The preparation of solids, particularly electronic solids in space is discussed. Particular attention is given to the effect of non-gravational environments on the development of homogeneous materials that cannot be manufactured on earth

Gatos, H. C.

English

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SPACE ENV I RONMENT - A NEW D IMENS ION 
IN THE PREPARATION OF UNIQUE SOLIDS 
By Dr. Harry C. Gatos 
Center for Materials Sciences and Engineering 
Massachusetts Institute of Technoloa (MI") 
This report is about solids ; primarily electronic 
solids, and what the absence of gravity in space can 
do in achieving homogenei.ty in materials that we can 
not achieve on earth. First ,  though, some remarks 
will be made regarding materials, in general, and 
solids, in particular. 
Up to about 15 years ago, the mawrlals were pri- 
marily a matter of art, of trial and orror. Science 
was  lagging behind. However, things have changed. 
Now, theoretical o r  scientific o r  fundamental pre- 
dictions about devices, structures, and components 
are way ahead of materials technology. We specu- 
late, in fact, we show, on sound basis, what we could 
do if materials were available; and during the last  
10 or  15 years, nearly all of our expectations were 
realized, when the right materials were available. 
This is  true even in the case of high-strength 
materials, the laser,  semiconductor devices, and 
many of the other solid-state electronics. 
The three parameters that worry us the most 
about materials are: first, structure - that i s ,  how 
perfect the materials are in terms of having their 
atoms in the right places; two, purity - how pure 
they a re ;  and, three, how homogeneous they are. In 
other words, what are the variations in purity and 
structural defects from one point of the material to 
the other '? Two of these problems have come a long 
way in the last 15 years. Regaraing structure, we 
can now prepare materials that are structurally per- 
fect. Particularly in the electronic area, we can pre- 
pare silicon crystals and others which have absolutely 
no structural defects; all the atoms are  where they 
should be. Regarding purity, we have achieved the 
purest materials a s  we know how to detect and identify. 
The purity in some of the electronic materials is in 
parts per trillion o r  better. However, we do not use 
very pure materials, for they are  not good for very 
much. They must have impurities in them, but those 
impurities must be very carefully controlled and be 
extremely homogeneously distributed. Here is the 
stumbling block: how do I identify this type of hetero- 
geneity? A s  w e  have been making progress in identi- 
fying them, we have been detecting more and more 
heterogeneities, which we can directly trace to gravi- 
ty and which we can do nothing about on earth. 
To be specific, you heard earlier that if one, 
on earth, is about to prepare a solid, he must use 
a liquid. One must first melt the solid for a solu- 
tion by heating it. There is no way that one can 
avoid the heat convection which forms gradients - 
changes in temperatures. The heavier things tend 
to settle in the liquids o r  fluids, and the lighter 
things tend to rise. Why is that harmful? When 
these convections take place, the temperature 
changes at the places where you need perfect control, 
that is, between the solid and the melt. This means 
that the rate at which the solid is formed changes, 
and once that happens, there is absolutely nothing 
that you can do to make sure that that solid is going 
to be homogeneous. Why is that important? It is 
important on a microscale, for example, in the case 
of the ordinary casting of metals, of superalloys, 
which we need for high-temperature/high-strength 
applications, as in the case of jet engines and the 
like. There is a serious problem that has to do 
with gravity. A s  you cast a metal, the bottom part  
of the metal o r  the container is going to cool; so it 
solidifies, but in between that solid and the melt 
above it, there is an intermediate region where you 
have both solid and melt. A s  the solid solidifies, in 
many instances some of the lighter elements of that 
liquid are not incorporated into the solid, but are  
left behind in the melt. The melt becomes lighter 
directly above the solid and shoots up in jetlike 
form through the intermediate stage which is a mix 
of solid and liquid. This mixture then thins out and 
destroys the solid configuration and creates what 
people now call freckles or channels. These defects 
are very detrimental to the strength and other charac- 
teristics of materials. 
Microscopic convection becomes of paramount 
importance in the electronic materials, the semi- 
conductors. These devices are  becoming smaller 
and smaller by the year. In the so-called integrated 
circuit, we assemble hundreds of devices the size of 
the order of microns. You can barely see them with 
the naked eye. This implies that on a wafer - a 
piece of silicon 0.75 in. diam by 1 in. , there are 
about I million devices. You have to make sure that 
all of these devices are of materials which have the 
same characteristics on the microscale, to do this. 
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https://ntrs.nasa.gov/search.jsp?R=19730005134 2020-03-17T07:37:27+00:00Z
This is possible, but only with a very small yield to- 
day, and it is believed that the limit is here, not in 
terms of what we could do, but in terms of what we 
can do now. A very large computer on a good-sized 
missile will have 2000 individual integrated circuits, 
which means that about 20, 30, o r  40 thousand devices 
can be accommodated in approximately 2 g of silicon. 
However, the end is not here. We ha?re a number of 
devices waiting on paper and proven on fundamental 
grounds which we cannot produce, because the re- 
