FR2559959A1 - Microwave diode with external connections taken by means of beams and its method of production. - Google Patents

Abstract

The invention relates to a microwave diode provided with external connections taken by means of beams and operating in the millimetre range. The diode is produced from a substrate 1 subsisting over a minimum thickness e only in the region of the semi-conductor wafer and covered on the back surface with a metallisation 10 coming into direct contact with one 8 of the beams, thus enabling the series resistance of the diode to be minimised. Application to "beam-lead" PIN diodes operating at 94 GHz.

Description

MICROWAVE DIODE WITH EXTERNAL CONNECTIONS TAKEN
USING BEAMS AND ITS MANUFACTURING METHOD
The present invention relates to a microwave diode with a so-called vertical structure working in the millimeter range, in particular at frequencies equal to or greater than 60 GHz, and the external connections of which are taken by means of flat metal beams, as well as a method of production. of such a diode. This diode is mainly intended to be used in switching.

By vertical structure diode is meant a diode whose layers forming it are arranged vertically with respect to one another. By way of example, mention may be made of the PIN diode produced from an N + type doped semiconductor substrate, for example on which a so-called I layer of high resistivity material is deposited, then a P + type layer, the substrate thus providing on the one hand the mechanical support and on the other hand the cathode contact layer of the diode. This vertical PIN diode structure is to be opposed to the so-called surface or horizontal structure according to which each of the P + and N + type layers is formed in a substrate of high resistivity material, as described in the French patent application filed on 9 December 1981 by the
Applicant and published under the number 2,517,384.

 On the other hand, in the microwave domain, the diodes used are frequently provided with external connections taken via flat beams, better known under the English terminology "beam-lead", because these beams of a on the other hand facilitate interconnection with the surrounding microwave circuit and on the other hand have a lower induction than the soldered wires.

 A microwave diode structure is already known, such as for example a PIN diode with vertical structure, working at high frequency and provided with external connections constituted by metal beams fixed flat according to the so-called "beam-lead" technology. This PIN diode, used as a switching diode mounted in shunt or in series in a transmission line, is for example produced from an N + type doped semiconductor substrate on which two superposed layers are formed by epitaxy, one of which is called I is made of material of the highest possible resistivity and the other of which is of the P + type. A chemical attack is carried out from the epitaxial layers into the substrate over a given depth equal, for example, to 20 microns, in order to create a cradle of rectangular shape, for example, under which a certain thickness of substrate remains. to delimit two semiconductor islands connected together by the intermediary of the substrate, one of which is for example circular in shape and the other of which is in the form of a crescent, the round island being opposite and at the center of I ' indentation of the crescent. The cradle is filled with a dielectric material, such as for example glass, and the crescent-shaped layers P + and I of it are then removed by chemical attack. An anode contact is ensured by a first metal beam s' pressing on the layer P + of the round dish, the latter constituting the diode proper and on the glass to end in cantilever, while a cathode contact is ensured by a second metal beam resting on the substrate N by marrying its crescent shape to make an ohmic contact and on the glass to end in cantilever.

 It can be noted that this particular diode structure working at high frequency according to the prior art was designed to solve a particular technical problem which consisted in greatly reducing the stray capacitance between the anode beam and the substrate: this problem was solved by realizing a deep cradle around the diode, which made it possible to fill it with a dielectric whose thick thickness contributed to a marked reduction in the stray capacitance. However, this type of diode does not solve another technical problem of great importance at high frequencies, which consists in also reducing, and notably, the resistance in series with the diode, also called resistance of access to the diode, which is formed by the substrate between the cathode beam and the N ± layer I substrate junction of this PIN diode.

 Indeed, at high frequency, the semiconductor material constituting the substrate of the PIN diode is subjected to the skin effect so that the current in the diode is localized in a certain thickness of skin of the substrate. However, as is well known, the higher the working frequency of the diode, the more the skin effect will contribute to increasing the series resistance of the diode. In addition, in the case where this switching diode is mounted in shunt in a transmission line, on the one hand the insertion losses at low level and with zero or reverse polarization are proportional to the product of this resistance by the square of the capacitance of the diode, and on the other hand the insulation provided by the diode in direct polarization is inversely proportional to this resistance; the same for a series mounting of the diode in the transmission line, with the insertion losses obtained in direct polarization of the diode and with the insulation with zero or reverse polarization. Consequently, it is necessary at high frequencies to also reduce, in addition to the capacitance of the diode, the series resistance of the latter in order to obtain the lowest possible insertion losses and / or the best possible insulation.

 The object of the present invention is to improve this type of microwave diode described above and working in the millimeter range so as to reduce its series resistance as much as possible, while guaranteeing it a stray capacitance which is greatly reduced and hence giving it low losses of insertion and high insulation in its use as a switching diode.

