Description:
Electrical probe stations are ubiquitous tools in the semiconductor electronics and data storage industries [1-6]. These instruments enable the probing of electrical properties of microfabricated electronics on silicon wafers or other planar substrates. This probing is used to determine whether the microfabrication was successful; if so, the silicon wafers are then cut into smaller pieces called dies that are packaged and integrated into more complex electronic assemblies. Typically, a silicon substrate contains many identical die so the electrical probes are translated over the substrate, aligned to the relevant features, placed in contact with the substrate, used for measurements, and then translated to the next die location. Probe station technology is well developed for electronics that function at room temperature. In particular, so-called probe cards allow a large number (hundreds) of temporary electrical connections to be made to a substrate using only mechanical pressure. However, there is an unmet need for a probe station with numerous closely packed electrical probes that can operate at temperatures near 4 K.
In recent years, the need for and variety of electronics that operate at temperatures near 4 K have expanded greatly. Examples include sensors for industrial materials analysis, nuclear security, concealed weapons detection, and astrophysics. The next generation of instruments to study the cosmic microwave background, for instance, may require 10s to 100s of silicon wafers containing cryogenic circuitry. Another example is classical computing using high speed, low power superconducting elements. Still another example is quantum computing using novel circuit components also based on superconducting films. Both research and commercial activity based on cryogenic electronics are growing. In order to aid the manufacture of cryogenic electronics, NIST is soliciting proposals for the development of a probe station optimized for this emerging market area.
Cryogenic electronics must be tested at low temperatures near their planned operating temperatures. Testing after microfabrication but before dicing and integration can save manufacturers and customers the enormous expense of packaging, shipping, cooling, and attempting to use flawed electronic components.
While cryogenic probe stations are already commercially available, these units do not have performance suitable for emerging applications. For example, niobium is a crucial material in cryogenic superconducting electronics. The transition temperature of niobium is 9.2 K and devices containing niobium must be probed at temperatures well below this value in order for the tests to accurately predict device behavior. Hence, the silicon substrate being tested should be at a temperature near 4.2 K or colder. Existing cryogenic probe stations are not able to achieve temperatures this low for the large substrates (up to 150 mm in diameter) that are now used to make superconducting circuits. As the complexity of cryogenic electronics has increased, so too has the number of circuit elements that need to be probed on a single die. However, existing cryogenic probe stations have only small numbers of probes (typically less than 10) that are physically large and therefore cannot be used to contact the closely spaced features that are increasingly used in cryogenic electronics. Further, existing probes are often optimized for much higher signal bandwidth than is now needed for basic tests of circuit functionality. Finally, these probes often contact the substrate under test from a warmer temperature stage and therefore are a major heat load that makes temperatures near 4.2 K difficult or impossible to achieve.
To aid the manufacture of cryogenic electronics for sensing and computing, NIST seeks proposals for a high-density cryogenic probe station that meets the following technical goals:
- Sample cooling to 4.5 K or below. This value refers to the temperature of the substrate under test and not the temperature of the underlying metal. Use of a mechanical cryocooler is preferred but liquid or gaseous helium are also acceptable.
- Rapid cooling and warming are desirable. A cool-down time from 300 K to base temperature below 2 hours is preferred. A warm-up time from base temperature to 300 K below 1 hour is preferred.
- Compatibility with substrates up to 150 mm in diameter.
- The ability to simultaneously make 100 or more electrical contacts to a die under test. Contacts to be made using mechanical pressure only, not wirebonding or other contact schemes that mechanically alter the test substrate.
- Electrical contacts must be pre-cooled at the cold stage of the probe station before contacting the substrate under test so as to preserve a sample temperature below 4.5 K.
- Electrical contacts should be low resistance with a best-effort goal of 10 milliOhm contact resistances.
- Electrical contacts should also be high-density with a best-effort goal of center-to-center pitches as small as 150 mm. Metallic contact features on the substrate are expected to be as small as 100 mm in diameter. The mechanical pattern of the contacts can be fixed so long as it is reconfigurable via use of alternate probe cards.
- Electrical contacts to be compatible with signal bandwidths below 500 kHz.
- Mechanical provisions to move the electrical contacts over the complete substrate under test while cold in order to probe multiple identical contact patterns on the substrate.
- Optical or other provisions to align the electrical connections to the contact pattern on the substrate.
- Provisions at room temperature to perform basic electrical measurements (continuity, current-voltage curves, etc.) among any combination of the electrical contacts.
Phase I expected results:
Develop a mechanical and electrical design that addresses the project goals described above.
Phase II expected results:
Construct a prototype high-density cryogenic probe station that is able to achieve the project goals described above.
NIST personnel will be available to assist the awardee in a variety of ways, including but not limited to:
- consulting on instrument design including participation in design reviews,
- sharing the results of NIST research to develop high-density probe cards suitable for cryogenic applications, including prototypes, with the awardee, and
- fabricating and providing at no cost substrates up to 150 mm in diameter with metal test patterns that can be used by the awardee to demonstrate cryogenic probing.
References:
[1] Applications for superconducting electronics: http://www.scientificcomputing.com/news/2014/12/iarpa-develop-superconducting-supercomputer.
[2] http://arxiv.org/pdf/1309.5383v3.pdf (for the motivation for arrays of 500,000 superconducting sensors).
[3] Examples of existing cryogenic probe stations: http://www.lakeshore.com/products/cryogenic-probe-stations/pages/cryogenic-probe-stations.aspx.
[4] Micro-manipulated Cryogenic & Vacuum Probe Systems for Chips, Wafers and Device Testing from ~3.5 K to 675 K, http://www.janis.com/ProbeStations_Home_KeySupplier.aspx.
[5] Examples of existing high-density probe cards for room temperature operation: https://www.cmicro.com/products/probe-cards.
[6] Cantilever Probe Cards - Well Beyond the State of the Art, http://www.technoprobe.com/cantilever-probe-card/.
Any mention of commercial products is for information only; it does not imply recommendation or endorsement by NIST.
