Measuring and testing – With fluid pressure – Dimension – shape – or size
Patent
1984-07-27
1986-08-12
Levy, Stewart J.
Measuring and testing
With fluid pressure
Dimension, shape, or size
G01B 1302
Patent
active
046048928
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a device for the automatic measurement without contact of the volume of a layer deposited on a substrate. It applies to deposits formed for electronic circuits by hybrid microelectronic technologies.
With the device of the invention, the equivalent of a volume can be measured; we could just as well measure a thickness, we would then have much richer information than the measurement of the thickness alone. The "thickness" measured could be called in this case "topological thickness".
2. Description of the Prior Art
In a general way it is known that a hybrid circuit for example is formed from an electrically insulated substrate on which are deposited different layers of electricity conducting, linearly resistive or not, dielectric or insulating materials.
If we take the case of resistances obtained from resistive inks deposited on a substrate, the thickness of the deposit needs to be accurately controlled for it determines the final value of the resistance according to the conventional equation:
If we consider that the resistance is rectangular in section, we may write:
We may therefore write, supposing L/1=N (number of squares)
Since .rho. and N are constants, it can be seen that the value of the resistance is directly related to that of the thickness: 1/R=f (e).
Up to now, the technician corrects his process by measuring (e). Now at least three basic problems arise from this way of operating:
1. It is necessary to measure the thickness as soon as the deposit has been formed, i.e. according to the terms used in the technique, to measure in the "wet" or "raw" state, so as not to wait for the end of the procedure (about 60 to 90 minutes later) and risk throwing away a large number of the substrates because they are outside the values.
2. The technician knows that the term thickness is extremely vague and canot be more imprecise. In fact, the shape of the deposit is not a parallelepiped and only Equation (1) is applicable since it only considers the section of the resistive element without making assumptions as to the shape of this section. Now, all the manufacturers of silk-screen printed hybrid circuits use the Equation (2) because the thickness is the only measurement which they may apprehend. It can be seen in FIG. 1 that this is an illusion and that no fine correlation can be obtained between this measurement of the layer thickness "e" (FIG. 1) and the value of the resistance obtained after baking and that consequently silk-screen printing systems cannot be built which would be self correcting and which would thus be really automatic. Depending on the widths of the resistances, and their length, different shapes in section may be obtained; these result from the balance of the surface tension forces and the forces of gravity. Consider what information can the measurement of "e" (illustrated at the left of FIG. 1) give in the case of a resistance, e.g. in the case of resistance 2 of small width l1 (.ltoreq.1 mm), or that of a resistance 3, or that of resistance 4 for which l3 (seen in FIG. 1) is approximately greater than 2 mm and is greater than l2.
3. The methods for measuring the thickness used at present do not even allow the section, or average, thickness to be measured.
The solutions known at present are not adapted to the problem of measuring the thicknesses of silk-screen printed deposits or deposits made by another method. The different methods of measurement at present available may be presented by showing their limits, which will more clearly bring out the advantage of the present invention.
Measurement by a fluorescence RX, though it is effectively without contact, is however related to the nature of the atoms forming the layer. Now in so far as resistive inks are concerned, from the low values (a few ohms per square) to very high values (a few tens of megohms per square), their composition is essentially variable. Finally, the measurement takes between 5 and 30 seconds.
Measurement by back-scattering of beta
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patent: 2953918 (1960-09-01), Fowler et al.
patent: 3213670 (1965-10-01), MacGeorge
patent: 3495442 (1970-02-01), Rejsa
patent: 3757563 (1973-09-01), Kampf et al.
patent: 3948082 (1976-04-01), Zumbach et al.
patent: 4364270 (1982-12-01), Heinzl et al.
Kahwati, Research Disclosure No. 137, 9-1975.
Lettvin Norman
Levy Stewart J.
Williams Hezron E.
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