Renesas White
Paper Specialty
Products Spaceand Defense

Wafer by Wafer Low Dose Rate Acceptance Testing in aProduction Environment

Abstract—This White Paper describes technical details of a wafer by wafer low dose rate
acceptance testing program being implemented for all Intersil radiation hardened products. We
also discuss the design and construction details of a vault-type 60Co irradiator facility currently
on line at the Intersil Palm Bay facility to support these acceptance testing activities.

July 2011
Revised 23 August 2011

I. INTRODUCTION
The low dose rate ionizing dose response of semiconductors has become a key issue inspace applications. Routine 60Co acceptance testing at high dose rate has been demonstrated
to be nonconservative for some technologies, and is increasingly being regarded as a classicalaccelerated test that has transitioned into a nonlinear range. The actual dose rate in space ismany orders of magnitude below these acceptance testing levels. Intersil is addressing thischanged market using a three-phase approach.
First, a low dose rate testing program of thirteen selected legacy parts was initiated in 2009
[1, 2] and has now been concluded. These tests were performed in accordance with the MILSTD-883 Test Method 1019.7 diagnostic protocol. In all cases the experimental matrix consistedof five samples irradiated at high dose rate with all pins grounded, five samples irradiated athigh dose rate under bias, five samples irradiated at low dose rate with all pins grounded, fivesamples irradiated at low dose rate under bias and one control unit. The majority of the Legacy
parts being tested under this program are implemented in dielectrically isolated complementarybipolar processes. Two of the Legacy ELDRS tests were performed on HS26CxRH parts, which
use junction-isolated (bulk) epi CMOS technology. At this time all thirteen tested parts havecompleted 150krad(Si) at low dose rate, and reports are posted on the Intersil Web site [3].
As a second phase, all new development parts such as the ISL70001SRH [4] and theISL7884xASRH [5] are now characterized at both high and low dose rate as a routine part ofthe product development. These parts use a 0.6um bulk BiCMOS power management processand represent an extension of the Intersil RHA product line into much higher levels ofintegration. Both parts were found to be ELDRS-free, and reports summarizing the results forthese two products are posted on the Intersil Web site [3] for ready reference.
As a third step, Intersil is introducing wafer by wafer low dose rate acceptance testing as acomplement to current high dose rate acceptance testing. A vault-type 60Co low dose rateirradiator to support this work has been completed at the Intersil Palm Bay facility. The balanceof this note discusses the design and construction of this facility and the test methodologies tobe used for acceptance testing work.
II. CURRENT HIGH DOSE RATE QUALIFICATION PROCEDURES
Current Intersil high dose rate total dose acceptance testing is performed on a wafer bywafer basis. This procedure provides RHA assurance at the wafer level, which is the most basic
level of the IC fabrication process. Following probe and die separation, sample die are selected
at random from each wafer; the sample size is based on transistor count (4 die/wafer for chips
with less than 4000 transistors or 2 die/wafer for chips with greater than 4000 transistors) as
outlined in MIL-STD-883, TM 5005, Group E, Subgroup 2. The samples are assembled andcharacterized using the production automated test equipment (ATE) and are then irradiated athigh dose rate to the maximum level specified in the applicable Standard Microcircuit Drawing(SMD). The irradiation is performed under bias. A second round of ATE testing is thenperformed, at room temperature only. Any rejects in the sample fail the wafer.
High dose rate testing at the Palm Bay, Florida Intersil facility has historically been
performed using Gammacell 220 60Co irradiators. Initial low dose rate testing at Intersil wasperformed using a J. L. Shepherd and Associates model 484 research 60Co low dose irradiator,
which is not suited to volume long-term testing. Low dose rate testing has also performed on anoutsourced basis at White Sands Missile Range SVAD (White Sands, NM).
An extension of the current high dose rate wafer acceptance procedure to the low dose rate
regime is not straightforward. Current high dose rate tests are performed to a level of300krad(Si), in nearly all cases, and such a test can be performed in an afternoon. An
equivalent 300krad(Si) test at 0.01rad(Si)/s takes some 347 days. The current low dose ratelevel of general interest to the user and research communities is in the 50 to 100krad(Si) range.
An initial decision was thus made to acceptance test all wafers to 50krad(Si) at low dose rate,
which strikes a balance between TID level, test duration and cost. Note that all products will beacceptance tested at low dose rate, including not only bipolar and BiCMOS products that areconsidered to be an ELDRS risk, but CMOS parts as well.
Since the exposure time at 50krad(Si) at .01rad(Si/s is some 60 days, roughly 40 tests mustbe run in parallel on average for current production volumes, requiring a large isodose test
volume irradiator. Supporting this large number of parallel tests required extensive test
automation, both from the scheduling and power supply control and sequencing standpoints.
To support the low dose rate wafer level RHA acceptance testing program, Intersil has
completed the construction and activation of a vault-type 60Co irradiator and associatedequipment and fixturing.
III. LOW DOSE RATE IRRADIATION FACILITY
Construction and activation of the Intersil Palm Bay, Florida low dose rate facility iscomplete. The prime contractor was Hopewell Designs, Inc. (5940 Gateway Drive, Alpharetta,GA 30004; www.hopewelldesigns.com). Figs. 1 - 3 provide conceptual CAD drawings of thefacility. The vault structure has 36” high-density concrete walls and a 48” roof, both constructedof high-density concrete. This shielding structure provides an estimated 10,000x reduction indose rate measured at 11 feet from the pop-up source.

