A matter of scale (ice spikes and silicon pillars)

One of the great things about science is how interlinked everything is once you start looking deeply enough. One example of this came a couple of months back when I was reviewing the electron micrographs from the latest experiment.

My work involves the growing of meso-scale (about a 100th of a millimeter) pillars on a silicon wafer by laser irradiation. A complex combination of melting, ablation (explosive boiling) and redeposition of material happens at the silicon surface, with the net result that pillars grow from the silicon towards the laser. The pillars spend most of their growing time bathed in a hot (>5,000 degree C) plasma, this tends to smooth out the tips and sides of the pillars by a combination of melting and sputtering.

In this particular experimental run, I’d been varying the conditions under which I grow silicon pillars. What I saw was most unexpected; nano-scale (around a billionth of a metre in size) spikes sticking out of the top of almost every pillar I’d grown. You can see in the pictures below, that the pillars are around a twenty microns long, and the tiny spikes stick out vertically from the tips.

forrest of spikes spike closeup
Cross sectional view of silicon pillars showing nano-scale spikes, and a closeup of two pillars with spikes at their tips

After a bit of head scratching, it was realised that what we were seeing was just a much smaller scale version of the ice spike formation process.

If you’ve ever tried to make ice cubes with filtered or very pure water, you will probably have noticed that you sometimes get a spike or lump that grows upwards from the free surface of the ice. A very good example of one formed from a distilled water ice cube is shown below, it is about a hundred million times larger than the silicon spikes in the pictures above.

ice spike
Ice spike growing from an ordinary ice cube. Image taken from the excellent ice spike site at Caltech

Rather amazingly, given the wildly different growing conditions, the process by which both ice spikes and silicon spikes grow is essentially the same. In both cases you start with a liquid – water or molten silicon which cools and begins to solidify on the outer edges until there is just a tiny space that remains unfrozen.

Both water and silicon expand as they freeze, liquid water is forced out of the center of the ice cube tray, up through the hole. If the conditions are correct, then water forced out of the hole will freeze into a hollow spike. The water freezes around the top of the spike, adding to its length. The spike continues to grow until all the water has frozen or the hollow in the spike freezes shut.

What happens with the silicon spikes is that the last pulse from the laser melts the tips of the pillars. These cool relatively slowly once the laser shuts off, the outside solidifying first and increasing the pressure on the liquid trapped inside the tips. The liquid silicon escapes though a hole or crack, forming a tube with frozen sides and a liquid centre. This grows like the ice spike until it freezes solid. In the case of the silicon spikes, the growth happens in a matter of microseconds, compared to the many minutes that ice spikes take to grow.

While you may have some trouble duplicating my work in your kitchen, you should be able to produce ice spikes quite easily. All the details are on the Caltech ice spike site

Death of a pioneer

On Friday 16th September, Gordon Gould the inventor of the laser died. Gould claimed that the idea and principles for the laser came to him one afternoon in 1957. His lab book details his ideas on a page under the title “Some rough calculations on the feasibility of a LASER: Light Amplification by Stimulated Emission of Radiation”.

Gould invented two types of laser, the gas laser and the optically pumped laser, but failed to get a patent granted on the inventions until the intervention of a judge in 1987. This worked to his advantage, he was able to collect license fees on the patent at a time when the laser was popular, had the patent been granted in 1957, then it would have expired before the laser was in significant use and the revenue from the patent would have been much less.

The laser I use in my work is loosely based on Gould’s first attempts at a gas laser, but pretty much all high power industrial cuttings lasers are direct descendants of Gould’s invention.

Open House

Next weekend is the London Open House weekend. Saturday sees me visiting family in Tiptree for the family tree project, so I’m looking for somewhere local to visit on Sunday. That’s off now, so I might be able to do both days.

The search on the website is, however, pretty badly done and, in fact, not actually working at the moment. Now I seem to remember there was a tour of the Becontree estate and Valance House had an open day. I think this may be a bit too local. I know both the museam and the estate very well. I want to see what is open towards the Eastern end of the District Line – Hornchurch and Upminster, mainly because I can easily cycle there, and it is downhill all the way on the return journey.

I wonder just how many punters some of the events will attract. I really can’t see the estate tour being very well subscribed…

Hmm.

I’m lucky (sad / geeky) enough to have several computers at home. So when one of them packed up on the 30 Aug, I just switched to using another one and thought about fixing it when the weather cooled down a bit.

I’ve just looked in that machine to see that the RAM is missing, not just one stick that could have fallen out, but two sticks that have very obviously been removed.

Now aside from the small top window that is pretty much always open, there is no way into the room from outside. The larger side window is firmly shut, while it may be possible to open the large window from outside, it is impossible to close it tightly again. No one else will admit to having been in the room, and to be honest, no one else here would really have a clue about computers anyway. Nothing else has gone as far as I can tell.

So where has the RAM gone? I’m rather puzzled by this.

Family tree research

Earlier this year, I started researching my family tree with my sister. I was something we’d both always wanted to do, but never told the other. When we both realised we wanted to do it, we found in the incentive to actually go about it. Things are much easier when you have someone else dragging you along…

As with most families I’d guess, there are various half remembered stories about great uncles or distant cousins that made good, died mysteriously or did something and were never spoken of again. The two best remembered stories in the family are quite good.

One involves either my great grandfather or his brother being an illegitimate son of one of the Mays from the Bryant and May match company. My Grandfather mentioned hearing about this when he was a kid, and there being quite a bit of money at stake should anyone ever be able to prove it. Of course it was never proven one way or the other when the money was still on the table, and there is no way to prove it now; the money is long gone anyway.

The other story is slightly more macabre. Again it involves my grandfather’s family. It seems that his father’s brother vanished in the first world war. Nothing too strange there, but it was later reported that he died on an ant hill in Africa and was stripped to the bone.

Well, now we are about 7 months into the research, we’ve traced back the immediate ancestors to around 1750 when they moved to the London area. Going back much further will probably be difficult and time consuming. So now we are starting to fill in the details of the lives of the people.

We visited the National Archives at Kew on Monday, to take a look at the army records from the first world war. Unfortunately, it seems that the majority of the records covering the time period we are interested in were destroyed during the bombing of London in the second world war.

We’ve been doing a bit of travelling around the East End, mainly Bow, Mile End and Hackney so we could see where the family has its roots. This should give me an excuse to post photos and maps and things…

The research has been fun and has expanded the number of living family members by around 50. Though these are all distant cousins, so apart from adding one or two the the xmas card list, I doubt we’ll have much further contact.

One good thing to come from all this has been discovering that on the whole, the family tends to long lives, aside from war and accidents, most of the menfolk make it to 70 with no problems, while the women get around another 10 years, even back in the 17 and 1800s.