ElectroOptical Innovationshttps://electrooptical.net/News/2022-08-05T01:19:40.609354+00:00Silicon Photomultiplier Module Design2021-01-25T16:30:28+00:002022-08-05T01:19:40.609354+00:00Simon Hobbshttps://electrooptical.net/News/author/simon/https://electrooptical.net/News/silicon-photomultiplier-module-design/<p>Internal Developments<br/><br/>In the last year or two we've been doing a lot of work aimed at replacing photomultiplier tubes (PMTs) in instruments, using <em>avalanche photodiodes</em> (APDs) and <em>silicon photomultipliers</em> (SiPMs). These devices are arrays of single-photon detectors, so they're also known as <em>multi-pixel photon counters</em> (MPPCs). Our main application areas include biomedical instruments such as flow cytometers and microplate readers, which have to measure low light levels very precisely but don't need the ultralow dark current of PMTs. (Follow-on articles will talk about our SiPM work in airborne lidar and SEM cathodoluminescence, as well as on improving the performance of actual PMTs.)<br/><br/>PMTs have been around since the 1930s, and remain the undisputed champs for the very lowest light levels. We love PMTs, but we have to admit that they're delicate and not that easy to use—they tend to be bulky, they need high voltage, and they need regular replacement. Most of all, PMTs are very expensive.</p>
<p><br/>We've been working with several customers on developing products using Hamamatsu, Broadcom, and On Semi (formerly SensL) SiPMs. They have different strengths, but all three series are excellent devices that have far better linearity in analog mode than we initially expected. (There's a fair amount of doom-and-gloom about that in the specialized technical literature.)<br/><br/>Our first product design used the Hamamatsu S13362, and can go from counting single photons to working in analog in dim room lights, with just the twist of a knob. Subsequently we've had the opportunity to do a couple of devices for time-of-flight lidar using OnSemi's MicroFCs, which we developed from our existing IP. Recently we've been consulting on microplate and flow cytometry applications. All of these applications have in common that they're moving to the newer solid state option and away from PMT-based designs.</p>
<p><br/>These applications are challenging enough without having to develop the photodetection hardware. With so much customer interest, we've been focusing on developing a series of SiPM modules that act as drop-in replacements for traditional PMT modules, including all their nice features such as wide-range voltage-controlled gain, ±5 V input, and selectable bandwidths from DC–200 kHz to DC–300 MHz. Our existing designs are available on a flexible licensing model that generates considerable savings compared with either purchased PMT modules or internally-funded development, and gives you complete control over your supply chain.<br/><br/>Because these technologies are new, we can provide customized proof-of-concept (POC) demos showing how they work in your exact application. We've delivered prototypes and POCs in as little as one week at low cost, so you can make a real-world engineering evaluation without sacrificing a lot of budget or schedule.<br/><br/>For more information on our SiPM/MPPC designs, or help with your low-light measurements, send us an <a href="mailto:pcdhobbs@electrooptical.net">email</a> or give us a call at <a href="tel:914-236-3005">+1 914 236 3005</a>—we're interested in solving your detection and system problems.</p>Silicon Photomultiplier (SiPM, MPPC) System for Cathodoluminescence2020-01-27T11:38:02+00:002022-01-20T13:36:29.010710+00:00Philip Hobbshttps://electrooptical.net/News/author/pcdh/https://electrooptical.net/News/silicon-photomultiplier-sipm-mppc-system-for-cathodoluminescence/<!-- 30 Jan 2020 12:20:00 -->
<p>In <a href="https://electrooptical.net/working-with-us/how-we-work/">How We Work</a>, we gave an overview of how we build instruments, from the initial feasibility calculation (or <i>photon budget</i>) to delivery of the first production units.</p>
<p>Each project is different, of course, but there are common themes. Here's a description of these steps from our most recent one at this writing (late January 2020), which is a low-cost cathodoluminescence detection system for use in scanning electron microscopes (SEMs).</p>