Fig. 1: CAD drawing of low dose rate irradiator vault floor plan showing the vault and source,
concentric fixturing, the labyrinth entrance and the adjacent control room. Doors are not shownfor clarity. The inside dimensions of the vault are 15 x 15 feet (5 x 5m).

Fig. 2: Conceptual CAD drawing (not to scale) of room layout showing the central ‘pop-up’ 60Co
source and the approximately spherical device under test (DUT) board mounting structure. The
source has a lower storage position in a 12” welded stainless steel well below grade. The 36”
concrete walls and 48” roof provide a 10,000x estimated reduction in outside exposure rate.

Fig. 3: Conceptual CAD drawing of two segments of the DUT board mounting system. The board
holders are arranged in a partial sphere surrounding the 60Co source. All DUT boards are
enclosed in individual PbAl spectrum hardening boxes to filter out secondary radiation. Note thatthe final version of this design has four DUT boards per arm for a total DUT board capacity of 64.

The radiation source consists of 50 Curie (Cy) of 60Co radioisotope in an air-actuatedaluminum ‘pop-up’ pellet. The pellet is housed in a stainless steel tube, which is in turn centrally
located in a 12” welded stainless steel well (Fig. 2). The recurved SS tube minimizes radiation
leakage while the source is in the ‘down’ position. The exposure rate inside the concrete vault
with the source in the shielded (‘down’) storage position is estimated at 0.1mR/hr. Estimatedexposure rate at the exterior of the vault is 0.45mR/hr while the irradiator is in operation. The
design field uniformity within the irradiator test volume is +/-10%, while the calculated uniformity
is +/-2.5%. The design dose rate is 0.01rad(Si)/s (10mrad(Si)/s) at the device under test (DUT)
board location. The separate source pellet design greatly simplifies source replacement.
The DUT boards are supported on aluminum structures; the horizontal distance between thesource and the board location is designed to be changed to maintain the required dose rate as
the source loses activity. The DUT board fixturing (Figs. 3, 7 and 8) is designed toaccommodate a total 64 9”x12” PC boards. All DUT boards are enclosed in a spectrum
hardening PbAl box to filter out secondary radiation. The fixturing system approximates anisodose sphere centered on the source pellet and consists of sixteen ‘wings’ each supportingone PbAl box enclosing four DUT board holders. Design calculations indicate that a new 50
Curie source will result in a dose rate of .0097rad(Si)/s at 48”, while a 22.5Cy source(representative of a 50Cy source after 5 years of decay) will result in the same dose rate at 36”.
Predicted total dose uncertainty was calculated at +/- 3% or better, depending on datacollection tools. All irradiations are overtested by 3% to compensate for this uncertainty.

Operator safety is a key issue in the design of any irradiation facility. All doors haveinterlocks and position sensors, and the facility is operated entirely from the control room.
Emergency stop buttons are provided, which will immediately drop the source into the shielding
well. Loss of power or air pressure, actu
actuation
ation of an emergency stop button or any attempts toopen any of the doors will drop the source and force the system into a safe state. All systems
including biasing power supplies and control computers are operated from dedicateduninterruptible power supplies (UPS), with long-term power outages (recall we’re in a hurricanezone) handled by on-site Diesel power generating facilities. Hopewell Designs have a long
history of building reliable and robust systems. Figs. 4 - 5 show images of the irradiation facility
taken during construction, while Fig. 6 shows the completed facility.

Fig. 4: Site excavation before pouring the vault foundation. The top of the source well is clearlyvisible to the left of image center. The PVC piping provides cable raceways between the vault
and the Intersil Palm Bay building 58 control room (right).

Fig. 5: View of wall forms before the second (wall) concrete pour. The labyrinth vault accessarchitecture can be clearly seen.

Fig. 6: External view of the completed low dose rate irradiator.

Fig. 7: View of ‘pop-up’ source (center) and surrounding test racks. The vertical test board holder
and their PbAl spectrum hardening filter boxes are shown. The holders are on rails and can bemoved towards the centrally located source as it loses activity. The curvature of the DUT rack is
not apparent as the rack is hidden by the PbAl box. The system supports 64 simultaneous tests.

Fig. 8: Internal view of a PbAl box and enclosed DUT board rack. The curvature of the test rackcan be clearly seen; this curvature combines with the circular arrangement of the racks togenerate an approximately spherical isodose profile at the DUT boards.

