MEMs oscillator sensitivity to helium (helium kills iPhones)

today on Applied Science we're going to take a look at this interesting and bizarre failure mode of modern cell phones if you take a phone and put it in a plastic bag and then filled the bag up with helium in about ten minutes the phone will completely go dead and it will take three or four days for the phone to recover sounds completely ridiculous but Kyle Wiens did a great blog and video on this on iFixit and it's totally true and the way that this was discovered was that someone working in an MRI facility had a whole bunch of Apple iPhones die all at the same time and the first someone thought it might have been like an electromagnetic pulse from the machine or something but they eventually figured out that it was probably the helium gas being emitted from the MRI machine and I became pretty skeptical at this because it is true that if you put it in a pure 100% helium environment the phone dies but how much helium could it really take so in this video I'm gonna measure exactly how much helium it takes to kill an iPhone and also tear down the device inside the phone that's actually sensitive to helium and take a look at it under the electron microscope so first the surprising result from all this testing even a two percent helium environment is enough to disable the part that is susceptible in these phones at 2% helium the phone will last about 30 minutes in that environment before being disabled this is really surprising and it actually makes the MRI story start to sound fairly plausible I was expecting this to be more like half an atmosphere or three quarters of an atmospheric helium but as it turns out somewhere between one and two percent is the magic point where this thing is as deadly and 0.2 this is not a swipe against Apple to their credit in their user manual they even say that you shouldn't hang out around boiling liquefied gases such as helium because it might affect your phone so they actually know about the problem and the chances that you're gonna run into this are so remote it's it's really not a problem let's talk about the actual component that's causing all of these problems in pretty much all electronics you need a frequency source called a clock and traditionally it's delivered by something that looks like this which is a quartz crystal inside here there is a little tiny pea of quartz with electrodes on it and it's physically vibrating at a frequency that's determined by its size and you can see that this is actually in a metal can here specifically to keep out environmental factors that may affect that piece of quartz over time and and cause its frequency to shift so one of the problems with this is that this this frequency is fairly high it's probably about 16 or 32 megahertz and that burns up a lot of power so in battery-operated electronics it's common to have two oscillators one at 16 or 32 megahertz for example and another one at a much lower clock frequency of just 32 kilohertz so now you have to have two quartz crystals in your battery-powered device the problem with having one or even two quartz crystals in your little device is that they take up a fair bit of board space this is not the smallest quartz crystal available but it's getting pretty close and it's a bit of a physical limitation because you just can't cut the quartz crystal small enough and still get good frequency stability and have everything that you want so there's a modern technology that replaces the quartz with a much smaller component in fact you can stack probably about two or three of these things on this quartz crystal and what I'm holding in the tweezers is a MEMS oscillator so there's no quartz inside this package there's actually a piece of silicon that is cut into a tuning fork and and that is actually oscillating instead of quartz it's a relatively recent innovation MEMS oscillators have been around for a while but the fact that devices are getting so small phones and watches for example are making this MEMS oscillator a better choice than quartz because it's smaller you might save a little bit on power as well so you get better battery life and a smaller device which is what everyone wants in mobile electronics the manufacturers know that the MEMS devices are sensitive to environmental factors too and so sometimes they're put into cases that are made of ceramic with a metal lid and this is sealed up specifically to keep environmental stuff out but the problem is if you want an oscillator that super super tiny for a wristwatch device for example you really can't make this in a metal lid package you need some other way to put it together and so the problem is that by miniaturizing it down so small it's opened up the susceptibility to helium and the main reason is that what this thing is made out of is it's almost pure silicon will do a teardown on this and show exactly what's inside here but the problem is that instead of metal and ceramic protecting it from the environment there is no protection it's just silicon hole the whole device is actually made from silicon and as it turns out helium can actually work its way through the silicon it's not like there's a crack or a seam or anything in there the gas is actually going through the material it's diffusing through it because it's such a small molecule getting inside the device and once the gas is inside there it causes the oscillator first to rise in frequency slightly probably because there's a compensation circuit in there and then when the concentration gets high enough it causes the oscillator to crash to a very low value essentially a total failure the component in question is the SI 1532 MEMS oscillator made by a company called sigh time and you can get these on digi-key again this isn't a swipe against side time I think this is a very weird edge case that probably isn't worth considering and side time claims this is the I think smallest lowest power 32 kilohertz oscillator that you can get so it's not a cost optimal footprint on the board and power I didn't bother making a circuit board that would connect up to this footprint and so to connect up the wiring to these parts which I knew I wanted to have in like a sealed container where I could control the helium atmosphere I tend a little bit of 40 gauge wire and just very quickly melt the solder ball and put the wire in so the whole device is kind of suspended by its wires and this works fine for just for connections or even three here's a look at the test setup I've got like a gas controlled manifold here and the blue hose is going to a vacuum pump that is on the floor with a valve here then T's into the test chamber and there's another valve here and the red hose connects to the source gas and of course helium is the one we're talking about today but I also tried hydrogen which which had