RADIATION TESTING
SDI'S MODEL 1527 ACCELEROMETER

Summary

This report documents tests conducted on Silicon Designs, Inc. (SDI) Model 1527 MEMS accelerometers to determine their sensitivity to total dose gamma radiation.  The Model 1527 was chosen for this test is because this one accelerometer model, once qualified, can perform the functions of all SDI analog accelerometers.  In particular the Model 1527 has the performance needed for tactical navigation, but it also shares the same architecture as SDI’s Model 1521, which was previously built and qualified for a military space application.

 

SDI develops and manufactures miniature accelerometers useful in a wide range of military and commercial systems, including measuring shock, vibration, impulse and other changes in velocity of aircraft, spacecraft and weapons.  SDI accelerometers are used on more than 13 US and NATO missiles, and the Model 1527 can be made in full scale ranges of 2, 5, 10, 25, 50, 100, 200 and 400g, all of which are expected to have similar performance after exposure to gamma radiation and are pin compatible allowing easy range changes during development.

 

The tests consisted of six steps:

  1. Fabricate a lot of 1527 accelerometers using current standard production methods;

  2. Measure their pre-radiation performance;

  3. Divide the units into six groups of three parts each;

  4. Expose each group at Oregon State University to six different total-dose levels of gamma radiation: 10K, 25K, 50K, 100K, 200K and 350K rads (Si);

  5. Measure the post-radiation performance of the Model 1527 accelerometers for general instrumentation uses; and

  6. Measure the post-radiation performance of the Model 1527 accelerometers for tactical navigation uses.

 

Three tests were intended to concentrate on measuring the adverse effects of gamma radiation to the accelerometers.  First, the standard production calibration test was run to measure each unit’s normal characteristics as general-purpose instrumentation accelerometers.  The second test evaluated each unit’s performance for navigation use at +1 and -1g to calculate bias and scale factor shifts, measured its output for eight hours at a constant 30°C temperature.  The third test measured each accelerometer’s output at +1 and -1 g over a temperature scan from 25 to 85°C, then to -40°C, and then back to 25°C to calculate bias and scale factor over temperature.  Plots of the output compensated for temperature were then generated to compare with the actual applied accelerations.

 

The test results show that all the units met all requirements as instrumentation accelerometers before and after radiation.  The two navigation performance tests show that the scale factor is only slightly affected by radiation.  All but one of the units showed a bias that remained within specification for all radiation levels.  Only one unit produced a bias shift of approximately 6 mg, about 0.024% of full scale, after 350K rads of radiation, and the unit retained its prior performance for tactical navigation.

Objectives

Accelerometers are important sensors used to measure changes in motion, shock and vibration for many applications. In space systems they are vital to detect and measure rocket firing impulse, determine velocity changes for guidance and navigation, detect the direction and magnitude of gravity or centripetal forces, measure vibration and shocks, and detect stage sequencing.  They must be robust to operate in the harsh radiation and thermal environment of space and after nuclear events.

 

Electronic parts are sensitive to a range of high energy particles in space that penetrate many materials.  It is important to identify those parts early in a project that are likely to operate in spite of the environments.  Small, light-weight accelerometers commonly used for commercial applications are generally not suitable for such applications.  The objective of this report is to describe the radiation test performed on an accelerometer model, similar to another model previously used in such applications, to determine if the current accelerometer model will likely operate after exposure to various levels of gamma radiation expected in future projects.

 

In 2016, Orbital Sciences tested SDI’s Model 1521 general-purpose accelerometers for radiation hardness, and SDI built some special units for their use, presumably, in GMD. However, Orbital did not share the test results, the radiation levels tested to, or any changes caused by the radiation.  Although some of SDI’s accelerometer components have changed in the last six years, this testing led SDI to believe that its newest products should have the same level of radiation hardness.  The objective of this test was to determine if this belief is well founded.