Control and scheduling of 64 two-month acceptance tests (maximum) in parallel while
taking source activity into account would be problematic at best. The Hopewell irradiator
design puts most operations under computer control. The operator continues to have fullcontrol over the system, but is able to focus on areas where expertise is most needed. The
automated irradiator control system is based on the National Instruments LabVIEW
environment. Features include automatic calibration and calibration report generation, dose
rate calculation and source decay correction. The irradiator calibration procedure uses an ionchamber and electrometer and is fully automated.
The irradiator has 64 board positions available, each of which can have up to 8 different
power supply voltages. Control and tracking of exposure time, total dose and dose rate arevitally important. Each DUT board is uniquely identified by a bar-coded label which enables thecontrol computer to track all boards while in the chamber. Each DUT board socket is verified
as meeting the ELDRS irradiation diagram (SMD or internal specification) before each new lot
is loaded, and DUT board supply currents are recorded before initiating irradiation.
Other capabilities are system status, system safety and interlock status, exposure status
and source position. Exposure rate and distance are linked so that when one is changed, theother is calculated and updated. Exposure and time are also linked. When an exposure rate isspecified, the system calculates a position, automatically compensating for source decay.
Products with multiple power supply requirements typically require individual power supply
power-up and power-down sequences. A dedicated power supply control computer determines
and controls the supply voltages and power sequencing for the individual board. The power
supply control computer records voltages and currents to insure correct system operation. Figs.
7 - 8 show the centrally located ‘pop-up’ 60Co source and DUT board fixturing detail.

IV. LOW DOSE RATE WAFER ACCEPTANCE TESTING PROCEDURE
The Intersil low dose rate acceptance testing procedure is similar to the current high dose
rate procedure. Four sample die are drawn from each probed wafer and are processed throughthe entire high-reliability Class V screening sequence. TM 5005 allows a sample size of 2 forparts with greater than 4000 transistors, but as both biased and unbiased tests are performed,
this would reduce the effective sample size to 1; hence, a uniform sample size of 4 is used forall products. The samples (2 die are biased and 2 die are grounded) are then irradiated to
50krad(Si) at 0.01rad(Si)/s. A second round of room temperature ATE testing is thenperformed, and any rejects fail the wafer.
It is important to note that all Intersil RHA parts will eventually be qualified using thisprocedure. This reflects the increased emphasis on low dose rate performance within ourcustomer base and also reflects recent results [6, 7] suggesting low dose rate sensitivity inadvanced shallow trench isolated CMOS processes. The existing high dose rate acceptancetesting procedure will continue, with a decision point some three years in the future when thecurrent Gammacell 220 will go out of dose rate specification.

As a part of the low dose rate qualification, Intersil will provide an enhanced data packagethat will accompany each shipment of QML class level V low dose rate qualified product. Thedata package will include an updated Radiation Effects Certificate of Compliance (C of C) for
high and low dose rate testing. It will also provide high and low dose rate room temperature
variables data for pre and post irradiation tests.
V. CONCLUSION
This document discusses high dose rate total dose wafer acceptance testing at Intersil andthe changes to these procedures necessitated by increased emphasis on low dose rate
performance at the system level. We also discuss planning for wafer acceptance testing ofIntersil parts at low dose rate, and the design and construction of a vault-type 60Co irradiator tosupport these plans.

VI. REFERENCES[1] N. W. van Vonno, L. G. Pearce, J. S. Gill, E. T. Thomson, and P. J. Chesley, ‘Low Dose Rate
Testing Results of Legacy Intersil Products’, presented at the 2010 Radiations et ses Effets sur
Composants et Systemes (RADECS) Workshop, Längenfeld, Austria (September 2010).
[2] N. W. van Vonno, L. G. Pearce, J. S. Gill, E. T. Thomson, and P. J. Chesley, ‘Results of Low DoseRate Testing of Legacy Intersil Products’, presented at the 2011 IEEE Radiation Effects Data Workshop(July 2011).
[3] http://www.intersil.com/space/ELDRS_test_report.aspx[4] N. W. van Vonno, L. W. Pearce, H. W. Satterfield, E. T. Thomson, A. P. Williams, T. E. Fobes, P. J.
Chesley and J. S. Gill, ‘Total Dose and Single Event Testing of a Hardened Point of Load Regulator’,2010 IEEE Radiation Effects Data Workshop Record (2010).
[5] N. W. van Vonno, L. G. Pearce, G. M. Wood, J. D. White, E. T. Thomson, T. M. Bernard, P.
Chesley and R. Hood, ‘Total Dose and Single Event Testing of a Hardened Single-Ended Current Mode
PWM Controller’, 2010 IEEE Radiation Effects Data Workshop Record (2010).
[6] S. T. Witczak, R. C. Lacoe, J. V. Osborn, J. M. Hutson and S. C. Moss, ‘Dose Rate Sensitivity ofModern nMOSFETs’, IEEE Trans. Nuc. Sci., vol. 52, no. 6 (December 2005).
[7] A. H. Johnston, R. T. Swimm and T. F. Miyahira, ‘Low Dose Rate Effects in Shallow Trench Isolation
Regions’, IEEE Trans. Nuc. Sci., vol. 57, no. 6 (December 2010).

Nick van VonnoJuly 2011
Revised 23 August 2011