quirements on the homogeneity are even greater 
than today's. High sensitivity detectors, for in- 
stance, which require very high controlled impurity 
distribution, cannot be produced today. There are 
instances where we require very high concentration 
on what we want to put into the material; but, in 
order to do that, we require very high thermal gra- 
dients in the metal from which we grow it. In simple 
terms, this means that we have to impose on the 
liquid very high thermal gradients, which are impos- 
sible to maintain on earth. 
What can be done on a microscale in space actu- 
ally - not on a speculative basis? If we take a crys- 
tal of silicon and look at the outside, it looks very 
heterogeneous, not at all uniform. You see stria- 
tions and heterogeneity. But out of that crystal, we 
must take sections to prepare the integrated circuit 
I was referring to before. Thus, it is no wonder 
that the yield of some of these devices is extremely 
small today. 
Let us open up the crystal now and look further 
into it. Under higher magnification we would see 
the striations become very pronounced. If we mag- 
nify further to 1000 times, we would see that be- 
tween the lines of striation there are many, many 
finer lines. Thus, at about 7 pm resolution you see 
many, many lines in between two striations; these 
are strictly the results of convections that take place 
at the interface and, in no way, can be completely 
avoided. 
What can we really do to understand, here on 
earth, that indeed these heterogeneities are due to 
this type convection; that is, they are because of the 
fact that the crystal or  solid does not solidify at the 
same rate throughout? It is not easy to prove this, 
but it has been done recently. To illustrate the 
principle, consider a tree. If you cut a tree across  
and look at the trunk, the rings represent the num- 
ber of years that the tree has lived. Now, if two 
rings are far apart, you know that the tree grew faster 
than in the year where the two rings a re  close togeth- 
er. That is, d e  have time markers here to tell u s  
how fast o r  how slow the tree was growing. We can 
do the same thing with the crystal. We put vibra- 
tions in it, and these vibrations would be the equiv- 
alent of the rings of the years in the t ree ' s  life. 
And those vibrations are of constant frequencies, a 
year each, although it happens to be seconds. The 
separation of the rings of vibration is not constant; 
where we cannot even tell them apart, the crystals 
were growing very, very slowly and where they 
are farther apart, the crystal grew faster. We see 
that the rate of growth of the crystal is not really 
uniform, and, in fact, it changes very drastically. 
I s  there anything here on earth that we can do to 
improve this? Yes. We can decrease the convec- 
tion current in two ways. One, we can turn the 
melt around and heat it on the top, not on the bottom; 
that means, from the bottom up, o r  you grow your 
crystal upside down. We certainly have done this, 
but you can appreciate the fact that you only can 
do it as a laboratory curiosity, that you cannot turn 
things upside down, and even then, note, you do not 
completely eliminate the convection configuration, 
only to some extent. The second method is by using 
a magnetic field. Most metal melts will behave like 
a viscous liquid if you put them in a transverse mag- 
netic field, that is, the viscosity increases and 
they will be less subjected to convective currents. 
We have done that very simply by putting our 
apparatus into a magnet. If the temperature is low, 
we can now achieve what we would expect to achieve 
when there was no gravity. We achieve complete 
homogeneity. Why do we not do that rather than go 
to space? Because we are very limited, in terms 
of size, in te rms  of materials, and in te rms  of 
temperature. W e  achieve good results only at low 
temperatures. If we go up in temperature, even 
the magnetic fields cannot do us  very much good. 
-'' ?y can change things but cannot eliminate the 
, -3,blems. 
What can we expect to achieve then in space ? 
I would like to believe that, in this particular area, 
one need not speculate. Once there is no gravity, 
there will be no convection, and there will be homo- 
geneity. But what do we do with homogeneous mate- 
rials? Do we just improve the yields of the things 
that we can do now? The answer is No. We will 
increase the density of what we can do now by a 
large factor, but more important, we will be able to 
put to use theoretical schemes of devices for higher 
power, higher sensitivity, smaller sizes, and in- 
cluding devices which are so badly needed in the bio- 
logical sciences, for  either detection of pressure 
fluctuations in bloodstreams, temperature flucuationr 
334 
in bloodstreams, or  things of that nature; not to 
mention the impacts of high-temperature semicon- 
ductors, like silicon carbide, zinc sulfide, zinc 
oxide, or exotic types of semiconductors. These 
exotic types a re  so difficult to prepare at all, in 
any applicable form on earth, we are  today sort of 
bypassing them or  a re  ignoring them outright. 
I hope this report has conveyed to you some of 
the reasons that make us look to space, to the use 
of space as  a truly new dimension in making materi- 
als which we can now only play with on paper. 
Having homogeneous materials does not have to do 
just with meeting the great expectations which are  
based on sound scientific and engineering basis. 
Beyond that, I believe, the implications of this type 
of homogeneous materials would even surpass the 
most far  -out science -fiction imagination today. 
Transcribed from tape 
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Space environment: A new dimension in the preparation of unique solids

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