Test Plan

  1. Fabricate a lot of 1527 accelerometers using standard production methods;

  2. Measure their pre-radiation performance;

  3. Divide the parts into six groups of three units each;

  4. Expose each group at Oregon State University to six different total-dose levels of gamma radiation: 10K, 25K, 50K, 100K, 200K and 350K rads (Si);

  5. Measure the post-radiation performance of the accelerometers for general instrumentation uses; and

  6. Measure the post-radiation performance of the accelerometers for tactical navigation uses.

 

Test Procedure

A lot of approximately 35 production Model 1527-025 accelerometers were built using the latest traveler and procedures.  The 25g range unit was selected because it is the range most commonly used in military applications.

 

All parts were screened in accordance with SDI’s standard production processes used for both 1521s and 1527s.  The first 24 parts in serial number sequence that passed all production tests were selected for radiation testing.  The standard production testing on a computer-controlled shaker system is used on all of SDI’s production parts to verify that the parts meet SDI’s specified performance for instrumentation applications. 

 

All 24 parts were then screened for navigation performance with two additional tests in a temperature tumble test system, recording each unit’s output.  First, the parts were run for eight hours at 30°C, periodically rotating the parts through +/- 1g positions.  Second, the parts spent four hours sweeping over a -40°C to +85°C range, periodically rotating between +/- 1g positions.  The purpose of these two tests is to verify that all units met the requirement for navigation.  All of the parts passed these tests.

 

The parts were then allocated to the six different radiation levels in sequence, in four-unit groups, with three units for testing as individual accelerometers and the fourth unit assembled into SDI’s Model 2227 Q-modules, which are configured for applications utilizing industry-standard quartz closed-loop accelerometers, the results of which will be analyzed later.

 

The six groups of three Model 1527 accelerometers and one Model 2227 Q-module accelerometer were packaged each in 2½ x 2½ x ½-inch static-resistant plastic boxes sandwiched between two layers of black conductive foam, and sealed with tape, which were then taped to cardboard rectangles (and labeled with the test level) so as to position the parts near the center of the Oregon State University (OSU) Gamma-cell test chamber.

 

The plastic boxes were sent by common carrier to OSU, where each box was irradiated to a different amount of gamma radiation based on the time in the Gamma-cell.  The levels applied were 10K, 25K, 50K, 100K, 200K and 350K Rads (Si).  Each group of parts was exposed individually at a dose rate of 348.45 K Rads per hour, with the total dose determined by the amount of time each group remained in the test chamber. The units remained sealed in the plastic boxes at all times while out of the SDI facility.  The unopened boxes were then returned to SDI.

 

When received, the boxes were opened and the parts were again tested using the same program and test stations as before where possible.  The same analysis was performed on the pre- and post-radiation results and compared with the original results.  NOTE: The pre-radiation temperature tests were declared a no-test because the A/D converter was replaced on the test system between the pre- and post-radiation testing.  

Results

All accelerometers passed the production room-temperature test conducted on the computer-controlled shaker, showing that all the parts met all requirements as instrumentation accelerometers before and after radiation. 

 

While no parts failed during the post-exposure testing, small changes to behavior were identified: 

  • The tests showed that the radiated accelerometers may have developed a small increase in current consumption. 

  • Some parts exposed to 200+ K Rads appear to have developed about 1 mg of temperature hysteresis.  

  • The bias, scale factor, linearity and frequency response appeared to be generally unaffected by gamma radiation up to 350K Rads, with bias showing only a minor shift. 

 

The bias/scale factor repeatability tests show that the scale factor is only slightly affected by radiation.  All but one of the parts showed a bias that remained within specification for all radiation levels.  After 350K rads of radiation one part produced a bias shift of approximately 6 mg, about 0.024% of full scale.

 

Temperature tumble tests found that all the parts met the requirements for tactical navigation after radiation at all levels.  The one part with a 6 mg bias shift, after the bias correction was made the part, recovered its prior performance for tactical navigation.

 

See the Appendices to view the production test reports for all 18 parts.

