Smallest ever advent calendar made

14 December 2007

Advent calendar
The world's smallest advent calendar?

© D Neumaier, J Biberger and F Goetz
A group of nanotech specialists in Germany have got into the Christmas spirit by making what they believe is the smallest ever Advent calendar. It would take about five million of the miniature calendars to cover a postage stamp. 

PhD student Daniel Neumaier, one of three members of the University of Regensburg's micro- and nanostructures group that created the calendar, told Chemistry World, 'We wanted to have a nice picture of Christmas on our home page. We waited until normal business was done for the day in the clean room. Then we went in and did it. We were just having fun.' 

The rectangular Advent calendar measures 8.4µm by 12.4µm and is etched onto a semi-conducting gallium arsenide wafer coated with Poly(methyl methacrylate) (PMMA) - used to make Perspex. The doors for 1 December through to 6 December are open, with six different images including Santa Claus, a bell, a snowman and a church. The smallest images on the calendar are the glass panes on the church windows, which measure about 20nm. At the bottom of the calendar, 'a Merry Christmas wish from Nanonic' is written in German. 

A snowman around 2 micrometres tall

© D Neumaier, J Biberger and F Goetz
The team used an electron microscope guided by a computer program from Nanonic- a start-up company founded by three Regensburg doctoral students. The microscope's electron beam breaks the bonds of the PMMA resist, etching the semiconductor below. 

But after the calendar was drawn by the electron beam and the remaining PMMA removed chemically, it was still difficult to see, Neumaier said. To improve the contrast of the image, the lines were etched in using an ion beam. 

The team needed two attempts to make the calendar. 'The whole process lasted about two hours,' Neumaier said, noting that the time stamp shows the image was completed shortly after 11:30pm on 4 December. 

Dieter Weiss, head of the working group that includes the three calendar makers, told Chemistry World he had suggested they might want to try something festive after he saw a news story in the German press about a 55mu.gifm-tall Christmas gingerbread man created by the Research Centre Jülich.

Church with windows 20nm across

© D Neumaier, J Biberger and F Goetz
 'That is huge,' Weiss said, adding that he e-mailed his team suggesting they could do better. Weiss believes his team's advent calendar is the smallest in the world, but admits he has no hard evidence to back that up. 'I searched on google and could find nothing smaller,' he said. 

Weiss admitted that several other labs around the world could make similar nano-scale images but said his lab is a global leader. 

'As far as precision of making such small structures, I think we are pretty good,' he says. 'For us, the calendar was a joke - but it is based on serious science.' 

Ned Stafford 


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