same as above
Results and discussion
Overall, we confirmed that the Ni–B micro-probes managed to keep their mechanical properties not only at room temperature but also at the high temperature of 95.5 ± 5 °C.
We presented Ni–B micro-probes with enhanced mechanical properties for fine pitch testing. We fabricated MEMS probes of Ni–B and Ni–Co with identical structures in order to compare their mechanical properties and prove the feasibility of Ni–B micro-probes. The hardness and elastic modulus of Ni–B and Ni–Co were measured by nanoindentation. Using our developed MEMS probe testing system, we tested the contact forces, deformation from the contact deflection, and thermal effect on the probes. All of the measurement and test results showed that the Ni–B micro-probe had superior mechanical properties. Based on these results, we concluded that the Ni–B micro-probes can sustain their mechanical performance for an extended service time and are useful for a large number of touchdown testing processes, which makes them potential replacements of conventional Ni–Co micro-probes for fine pitch testing.
YJK conceived the idea and supervised the project. YJK and HBG discussed the design and the fabrication process of the gas sensor array. KK and HRA performed the experimental measurements and analysis of the results. YJK, KK and HBG drafted the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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