 To this end, the subject of the invention is a microwave diode comprising a semiconductor substrate having two opposite parallel faces respectively called front face and rear face, at least two superimposed layers, one of which is made of semiconductor material and partially covers the front face of the substrate and the other of which is made of semiconductor material or metal, these layers forming an island delimited by a cradle extending in the substrate over a given depth and being filled with a dielectric material, the cradle being produced so as to delimit in the form of a crescent, the part of the front face not covered with the substrate, and two metal beams of external connections, one of which making a first contact rests flat on the pilot and on the dielectric material to end in carrier false and the other taking a second ohmic contact rests flat on the front face of the substrate by marrying its crescent shape and on the dielectric material to end in a cantilever characterized in that the rear face of the substrate is covered with a metallization.

 According to a variant, the substrate is thinned to a minimum thickness and remains, once metallized on the rear face, on the one hand at its junction with the semiconductor layer covering it and on the other hand at the level of the metal beam taking the ohmic contact.

 According to another variant which follows from the previous one, the metallized substrate on the rear face remains over a minimum thickness and only at its junction with the semiconductor layer covering it, so that the metal beam taking the ohmic contact and the metallization in rear side are in contact with each other. This configuration has the advantage of minimizing the series resistance between the beam taking the ohmic contact and the junction of the diode.

 The invention also relates to a method for producing such a microwave frequency diode, and which is more particularly intended to be used for obtaining a maximum reduction in the series resistance of the diode.

Other characteristics and advantages of the invention will appear more clearly in the detailed description which follows and which refers to the appended drawings, given solely by way of example and in which:
- Figures la and lb respectively represent a top view and a sectional view along line 1-1 of an embodiment of a PIN diode according to the invention;
- Figure 2 shows a sectional view identical to Figure 1b of a variant of a PIN diode according to the invention
- Figure 3 shows a sectional view identical to Figure 1b of another variant of a PIN diode according to the invention; and
FIGS. 4 to 10 represent sectional views illustrating the different stages of the method for producing the PIN diode according to FIG. 3.

 In these different figures, the same references relate to the same elements which fulfill the same functions for the same results.

 Figures la and lb show respectively a top view and a cross-sectional view of a microwave diode according to a first embodiment of the invention, such as for example a silicon PIN diode with vertical structure, provided with external connections beam-lead, and intended to work in the millimeter domain, at 94 GHz for example. This diode is mainly intended for use in switching by being mounted either in shunt or in series in a transmission line.

 In FIGS. 1a and 1b, this PIN diode, as final product comprises a doped silicon substrate 1 of N + type for example, of thickness equal for example to 25 microns, and having two opposite opposite faces respectively called front face or upper and rear side or lower lb. The substrate is partially covered on the front face with two superimposed semiconductor layers, one of which 2, called I, is made of silicon of the highest possible resistivity and the other 3 of which is made of P + type doped silicon. The assembly formed by the substrate 1 and the two layers 2 and 3 therefore forms the actual diode, the substrate 1 thus providing on the one hand a mechanical support and on the other hand the cathode contact layer, and the layer 3 providing the anode contact layer.

The diode is delimited by a deep cradle forming a box 5 having an outer periphery, for example rectangular (FIG.
la) and extending in the substrate 1 over a given depth, for example equal to 10 microns, so as to allow the latter to remain over a certain thickness E ′, of the order for example of 15 microns, between its rear face lb and the bottom of the cradle.

 In addition, the cradle 5 is produced so as to delimit in the form of an ilil, for example circular (figure la) or square, the diode proper, and in the form of a crescent (figure la) the part of the front face. not covered with the substrate. For reasons relating to the manufacture of the diode, the crescent-shaped front face of the substrate, identified in la, is not exactly at the same level as that covered with layer 2.

 As it appears in FIG. 1 a, the semiconductor island of circular shape is placed opposite and substantially at the center of the notch of the crescent formed by the substrate.

 The cradle 5 is filled with a dielectric material endowed with good mechanical qualities, such as for example molten glass.

Once filled, the glass cradle has a slightly curved surface.

 A first contact, or anode contact, is provided by a first metal beam 6 resting flat on the P + layer of the diode and on the surface of the glass cradle 5 to end in overhang. This 6 beam-lead anode beam is constituted for example by successive deposits of titanium, platinum and gold.

 Thus, having a large thickness of glass in the cradle 5 makes it possible to significantly reduce the parasitic capacity, symbolically represented in FIG. 1b, between the anode beam 6 and the substrate 1.

 A second ohmic contact, or cathode contact, is ensured by a second metal beam 8 resting flat on the front face of the substrate by matching its crescent shape (FIG. 1a) to make the ohmic contact there and on the surface. glass cradle 5 to end in overhang. As with the anode beam, the 8 beam-lead cathode beam consists of the same chain of successive deposits of titanium, platinum and gold.