no effect on it and I'll talk about the results later and then the gas manifold itself has this pressure gauge here this thing reads a little strangely it reads in gauge pressure below atmospheric so when it reads 101 that's actually zero absolute because we're at sea level here strange units but just kind of go with it at least it's kPa and then the actual chamber itself is made from an old vacuum gauge that I took apart basically to get the pass through so this is a NW flange I think this is called and inside here is the wires holding up the tiny little oscillator and it's just got a bunch of pin connection so it's basically just a sealed chamber with the electrical pass throughs and this convenient way to open and close it so the trick with helium poisoning these things is that they don't recover very quickly and so if I test a device I need to quickly get it out and put a fresh one in so that I'm always starting with a fresh oscillator I should point out that throughout these tests you'll see the draw of the current draws about 7.5 micro amp which is way higher than the datasheet spec and most of the current is actually going into the frequency counter so if I disconnect the counter so now the oscillator is still running it's just not connected to the frequency counter it drops down to 2.7 micro amp which is still higher than the datasheet but there's probably other straight capacitance in here that I haven't worked away the trick is that these ultra-low power draws even at 32 kilohertz a straight capacitance ends up pulling more current than the entire supply current for the device its health so when you get down to these crazy low currents it's kind of tough to chase away the micro amps and the single digits like that okay let's talk about the tests to start off I didn't mix up air environments with helium like one way to test this would be to fill up a container one percent with helium by pressure and then fill up the remaining space with air and then do another test fill out that chamber to two percent helium by pressure and then fill up the remainder with air however since the air is not really playing a part in this at all it's really only the helium that matters we can sort of approach the problem by just thinking about the partial pressure of helium so you know think of like a strainer where the helium is small enough to go through the strainer but the air is too big to go through the strainer it doesn't really matter if the air is there or not if you have the same partial pressure of helium so basically what I want to do is evacuate the chamber entirely to pure vacuum and then introduce a small amount of helium to a pressure that is 1% of an atmosphere let's say 1 kPa about in the first experiment I added about 1% of an atmosphere helium pressure and let that sit for a while and notice that the frequency was actually raising very slightly and I was watching the graph on the frequency counter and thought it was kind of leveling off and so initially what I thought I was going to do is introduce 1% helium and then go back to vacuum and watch the frequency return to normal then go back to 1% helium and try to characterize sort of the diffusion constant based on how quickly the frequency was changing versus how quickly I was adding and subtracting this 1% helium unfortunately it didn't work out that way because the frequency never returned there's something going on with the way the helium diffuses into and out of this device that I don't fully understand so it takes a long time for the helium to diffuse back out even in full vacuum and this is true in other testing too so for example in the iFixit blog it only takes 10 minutes in pure helium to disable the device but it takes days for the thing to recover and I'm not really sure why that is and in my testing I saw the same thing it takes 2 or 3 days to get back to a functioning oscillator even taking the power off and putting the power back on periodically even in vacuum and I'm not really quite sure why so after giving up on measuring the diffusion constant this way I decided maybe just to try to figure out like the minimum kill concentration so I went to 2 percent helium thinking that you would be about the same as 1% but to my surprise the frequency kept drifting upward and after about 30 minutes or so there was a sudden crash where the frequency started dropping precipitously and worked its way all the way down to 3 kilohertz basically a total failure if the oscillator very surprising I was expecting this to go all the way hit a 50% or 60% helium but it really only took 2% and I think it's possible that even less would do it obviously two kills it so somewhere between 1 and 2% is probably the unacceptable range for this device pretty interesting and then also I tested one atmosphere of helium just to see and sure enough in just a few minutes it actually causes the same failure mode first a frequency rise and then a sudden crash and if you think about this wave of helium diffusing through the silicon inside the device if the wave is really steep because there's a lot of helium on the outside of the device then these this sort of crash scenario happens more quickly as well so first the frequency rises kind of do the same failure point maybe 1 or 2 Hertz higher than its specified frequency and then the crash is more abrupt because this wave of helium is much taller basically for the lower concentrations one or two percent the failure is more gradual because the helium is diffusing in at the same speed but the the height of the wave is lower this concept of diffusion in engineering in general is sometimes tricky to grasp and so a good way to think about it is imagine taking like a frozen pie out of your freezer and putting it in the oven it doesn't matter if your oven is a hundred degrees or a thousand degrees if you take it out after 5 minutes the center is still going to be frozen you could have the hottest oven in the world and it's still not going to be able to thaw the middle out through conventional heat exchange and so the same thing is going on here the helium is actually diffusing through the material just like the heat is diffusing through your frozen pie and what you can do is change the height of the wave that is diffusing through the material but you can't change the speed at least not by adding more helium to the equation you can heat the whole thing up and that might increase the diffusion constant but the the actual pressure the thing that you're actually do using through the substance doesn't affect the speed at which it goes through also its III didn't test high-temperature diffusion for these MEMS oscillators it's about 15 degrees C and all my tests were done at that temperature let's tear one of these devices down and see what's inside there that's so sensitive to helium this is my setup