Attachment WP5_TlBr_Production_Ext_FINAL.pdf

White Paper #5 on TlBr Production (External):

Production: Supply Chain and Scaling-Up Production of Material and Crystals for TlBr

(Version: 200825)

This white paper summarizes the supply chain and what it would take to scale-up production of crystals in order to provide enough to produce 300 pixelated Radio-Isotope Identification Device (RIIDs) per year as well as 300 position-sensitive capacitive Frisch Grid (PS-CFG) RIIDs per year.

To achieve this production level, RMD plans to purchase a few additional items of equipment, as described below, to supplement RMD’s already significant facilities for TlBr. Current RMD staffing is sufficient for this production level. Labor requirements for each process step are also described below.

Figure 1-Left shows the packaged TlBr pixelated-array and Fig 1-Right shows a single PS-CFG crystal on the right on top of the “carton” that holds up to 16 wrapped PS-CFG crystals. A Pixelated RIID design would probably use two of the packages on the left of Figure 1, whereas the CFG design would only require one of the packages on the right. The size of the packaged pixelated array is 25 mm x 25 mm x 29 mm. The frame that holds the CFG devices in the picture on the right is 46 mm × 46 mm x 25 mm.

Both styles of packaging will probably be modified (for size and ease of use) by the time they are provided as GFE.

Figure 1. Left: Photograph of packaged TlBr pixelated array, 25 mm × 25 mm × 29 mm, next to a quarter as a size reference. The top face shows the connections for the bias voltage, while the connectors for the pixel array are on the bottom, not seen in the photograph. Note that two of these packages would probably be used in a RIID. Right: Photograph of a single “wrapped” PS-CFG crystal next to a dime, as a size reference, on top of the “carton” that holds 16 wrapped PS-CFG crystals, with the carton having a dimension of 46 mm × 46 mm × 25 mm. Note that only one of these packages would probably be used in a RIID.

1. Supply Chain and Materials

RMD currently purchases high purity TlBr material from multiple vendors. The material is often purchased in lots of 4 kg, which is enough material to make two ingots (see below regarding ingots);

each ingot weighs approximately 2 kg. The lead-time is 4 to 8 weeks for all vendors for this type of order size. The prices for the selected vendors are within 10% of each other. RMD maintains enough safety stock to have smooth and continuous crystal growth operation.

In production, TlBr is grown into ingots. An ingot is an approximately 30-cm long cylinder with a diameter of approximately 3 cm. The ingot is transversely sliced into disks, referred to as “slices”, with a diameter of approximately 3 cm.

Pixelated Version:

Each ingot will produce up to twenty 1-cm-thick slices, after polishing, with each slice capable of producing a 2 cm × 2 cm pixelated TlBr array. Before polishing, the thickness is approximately

1.2 cm for the arrays.

CFG:

Each ingot will produce up to seventeen 1.2-cm-thick slices, after polishing, with each slice capable of producing about 6 CFG crystals. Before polishing, the thickness is approximately

1.4 cm for the PS-CFGs.

RIIDs per Year:

Pixelated:

To fabricate 300 Pixelated RIID units in a year (with two pixelated arrays per RIID unit), RMD estimates the need to fabricate approximately 30 ingots, which is approximately three ingots per month, depending on yields. The fabrication of ingots requires ~8 kg of TlBr per month, or ~100 kg of TlBr material per year.

CFG:

To fabricate 300 PS-CFG RIIDs units in a year, where each RIID requires sixteen 0.5 cm × 0.5 cm ×

1.2 cm TlBr crystals, would require the fabrication of an additional ingot per month.

Other consumable materials used in the production of TlBr crystal ingots are readily available in good quality from standard commercial suppliers.

2. Crystal purification and fabrication

RMD currently has two zone refining (ZR) systems configured for TlBr purification and crystal growth, shown in Figure 2. RMD plans to bring on-line two additional zone refining systems in the same fume hood enclosure to enable the required number of ingots to be produced on a monthly basis. The preparation of the charge and zone refinement and growth of the crystal takes approximately 3 to 4 weeks with the current process. Thus, the throughput of each zone refining system is about 10-12 ingots per year.

Figure 2. Left: Photograph of horizontal furnaces in the fume enclosure. The top left shows a furnace with an ingot of TlBr, while the bottom left shows two furnaces ready for loading. Right: the fume enclosure;

the furnace controllers in the electronics cabinet are out of the frame of view (not seen) at the right of the photograph.

As part of the scale-up effort, RMD anticipates modifying the process to improve the yield and reduce the costs. These modifications include reuse of material and implementing growth techniques to reduce the number of grain boundaries.

Pixelated:

As mentioned above, RMD currently anticipates that each ingot will yield approximately 20 crystals for pixelated arrays to provide 600 crystals per year for fabricating 300 RIIDs.

CFG:

For CFG RIIDs, RMD anticipates that each ingot will yield sixteen 14-mm thick slices, where each “slice” produces 16 to 20 PS-CFG crystals. The smaller area of the PS-CFG crystals improves the yields with ingots containing grain boundaries. This would be adequate for supplying 300 CFG RIIDs per year.