<h3>Photon Budget</h3>
<h4>Cathodoluminescence Principles</h4>
<p>A SEM works by scanning a tightly-focused beam of high-energy electrons (1 keV - 30 keV) across a sample, and looking at the stuff that comes out. For ordinary imaging you usually look at backscattered and secondary electrons, but there are other modes. For instance, you can get a lot of information about the sample's chemical composition by looking at the x-rays it emits. Most samples will also emit some amount of light, a process called <a href="https://en.wikipedia.org/wiki/Cathodoluminescence"><i>cathodoluminescence </i></a>.</p>
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<p>Of course, some samples are a great deal brighter than others. The <i>luminescent yield</i> is the average number of photons escaping from the surface of the sample per incident electron. It ranges from about 10<sup>-5</sup> for some metals to more than 100 for LED chips.</p>
<h4>Signal-To-Noise Ratio</h4>
<p>To make a halfway decent image, you need a signal-to-noise ratio (SNR) of at least 20 dB, which means at least 100 detected photons per pixel on average. The smallest vaguely useful image size is around 300x300 pixels, so a minimally acceptable image needs at least 100×300×300 ~10<sup>7</sup> detected photons.</p>
<p><i>Good</i> images, ones you might want to publish, would have 10 times that many pixels and would need 10 or even 100 times more photons per pixel to reduce the visual noise. To make the system pleasant to use, you want the frame time to be at most a few seconds during setup and no more than a minute for a high quality image. Thus we need a count rate ≳10<sup>7</sup>e<sup>-</sup> / 3 s or 3 MHz for low SNR and ≳10<sup>9</sup> - 10<sup>10</sup> e<sup>-</sup> / 60 s or 17-170 MHz for high quality.</p>
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<p>A typical beam SEM beam current is on the order of 100 nA, which is roughly 10<sup>12</sup> electrons per second. The total photon emission rate will thus be between 10<sup>10</sup> and 10<sup>14</sup> per second for our intended range of samples. Fancy cathodoluminescence systems use large ellipsoidal or parabolic mirrors to collect nearly all of the emitted light, but those are a huge pain to align, and they get in the way of the other detectors for secondary electrons and x-rays.</p>
<p>This low-cost system is intended to be inexpensive and easy to use. The client was willing to specialize it for somewhat brighter samples, those with yields of ~0.1% or higher. We therefore relied on putting the sensor as close as possible to the sample without running into anything or blocking other detectors.</p>
<p>The result was that we collect about 1% of the emitted light. With 35% peak overall efficiency (48% area efficiency and 72% detection probability), we have about 4×10<sup>7</sup> to 4×10<sup>11</sup> detection events per second. With an ordinary photodiode with a gain of 1, that's a current range of about 6 pA to 60 nA, and that's for light near the peak sensitivity wavelength of the detector. It's hard to get good results in a wide bandwidth with that sort of current, so an electron-multiplying detector would help a lot.</p>
<p>Together with the client, we chose a tiled array of Hamamatsu <i>multi-pixel photon counters</i> (MPPCs), also known as <i>silicon photomultipliers</i> (SiPMs). These devices are sensitive to single photons, and have about the same overall quantum efficiency as a PMT (10-40% or so, depending on wavelength). They consist of an array of hundreds or thousands of individual avalanche photodiodes (APDs) wired in parallel, each with a series resistor to recharge it when it fires. At the right bias voltage, a single detected photon will cause one of these APD pixels to avalanche, dumping a fixed amount of charge into the external circuit. They recharge pretty fast (~20ns), which makes MPPCs useful in analogue mode as well as photon counting mode.</p>
<p>In <i>Building Electro-Optical Systems</i> I'm quite critical of avalanche photodiodes in general, because on an apples-to-apples basis they have around a million times higher dark count rates than photomultiplier tubes (PMTs). That is, a 100-μm silicon APD has about the same dark count rate as a <i>four inch</i> bialkali PMT. That's really bad for the lowest-light measurements.</p>