here I've got a really small ceramic hot plate under the microscope and then what I'm going to do is take one of these devices and put it on the hot plate and run about 50 or 60 watts through this little tiny ceramic hot plate just from the bench supply and that will allow us to separate the two dyes that make this device up it has got an interesting construction there's one piece of silicon that has the electrical circuitry on it and that's the more oblong rectangular shaped piece of silicon and then there's a smaller square piece of silicon that has the MEMS structure in it and the reason that it's two separate dies is because if you build a production line to build MEMS devices it's kind of set up only for MEMS and if you build a production line that's only for Simo circuitry then that's all you get so it's really two separate dies because they came from separate production lines I mounted some of these devices in epoxy and then sanded them down taking photos incrementally as I sanded through and that's actually how I discovered that there were two separate pieces of silicon with like an under bonding layer like a an adhesive or something that's helping them stay together there's also a four ball I think solder ball connecting them together as well the trouble is this adhesive is incredibly good and so to get them apart I put them on the hot plate and then twisted them with the tweezers to pull the to die apart which is not so easy since I knew I was going to do some scanning electron microscopy on this I also used conductive epoxy to hold a device down to the little aluminum stub that goes into the microscope and then just really carefully use sandpaper with a screwdriver and took off tiny tiny amounts of material looking at it under the microscope while it was doing this here are some images from the electron microscope these were obtained by mounting the entire MEMS oscillator in of epoxy and then grinding from the bottom into the MEMS device so in theory you can pull that MEMS die off and mount it either way and it's actually built as far as I can tell symmetrically so it doesn't matter if you grind from the bottom or the top which is very curious actually how do you make a MEMS device that's entirely encased in silicon it's almost like a bizarre sort of puzzle I mean it's there's no openings to this thing there's no seams there's no lid or anything I mean how do you make it so let's switch the cartoon view and we can talk about how this thing is made it may have been a little hard to tell in the SEM view but this is kind of how it looks in schematic form and the the thing that's actually moving the tuning fork is this darker object that I've labeled 3 and it probably Wiggles in like all the legs are probably moving in and out at the same rate but it's possible they did something tricky in its movement you know oscillating in a different mode the actuators are almost certainly electrostatic so if you put a positive voltage here in a negative voltage on the tuning fork it's going to cause this to be attracted here because opposite charges attract and you end up with a total of three leads basically if we were to boil this down even further it looks kind of like this and so if you put your attraction voltage here the thing moves this way and then when you release it it'll have a characteristic wiggle and you can use your other electrode to sense what's going on in there okay so we saw that this tuning fork structure has to be freely moving in here so imagine these non shaded parts are permanently fixed but this has to move it has to be completely open you know free to move and then maybe the tuning fork is pinned kind of at a couple spots like here and here so it's it's fixed here and here but everything else has to be free-floating and this entire thing is encased in silicon how do they do it so I found a couple of references that I'll put links to in the description but here's the basic idea if you start with a silicon wafer we can etch a little piece out of it then you can grow an oxide layer on it and then you can deposit silicon onto the oxide and I'm not exactly sure of the process I think it's poly silicon but you can use a patterning system to basically put silicon down on top of the oxide then you can put grow oxide or deposit or grow oxide on top of the thing that you just deposited and here's the step that I didn't believe was possible but apparently it is you can actually bridge the gap with a new silicon layer so in my mind if you're going to deposit silicon it seems like it would just cover everything but if you make the gap the right size and you use the right process you can actually get a cap of silicon to grow across your entire device and essentially seal off the chamber that's in there but then how do you get the device free I mean it has to be supported on something we've got these oxide supports holding the tuning fork there the trick is that you purposefully leave some very small vents in the outside going to the outside of your entire case here and you use a hydrofluoric vapor etch process so the hydrofluoric vapor will etch away all the oxide but not touch the silicon itself so you essentially remove the supports that were holding the tuning fork in place by letting the HF vapor go in and then you have to seal off these little vents and I think as it happens the vents can be coordinated with your electrical contacts which you also have to make through this top layer so that the the way that you connect electrically to the tuning fork and the other electrodes that are inside this thing are done through these vents you also need vents in the tuning fork itself to let the HF gas get through this entire device so when you look at the image in the SEM all those weird little slots and stuff cut in there might be there for structural reasons but it's also likely that those slots are there specifically to let this HF gas process work pretty pretty cool actually did not though this was possible and you end up with a completely hermetically sealed box after you close off these vents and I think the way this is done in the factory is that you use hydrogen to prevent this whole thing from getting oxidized and seal it up with the hydrogen inside it and then you put it into a really hot oven in vacuum and the hydrogen actually diffuses out through the so lookin at those high temperatures but as I found out I tested it myself and at room temperature after one day of one atmosphere hydrogen I couldn't detect any shift in frequency at all so at room temperature the hydrogen diffusion is so slow that you know they take millennia or whatever did to affect it or something but in a hot oven in vacuum that hydrogen diffuses out in a timescale that makes manufacturing these things possible okay well I hope you found that interesting see you next time bye

