Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
1998-10-05
2001-04-03
Brown, Glenn W. (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S754090
Reexamination Certificate
active
06211688
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of electronics and microelectronics, and more particularly, to test areas for devices with integrated electronic circuits, and related methods.
BACKGROUND OF THE INVENTION
Test areas, generally formed on the surface of substrates are electrically connected to different parts of circuits in order to carry out checks on the correct operation of these circuits. The checks typically include measuring continuous control voltages at the test area and are carried out by means of microprobes. These microprobes are connected to suitable measurement equipment and are temporarily applied to contact points on the test area to carry out the measurements.
The known type of test areas for electronic circuits are in the form of metal contact points. These contact points, generally square in shape, have dimensions of the order of 20 &mgr;m×20 &mgr;m and are arranged on the surface of a substrate in which one or more electronic circuits are formed. The contact points are surrounded by an insulating layer called a passivation layer which forms a slight rounded mass in the surface of the substrate.
The contact points or pads can be connected to parts of an electronic circuit formed in the surface of the substrate, but also to parts of a circuit formed deep in the substrate. This is notably the case when the substrate is a multi-layer substrate comprising a stack of conducting layers separated by insulating layers. In this case, a contact point of a test area is formed in the surface of the substrate and is connected to a component of a deep conducting layer by means of conductor filled vias which pass through the insulating layers.
In order to carry out the control measurements on the electronic circuits, a tool fitted with one or more microprobes is brought to the surface of the substrate to a distance close enough for the ends of the microprobes to make contact with the substrate. The microprobes take the form of fine metal strips which are pressed onto the surface of the substrate. These fine metal strips are generally flexible and are comparable to hair.
A micrometric displacement device, such as a micropositioner, is then actuated to cause a relative displacement of the probes and the substrate along the plane of the substrate. This displacement is intended to bring the ends of the probes into coincidence with the contact points on the test areas. The relative displacement movement of the probes and the substrate can be controlled by various devices, such as optical location systems or viewing systems that use a CCD camera.
The main difficulty that is apparent during the control operation of the circuits described above is positioning the ends of the probes exactly on the contact points. Incorrect positioning of the probes can cause erroneous control measurements. Another difficulty is to keep the ends of the probes on the contact points. This is attributed to the fact that the contact points protrude from the surface of the substrate.
To facilitate the positioning and holding of the microprobes on the contact points, the dimensions of the contact points are commonly increased. However, increasing the dimensions, and hence the area of metal contact points on the surface of the substrate, is accompanied by the creation of or the increase of parasitic capacitances.
In effect, the contact points of the test areas define capacitors with the other conductive parts of the integrated circuits of the substrate. These capacitors have a capacitance that increases with the area of the contact points. These capacitances are parasitic capacitances which modify and impair the behavior of electronic circuits. This problem is a particular nuisance for electronic circuits operating at high frequency.
On the other hand, when the dimensions of the contact points are reduced so as to limit the parasitic capacitances, expensive equipment for adjusting the probes on the contact points must be installed and good electrical contact between the probes and the contact points is not always guaranteed. Furthermore, location of contact points of small dimensions on the surface of the substrate proves to be difficult.
SUMMARY OF THE INVENTION
An object of the invention is to provide a type of test area for electronic circuits that resolves the difficulties mentioned above. A particular object is to provide a test area that permits easy and automatic adjustment of the microprobes on the contact points without having to use complex and expensive alignment devices.
Another object of the invention is to provide test areas that guarantee that the ends of the microprobes are held on the contact points. A further object of the invention is to provide test areas in which location on the surface of the substrate is facilitated. A still further object of the invention is to provide a particularly simple and inexpensive method for producing test areas.
In order to achieve these objectives, a more precise aspect of the invention is to provide a test area for an electronic circuit comprising a contact point formed on the surface of the substrate, and guide means displaced on the surface of the substrate for guiding a test probe towards the contact point. In the context of this description, an electronic circuit is understood to mean both complete and complex circuits and portions of a circuit comprising one or more electronic components.
Because the guide means are arranged on the surface of the substrate, they are sufficient for the probes to come close to the contact point such that they rest against the guide means. In effect, whenever a probe is resting against the guide means for a contact point, any displacement, however imprecise, of the probe in the direction of the contact point results in the end of the probe automatically making contact with the contact point.
Because of this characteristic, contact points having a very small surface area can be created without compromising the ease of positioning the microprobes on the points. By reducing the surface area of the points, the size of the parasitic capacitances that the contact points are capable of generating is also reduced.
According to one particular embodiment, the guide means comprises, in proximity to the contact point, at least one elongated boss formed on the surface of the substrate that is oriented towards the contact point. In particular, the guide means may comprise a plurality of elongated bosses arranged in a radial fashion around the contact point.
By being arranged all around the contact point, the guide bosses permit, to a certain extent, that when the end of a probe is applied onto a contact point, it does not slide to the side of this point. Hence they constitute, in a way, means of locking the probe onto the contact point. This function can be further improved, according to another particular aspect of the invention, when the contact point is designed in such a way as to form a depression in relation to the surface of the substrate.
The elongated bosses can have a metal core, preferably insulated from the contact point. They can furthermore have an insulating covering in the form of a passivation layer. The insulating covering, has for example, a surface layer of silicon nitride. The surface layer of silicon nitride provides not only good electrical insulation properties, but also good resistance to abrasion and wear.
An object of the invention is also an electronic circuit including one or more test areas of the kind described herein. Another object of the invention is a method of producing a test area of an electronic circuit on a surface of a substrate. The method preferably comprises the following steps:
a) forming on the surface of the substrate of a layer of electrically conducting material in electrical contact with at least one part of an electronic circuit of the substrate;
b) shaping the layer of electrically conducting material to define a contact point in electrical contact with part of the electronic circuit, and to define elongated bosses surrounding the contact point
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Brown Glenn W.
Galanthay Theodore E.
STMicroelectronics S.A.
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