Irradiation using individual pixel control

To test the individual pixel control capability of our system the glass substrate based multi-pixel cathode (Figure 4.6(b)) was installed along with a new multi-pixel window. In this case several pixels were wired individually as seen in Figure 4.9 (a) with effort made to avoid any intermittent contact to other pixels. The vacuum chamber was fitted with a high voltage multi-pin feedthrough flange (Figure 4.26), and the Kapton lead wires for these contacts connected via the feedthrough. With all of the wires connected on the inside it was possible to connect select wires on the outside to address particular pixels on the inside.

(a) (b)

Figure 4.26: Views of the multi-pin feedthrough wiring scheme. (a) Multiple wires are connected on the vacuum side of the flange. (b) A single wire connected to a grounded multimeter on the outside in order to activate the pixel.

Pixels were connected individually and in a few cases several pixels were connected

simultaneously. Unfortunately it was not possible under this arrangement to tell which pixels were emitting and which were not when several were connected. Also different responses to the gate voltage made this mode of operation less useful. In one case some damage was done to some of the pixels because of arcing or some form. Therefore more caution was taken in gathering the data.

For the multi-pixel window a modification was made in order to enhance the window collimation performance. As discussed earlier a single KOH etched window provides relatively low screening as compared with a higher aspect ratio vertical collimator. To address this difficulty two KOH etched windows were stacked together as an initial, simple, and cost effective means of forming a higher aspect ratio collimator. In this case two 5x5 window arrays were used. The top silicon array was left in its native state with the 9-13 um nitride membranes intact, with the exception of a few which had been broken. The bottom wafer was etched in the RIE to remove the nitride layer, and then etched for a few minutes in KOH to expand the size of the aperture. These ranged from 10-30 um. The two wafers were stacked and glued with vacuum epoxy wafer backside to wafer backside as shown in figure 4.27 to form a new high aspect ratio collimator with a thickness of 800 um and an aspect ratio of perhaps 25:1. Because of the broken windows a piece of Mylar was attached to the top surface and the entire piece was glued with the epoxy to a multi-pixel mini-flange having 1 mm holes for each window. The chips were aligned by placing their edges evenly with one another, the edge positions having been determined by the KOH trench etching technique described earlier. Alignment was checked via an approximately orthogonal visible light source. Light was seen to shine through all 25 apertures in the array.

Figure 4.27: The double-stacked 5x5 array electron window collimator: schematic, picture, and optical microscope images of sample windows from the top and bottom chips.

For the film irradiation test of individual pixel control it was found that high voltages above ~20 kV would induce arcing. In addition to the 20 kV acceleration voltage a pixel current of 5 uA was used at 10% duty cycle. Because of the low acceleration voltage much longer irradiation times were necessary to obtain recognizable irradiation spots on the film. Exposure times on the order of 5-10 minutes were used. This length of time is clearly outside of the reasonable length for irradiations, but in future work it should be possible to increase the voltage and reduce the time significantly. The 20 kV acceleration potential reduced the electron transmission and dose delivery. Also higher aspect ratio collimation reduces the number of electrons passing through. These issues are related to the arcing and electrical challenges that have occurred, and should be addressable so as to increase current and voltage and thereby reduce the irradiation time.

For simplicity of evaluation corner pixels were used to irradiate the film. Using the parameters listed above cathode pixels in the upper right and lower right of the chip layout as seen in Figure 4.6 were activated and used for irradiation. Both cathode pixels showed the strongest irradiation over the window corresponding to their pixel location, but some irradiation took place around neighboring windows (Figure 4.28). The specific values obtained for the detectable irradiation spots are marked on the figure. For both data sets the

Figure 4.28: (Top) Irradiation pattern from upper right pixel. (Bottom) Irradiation pattern from lower right pixel. The black box shows the approximate location of the 5x5 array. The orange box shows the recognizable irradiation spots. The spots have been analyzed and the peak dose at each detectable spot is listed.

dose obtained at nearest neighbors was about half that received at the corner window. For the next nearest neighbor the dose was around one quarter of the corner dose. The values of one half and one quarter are not particularly meaningful other than the fact that they

generally demonstrate that some form or forms of crosstalk are taking place in the multi-pixel system, such as beam crosstalk (a collimation challenge) and/or electrical crosstalk (an insulation and breakdown challenge). For biological experiments crosstalk of this magnitude is undesirable and unacceptable as it does not represent true individual pixel control. In some cases crosstalk of a few percent might be acceptable, but ultimately this requires some form of alleviation.