# Presentation: 516563

## Gary Swift

**Gary Swift**
**Jet Propulsion Lab**
**California Institute Of Technology**

**Doug Millward**
**Millward Research**

**(not pictured)**
**Henry Clark**
**Cyclotron Institute**
**Texas A&M University**

- 9/28/01

## Single-event testing is always conducted by “beaming” through a “dead” layer of material, albeit normally quite thin (passivation and metal layers).  In this paper, we consider testing by irradiating through thicker layers of material.  This may be out of necessity (e.g., when a DUT is in “flip-chip” packaging), due to practical issues (e.g., in the case of SDRAMs whose lead frames require extreme effort in re-packaging often with low yield to enable short-range top-side irradiation), or for simple convenience (such as using beam degraders to adjust LET’s, instead of or in addition to changing the angle of incidence.)

## Modern microprocessors are only available in packages that present huge logistic problems for SEU testers.  Getting the ions into the active region with known LET can really only be done through the silicon substrate on “backside up” packaged devices, so called “flip-chips”, because, even if ions with a long enough range are available, the bump bonds and the solder balls of ball-grid array (BGA) packages make for an extremely lumpy pattern of many material densities.  Further, the backside of the silicon can be thinned greatly without impacting the electrical characteristics.  Photo 1 shows an example of a working “G4” processor thinned to 100 microns.  That the charge collection characteristics are also unchanged is expected because the use of a thin-epitaxial layer is almost imperative for advanced processors.  Figure 1 compares an example set of results obtained from three thickness’ of Motorola XPC750.  This confirms that, for this device, the charge collection is not significantly affected by the removal of more than 90% of the silicon bulk.

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## Mechanical thinning can be quite precise, resulting in a fairly uniform thickness of Silicon (±1μm) but the accuracy of gauging the thickness is usually over an order of magnitude worse.  For DUT with a large number of upsettable bits, it is straightforward to use the beam as the depth gauge. Statistically significant numbers of upsets are counted for a range of incident energies surrounding the Bragg Peak, as shown in Figure 2a, for an IBM power PC750 that was thinned to 120 μm.  For the incident energy of the cross-section peak (2040MeV), the range is 170 μm.  Subtracting the range of the Bragg peak of 33 μm results in a thickness of 137 μm.  A range base correction technique is illustrated in Figure 2b, using the nominal thickness of 120 μm.  The peak upset rate corresponds to a residual range of 55 μm, but the range of ions at the Brigg peak is about 33 μm.  The difference of 22 μm is added to the nominal 120 μm for a resulting thickness of 142 μm which is clearly more accurate.

- Mechanical thinning can be quite precise, resulting in a fairly uniform thickness of Silicon (±1μm) but the accuracy of gauging the thickness is usually over an order of magnitude worse.  For DUT with a large number of upsettable bits, it is straightforward to use the beam as the depth gauge. Statistically significant numbers of upsets are counted for a range of incident energies surrounding the Bragg Peak, as shown in Figure 2a, for an IBM power PC750 that was thinned to 120 μm.  For the incident energy of the cross-section peak (2040MeV), the range is 170 μm.  Subtracting the range of the Bragg peak of 33 μm results in a thickness of 137 μm.  A range base correction technique is illustrated in Figure 2b, using the nominal thickness of 120 μm.  The peak upset rate corresponds to a residual range of 55 μm, but the range of ions at the Brigg peak is about 33 μm.  The difference of 22 μm is added to the nominal 120 μm for a resulting thickness of 142 μm which is clearly more accurate.

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## By de-packaging and re-bonding, DRAMs can be irradiated in the “normal” front side fashion, as well as through the backside.  Figure 3 shows a comparison of front side results (Brookhaven ions) to backside irradiation results obtained using ions from Texas A&M cyclotron.

- At some point, thinning can be taken too far and affect charge collection characteristics.  DRAMs typically are fabricated on bulk wafers and their upsets result from total charge collected over relatively long times.  Thus, they are excellent choices for investigating the limits of the thinning technique.  Figure 4 compares measured SEU cross sections in the LET threshold region for Samsung 64Mb devices of four different thickness.  It is clear that the 50 micron device is not collecting as much charge, yielding an incorrectly low set of cross sections.

## The Texas A&M cyclotron produces the very long-range ions needed for upset testing through thick materials.  Particularly, the 25 and 40 MeV/amu beams are quite penetrating. The facility takes advantage of this in two ways.  First, it is practical to do irradiations in air rather than in a vacuum.  Contrast the more difficult set up of the PowerPC test board in the vacuum chamber (shown in Photo 2) with that of the in-air, shown in Photo 3. Second, fewer beam changes are needed (saving time and money) because degraders are used to extend the LET coverage because of ions at normal incidence.  For example, the 3.2 GeV 129Xe ion has an undegraded LET of ~39 MeV per mg/cm2, but the judicious use of degrader foils extends the normal incidence LET to above 60.  By rotating the foils to change their effective thickness, the LET is continuously adjustable.  

- The Texas A&M cyclotron produces the very long-range ions needed for upset testing through thick materials.  Particularly, the 25 and 40 MeV/amu beams are quite penetrating. The facility takes advantage of this in two ways.  First, it is practical to do irradiations in air rather than in a vacuum.  Contrast the more difficult set up of the PowerPC test board in the vacuum chamber (shown in Photo 2) with that of the in-air, shown in Photo 3. Second, fewer beam changes are needed (saving time and money) because degraders are used to extend the LET coverage because of ions at normal incidence.  For example, the 3.2 GeV 129Xe ion has an undegraded LET of ~39 MeV per mg/cm2, but the judicious use of degrader foils extends the normal incidence LET to above 60.  By rotating the foils to change their effective thickness, the LET is continuously adjustable.
- However, this raises a concern about beam straggling affecting the results, but, while energy straggling may be large, LET straggling most definitely is not; see Figures 5a & 5b.
- Another concern is beam uniformity, since the cyclotron attains it by de-focusing.  Figures 6a & 6b show what is achievable.

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## Example results presented here show the successful application of the use of degraded heavy ion beams and demonstrate some of their advantages for SEE testing.  Quality data requires that care must be taken to assign LETs correctly and to avoid thinning to the point that charge collection is affected.