<p>MPPCs do have some very important advantages, though: they're much less delicate, easier to drive, longer-lived, and considerably cheaper. In this case that turned out to be a big win, because the minimum useful signal for imaging (3 MHz count rate) is more than three times the maximum dark count rate (about 900 kHz in our operating conditions). Thus for imaging, the dark count rate has only a minor effect on the system performance.</p>
<h3>Proof of Concept</h3>
<p>The POC prototype was composed of our high bandwidth voltage controlled amplifier (0.5×-64×, 50 MHz BW), low dissipation thermoelectric cooler (TEC) driver, and avalanche photodiode bias supply; a custom front end based on previous designs, with a bootstrap based on a Mini Circuits <a href="https://www.minicircuits.com/pdfs/SAV-551+.pdf">SAV-551+</a> pHEMT; a collection of power supplies out of the drawer; and of course a very nice Hamamatsu MPPC with built-in thermoelectric cooler (TEC). The whole thing was controlled with a modified version of one of our older laser driver models. The firmware and PC software were derived from these products as well.</p>
<p><a href="https://www.electrooptical.net/static/media/uploads/videos/MPPCphotonCounting.mpg">The result</a> was a system that could resolve individual photon detection events at high gain, and worked in room light at low gain, all with the twist of a knob. Actually there are two knobs in this version—one for the MPPC bias and one for the voltage-controlled amplifier. The production version has one knob that controls both via software, with the "knob feel" designed to mimic that of a photomultiplier. (The bias control and the VCA are actually implemented with a microcontroller. It turns out to be very hard to do a VCA in analogue that maintains low noise at low gain, at least with a reasonable number of parts.)</p>
<h3>Productizing</h3>
<p>The POC hardware design, software, and construction took a bit over three weeks' work. Calendar time from contract signing to delivery: eight weeks, including a week each for the photon budget and the client's evaluation process.</p>
<p>The client was happy with the POC system, so we agreed informally on license terms and moved forward with productizing it. This was a bit more involved than usual. The system needed to operate inside the SEM's vacuum chamber, and had to be very small in order to get the detector as close to the sample as possible so that we'd get more light. It also had to have an adjustable length so as to fit as many SEM brands as possible, and had to minimize stray magnetic fields.</p>
<p>All of this meant a fair amount of back-and-forth with the client's engineers to arrive at a workable optical/electrical/mechanical/thermal design. We wound up with a three-board solution.</p>
<p>The MPPC itself mounts on a 15-mm square board that sits on top of the TEC, along with a SMT thermistor. It connects via a Kapton flex circuit with a 300 μm pitch. This minimizes the heat leak from the wiring, which is a serious issue in cooled circuitry.</p>
<p>A very small front-end board (20 × 70 mm) also goes inside the chamber, with the front end amplifier, variable gain stages, various sensors for ambient conditions, and a small microcontroller (ARM Cortex M0+). Cooling electronics is tough <i>in vacuo</i>, so power dissipation had to be kept low. The main cooling issue is getting rid of the waste heat from the TEC, which is several watts, but that flows down the aluminum mounting bracket to the vacuum flange.</p>
<p>The third board, which mounts on the outside of the vacuum flange, has the bias generator for the MPPC, DC-DC converters, <a href="https://electrooptical.net/News/technology-low-noise-thermoelectric-cooler-tec-controllers/">low noise thermoelectric cooler controller</a>, configurable output amplifiers, and communications: serial to the front end board and USB to the user interface box.</p>
<p>All three boards worked fine on the first iteration. (A couple of resistor values needed changing, but that was it.) The result was the <a href="https://www.delmic.com/sparc-jolt-detection">Delmic JOLT system.</a></p>
<p>Overall, a very pleasant and successful project, working with some great people.</p>