Herramienta número dos: el trabajo cooperativo

The goal o f the m odelling was to reproduce a link, therm ally com parable to two 0.2mm long 8|Ltm diam eter brass links, but making the detector much easier to fabricate and place in an array. A second way to achieve this is to dispense with the thermal links altogether, and use the therm al boundary resistance o f detector bonded directly to the heat sink as the thermal link. As m entioned above we cannot use a H2 0 epoxy bond to do this. This creates a problem

since using a metal filled epoxy is an easy way to make thermal and electrical contact to the thermistor.

A bsorber

Electrically insulating layer

Palladium layer _ Gold layer

Boron ion implant

Electrical sense wire

I NTD Ge H2 0 epoxy F i g u r e 5 . 3 A m o d i f i e d d e t e c t o r in w h i c h t h e r o l e s o f s e n s e w i r e a n d t h e r m a l l i n k h a v e b e e n s e p a r a t e d . T h i s s c h e m e u s e s a h i g h t h e r m a l c o n d u c t i v i t y e p o x y b o n d to p r o v i d e t h e t h e r m a l l i n k b e t w e e n t h e a b s o r b e r , d e t e c t o r a n d h e a t s i n k . L o w t h e r m a l c o n d u c t i v i t y e l e c t r i c a l l y c o n d u c t i n g w i r e s b o n d e d to t h e e l e c t r i c a l c o n t a c t s as s e n s e w i r e s .

A way around this problem is to separate the roles o f thermal link and electrical sense wires. This technique has been used before The scheme envisioned here differs in the nature o f the thermal links used and is shown in figure 5.3. It is assum ed the electrical contact is a thin electrically conducting strip having a low thermal conductivity and heat capacity and thus plays no thermal role in the detector. A small amount o f H20 epoxy is used to make electrical

contact betw een the detector and contact strip. A non-metal filled epoxy is chosen to bond the absorber, detector and link to the heat sink. These joints need to have a low thermal boundary resistance. For this reason a non metal filled high thermal conductivity epoxy can be used, for example Epibond-121. No data are available on the thermal resistance o f a specific {NTD G e\E pibond-121} interface at lOmK. However, there is data on a range o f {metalVEpibond- 121\ MylarXEpibond-121 \M ylar\metal} interfaces down to 50mK These interfaces give a range o f boundary resistances, and since the {E pibond-121 \Mylar} interface is common to all the bonds tested, any change must be attributable to the {metalVEpibond-121} region o f the bond. The different metals in the bond alter its thermal conductivity.

l.E-08 l.E-09 l.E-10 E i.E-11 k-. <D I l.E-12 - o Cu l.E-13 l.E-14 l.E-15

rass wire 0 = 8 )im length 200gm Brass wire 0 = 8 g m length 1000|im

(200x40)|im ^ Copper\Epibond 121\Mylar (200x40)|am^ Lead\Epibond 121\M yhr (200x40)gm ^ Gold\H2o\Gold 0.01 0.02 0.04 0.05 0.06 F i g u r e 5 . 4 T h e e x p e c t e d p o w e r a c r o s s t h e t h e r m a l b o u n d a r y r e s i s t a n c e s o f h i g h c o n d u c t i v i t y e p o x y i n t e r f a c e s . C o m p a r e d t o b r a s s w i r e s a n d t h e t h e r m a l b o u n d a r y r e s i s t a n c e o f a H2 0 e p o x y b o n d . A l l c a l c u l a t e d a s a f u n c t i o n o f t h e t e m p e r a t u r e d i f f e r e n c e d T a c r o s s t h e l i n k at l O m K .

If the 50mK values are extrapolated to lOmK as shown in figure 5.4, an idea o f the expected pow er through a {NTD G e/E pibond-121} joint can be obtained. In the figure these values are

compared with brass wires having lengths, 0.2 and 1mm long (these lengths are close to the

optimal range of values indicated by the optimisation models). The thermal power through

such interfaces is found to be comparable to that through the wires. The area o f the Epibond

121 bond has been scaled for the area presented by the detector to the heat sink 40x200pm^,

shown in figure 5.3. The values are also compared with the extrapolated values for

gold\H20\gold bond of the same area, the conductivity o f this bond is much lower then the

Epibond 121 joints.

The conclusion is that an Epibond 121 (or another high conductivity epoxy) bond can be

tailored to have thermal conductivity comparable to that o f an ideal thermal link. This scheme

has advantages that make it much simpler to create a single detector and construct large

arrays. This scheme is described in section 5.4.2. Overall the boundary resistance link looks

favourable from the initial modelling results and needs experimental verification. It may

prove very hard to manufacture a consistent thermally specific boundary resistance, since the

exact thermal properties o f any boundary resistance are related to the surface properties of the