Conclusions

Preliminary radiation testing found that the SDI Model 1527 MEMS accelerometers are a good candidate for instrumenting shock and vibration measurements during tests and for arming and fuzing operations with the potential of gamma radiation.  In addition, the Model 1527 accelerometers can be considered reliable before and after exposure to gamma radiation for measuring varying accelerations to compute velocity changes for tactical navigation.

​SDI's Accelerometer Design

The current accelerometers are designed for ruggedness, reliability and technical performance over a military temperature range, -55 to 125 degrees C.  Space applications and new missiles for hypersonic offensive and defensive weapons now need also to operate in severe temperature environments. 

 

All SDI MEMS accelerometers are about 9 mm on a side and 3 mm high with a mass less than 1g, consisting of the following six major components: 1) one of eight different silicon sense elements that determine the g-level, 2) one of four similar CMOS application-specific integrated circuits (ASICs) that determine the output, accuracy and stability, 3) one of three different 20-pin ceramic chip carrier packages based on the intended application, 4) either gold or aluminum wire bonds, 5) a solder-sealed, gold-plated kovar lid and 6) an adhesive.  The sense element and ASIC are the main accelerometer components.

 

Closed-loop accelerometers can offer excellent bias stability, good linearity and low noise. However, many closed- and open-loop designs, are designed for one or two specific ranges of operation. When fielded in applications with peak accelerations less than the designed range, their gain is often increased, giving instability and noise higher than it would be if the accelerometers were redesigned for that g-level. Better system performance can be obtained from a family of interchangeable models of accelerometers, each of which is optimized for a particular g-level and together cover a wide range of g-levels needed for navigation.  The optimum g-range of SDI accelerometers can be modified for different hypersonic or other military applications by simply using sense elements for that range.  SDI makes pin-compatible accelerometers with full scale ranges of 2, 5, 10, 25, 50,100, 200 and 400g, and it can make optimum units for any specific value between 2 and 400g for special applications.

 

On each sensor dice are located two pendulous, capacitive, MEMS sense elements. Each sense element consists of a flat plate of silicon supported above the substrate surface by a rugged but sensitive flexure attached to a central pedestal.  The structure is asymmetrically shaped so that one side is larger than the other, resulting in a center of mass that is offset from the flexure axis. Some g-levels have an additional mass on the larger end. When an acceleration perpendicular to the substrate produces a moment around the flexure axis, the plate is free to rotate around that axis, constrained only by the tensional spring constant of the flexure.

 

On the substrate beneath each sense element are two equal-sized capacitor plates, one on each side of the flexure axis. The sense element structure and the air gap with the equal-sized capacitor plates on the substrate form two nominally equal, differential, variable capacitors, with the sense element forming a common connection. Under acceleration the average distance between the sense element and one surface plate decreases, increasing its capacitance, while the distance to the other plate increases, decreasing its capacitance. The two sense elements on the sensor die, each with their two differential capacitors, together form a fully active capacitance bridge. Sense elements are about 2200 by 1000 microns in size and about 10 microns thick. The sense elements are spaced about 8-12 microns from the fixed capacitor plates on the substrate, forming a capacitor with the sense element to each plate of about 150 femtofarads. For different g-levels the sensitivity of the sense elements, the ratio of deflection to acceleration, is determined mainly by the flexure stiffness and the substrate spacing. Squeeze-film damping is controlled by the substrate spacing and the holes in the sense element control the damping ratio. The number and spacing of holes are designed to give near critical damping. The two identical sense elements are laid out rotated 180 degrees relative to each other to convert any cross-axis acceleration into a common mode signal that is cancelled out in the ASIC.

 

The ASIC is designed and currently built with a bulk CMOS process. SDI’s ASICs are designed for minimum risk of latch-up, and their relatively large gate lengths (1-5 microns) make them relatively insensitive to single event bit upset.  If it does occur, however, they are designed to recover quickly (in less than 100 microseconds).

 

To achieve the ASICs high stability, the ASIC architecture also takes advantage of fully differential and chopper-stabilization techniques, that cancel out much of the drift, temperature effects, component mismatch and 1/f noise present in all silicon parts.

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