3. Sensor Fabrication (crystal dicing, electrode deposition, and annealing)

After crystal growth, the ingots are cut to the desired dimensions for either pixelated arrays or CFG detectors. RMD has two active wire-saws for dicing the ingot. Figure 3 shows a photograph of the wire saw. RMD plans to purchase an additional wire-saw as a backup. RMD performs the dicing in a fume hood while manually applying the cutting slurry, and each cut takes approximately ½ hour, with an additional 10-minute setup and clean-up time.

After cutting the crystals to the required dimensions, the technician then laps, polishes, and chemically etches the diced crystals in a fume hood. Currently, the lapping and polishing is a sequential process done manually. The process begins with lapping using rough grit to remove saw damage and provide smooth flat surfaces. Lapping is then followed by polishing with fine grit.

It should be noted that RMD is leveraging advances in “wafer-level processing” techniques that automate dicing and polishing, and uses jigs to improve dimensional tolerances. The wafer-level processing technique focuses on optimizing the manufacturability of TlBr detectors by streamlining the fabrication process and exploring those approaches used to process silicon devices at the wafer-level, such as the application of electrodes and the qualification of devices. The automation of key processing steps, including dicing, annealing, and testing, enabled by the wafer-level processing advances, provides a means to reduce the labor cost associated with production as well as improve the quality and repeatability of the finished product. While the wire saw minimizes the stress of the dicing process, it requires the attention of a technician, and the use of a programmable diamond saw was developed as part of the wafer-level processing effort.

RMD next applies the electrodes to the crystals using either thermal vapor deposition or electron-beam vapor deposition, shown in the photographs of Figure 4.

Figure 4. Left: thermal evaporation system. Right: e-beam deposition system.

Both of the electrode deposition techniques use high vacuum systems, each of which require time for pump-down to the correct pressure after placing the samples in the vacuum chamber. The TlBr samples are first placed into a holder for alignment to a shadow mask that defines the electrodes. The pumping

Figure 3. South Bay Technology Wire saw (Model

850) for cutting TlBr wafers in a fume hood.

typically requires two hours, and the deposition requires 4 to 5 hours, so the whole process takes about 1 day. The holder accommodates four of the pixelated TlBr crystals, or 64 of the CFG crystals.

Depositing electrodes on 60 crystals in 1 month, for the 300 pixelated TlBr RIIDs, will require 15 days/month; an equal amount of time is needed for the CFG RIIDs.

Note that as part of the wafer-level processing effort, RMD developed mounting fixtures to increase the number of samples processed in the electrode deposition apparatus, which increases the processing throughput of devices.

After depositing the electrodes, the samples are annealed. The current annealing furnace is shown in Figure 5.

4. Packaging

Pixelated:

After depositing the electrodes and annealing the pixelated samples, RMD sends the arrays, along with a printed circuit interposer board, to Polymer Assembly Technology for flip-chip bonding. After flip-chip bonding, RMD packages the crystal to facilitate the handling, testing, and shipping of the array. Figure 1 shows the packaged TlBr pixelated array in the left photograph.

CFG:

Currently, the PS-CFG package consists of wrapping the crystal in a flex circuit with spring contacts at the anode and cathode, and placing the wrapped crystals in a segmented container to route the electrical connections to a connector. RMD currently uses the same connectors for the flip-chip-bonded pixelated samples and the PS-CFG container, although the number of connectors and pin-outs are different. RMD can package 16 CFG crystals in approximately two hours using the current flex-circuit design. Figure 1, right, shows a photograph of the wrapped PS-CFG crystal on top of the “carton” that holds 16 wrapped PS-CFG crystals.

The packaging for both the pixelated arrays and the PS-CFG carton facilitates the handling and testing of the TlBr detectors; the enclosure for the pixelated array can be removed if necessary for mounting in the RIID, and the wrapped PS-CFG crystals can removed from carton or replaced individually if necessary.

RMD conditions the samples by applying a high voltage with a programmed ramp. RMD is currently designing readout boxes for both the pixelated TlBr arrays and the CFG detectors. The designed readout boxes contain a high voltage supply. The anode signal qualifies the TlBr crystals. For the pixelated arrays, a sample of pixels, between 8 and 16, are used to qualify the array.

Summary:

In summary, RMD will be able to provide TlBr material for 300 pixelated Radio-Isotope Identification Device (RIIDs) per year as well as 300 position-sensitive capacitive Frisch grid (PS-CFG) RIIDs per year using existing facilities supplemented with some additional dedicated equipment, such as furnaces, Figure 5. Current annealing furnace.

wire saws, deposition system, an environmental chamber, and burn-in, conditioning, test and measurement electronics.

Final Note: It is assumed that if a supply of more than 300 RIIDs of each type (Pixelated and CFG) per year were to be needed, then the production chain would be enhanced in a proportional manner.