Channel: Applied Science

Views: 382,744

Rating: 4.9615436 out of 5

Keywords: Si1532, MEMs, oscillator, 32KHz, iphone, helium, allergic, sensitivity, vacuum, silicon, die, applied science

Id: vvzWaVvB908

Length: 20min 42sec (1242 seconds)

Published: Sun Nov 18 2018

Reddit Comments

[u/James-Lerch]

Do you like dead iPhones?

Do you like Electron Microscopes?

Do you like Oscilloscopes, frequency counters, and related electronic test setups?

Do you like learning about semi-conductor fabrications methods?

​

If so, this video is for you!

[u/Gnarlodious]

He doesn’t state the reason clearly in the video, but deep in the comments he answers a question like this:

My guess is that the gas pressure inside the device causes friction between the tuning fork and the stationary electrodes, and this friction causes energy loss. If the energy loss is high enough, the oscillator will not run. It's like slowing down the pendulum of a clock with your hand. It will work with some amount of energy loss (friction), but there is a point at which it will stop due to design limits on how much energy can be put into the oscillator. Normally, there is vacuum inside the device.

Other people speculate that the helium atoms interact very slippery-like, as it is known to do. The oscillator surrounded by helium causes a slight increase in frequency, due to reduced atomic resistance.

Others seem to think the helium infiltrates the vibrating silicon oscillator, causing a change in mass.

In any case, its likely the helium is more mobile while the oscillator is vibrating, and that explains why it takes so long to dissipate the offending atoms when the oscillator is off.

[u/Ramast]

Great, one could put his cellphone in helium before flying to one of the countries that force you unlock your phone so they can take a peek at your private life.

[u/bobhert1]

We had a similar issue with a sensor design. We used a MEMs oscillator on a PWB in a hermetically sealed housing. Mysterious failures started occurring when the clock would stop. Within hours of opening up the package to troubleshoot, the clock would start again. It was maddening and we spent over $100k chasing the problem. After several months we found out that the helium tracer used to verify hermeticity was killing the oscillator (temporarily). Still our favorite failure story of all time.

[u/doodle77]

I wonder if applying 50% or 100% helium for a few seconds, then returning to regular atmosphere will still cause a "wave" of helium to reach the inside of the device and disable it.

[u/Renovatio_]

Reports are it doesn't kill them but puts them into a week long coma

[u/corsecprops]

Can a person notice a 5% helium environment? Is it enough to change voice pitch or otherwise be noticable?