According to one aspect of the invention, the rear face 1b of the substrate is entirely covered with a metallization 10, for example of the same type as that forming the beams 6 and 8, that is to say by means of titanium, platinum and Golden. Thus, this metallization 10 forms a resistance of low value in parallel with that formed by the substrate, which is of high value due to the skin effect at the high working frequency. Consequently, the current in the diode, between cathode and anode beams, will pass for a very large part through metallization 10, which therefore already makes it possible to reduce the series resistance of the diode between the cathode beam and the N + junction
I. Consequently, the combination of a large reduction in the parasitic capacitance and a first notable reduction in the series resistance of the diode already makes it possible to obtain a substantial reduction in its insertion losses and an increase in its insulation. as a switching diode.

 An even greater reduction than the previous reduction in the series resistance of the diode is obtained in the context of the variant shown in Figure 2 which is also a sectional view along line I-I of Figure la.

 In fact, according to this variant of PIN diode in FIG. 2, and the basic structure of which repeats that described above, the substrate 1 is thinned to a minimum thickness so as to completely release the bottom of the cradle 5. Thus, the thinned substrate I remains in a first part of minimum thickness e at its junction with layer 2, and in a second part in the form of a crescent separated from the first and of thickness e very little different from e under the cathode beam 8 taking ohmic contact. As for metallization 10, it covers the two parts of substrate 1 thus produced on their rear face 1b as well as the bottom of the part of the cradle 5 situated between these two parts of substrate.

 With such a substrate of minimum thickness under the cathode beam 8 and under the layer 2, the current in the diode, between cathode beams and anode, will pass in the part of substrate under the cathode beam, then entirely in metallization 10 and finally in the substrate part under layer I, which makes it possible to further reduce the series resistance of the diode between the cathode beam and the N + I junction. As the stray capacitance according to this variant is lower than previously due to the thinning of the substrate under layer I, this second stronger reduction in the series resistance of the diode therefore makes it possible to further reduce its insertion losses and to increase its insulation.

 According to another preferred variant shown in FIG. 3, which follows from the previous one and which is also a sectional view along line II of FIG. 1a, the substrate 1 is always thinned to a minimum thickness e at its junction with layer 2, but is completely eliminated under the cathode beam 8, so that said substrate now remains only at the level of the diode itself. As for metallization 10, it always covers the substrate on its rear face 1b as well as the bottom of the cradle 5 and comes into direct contact with the cathode beam 8, thus forming a short circuit between them.

 Of course the ohmic contact on the substrate 1 is always taken by the cathode beam 8 but through its electrical connection with the metallization 10 which covers the rear face of the substrate.

Under these conditions, having a short circuit between the cathode beam 8 and the metallization 10 allows the current in the diode to pass directly, therefore entirely, into the metallization 10. With this short circuit and a thickness minimum of substrate 1, the series resistance of the diode between the cathode beam and the N + junction
I is therefore reduced to the maximum. Consequently, this third reduction, termed maximum, of the series resistance of the diode, associated as previously with a large reduction in the parasitic capacitance, makes it possible to obtain very low insertion losses as well as very high insulation.

 It will be noted that having a metallization on the rear face of the beam-lead diode according to the invention advantageously makes it possible to respond to a possible problem of heat dissipation: in fact, in this case, the diode can be directly reported by its metallization on the rear face, for example by soldering, on a conventional metal base, for example copper, so that the heat released by the beam-lead diode during its use is dissipated by its metallization and the base; in this case, the cathode beam can be omitted.

 We will now describe with reference to FIGS. 4 to 10 a method for producing a beam-lead microwave diode according to the invention, and more particularly that which is technologically the most difficult to produce, namely the minimum series resistance diode according to the preferred variant of Figure 3. Of course, the production assumes a collective production process, which is normal for this type of diodes.

 In these different FIGS. 4 to 10, the same references have been used as those in FIG. 3 for the same elements which fulfill the same functions for the same results.

 This process for the collective production of PIN beam-lead silicon diodes is implemented from a substrate 1 of N + doped silicon (doping 4.1019 atoms / cm3) in the form of a washer with a thickness of approximately 200 microns and having a very low resistivity, of the order of 0.002 ohm.cm, to introduce minimum losses at the frequency of use of the diodes, at 94 GHz for example.

The first step of the process, represented in FIG. 4, consists in depositing a barrier layer 12, of thickness approximately 0.5 micron, on the rear or lower face 1b of the substrate. This barrier layer can be produced for example from silicon oxide
SiO2 obtained by thermal oxidation of the silicon substrate. Then, in a conventional reactor, a polycrystalline silicon layer 13 is deposited on the barrier layer 12 over a thickness of the order of 100 microns.

 It will be noted that the advantage of this barrier layer 12 deposited between the substrate 1 and the thick polycrystalline silicon layer 13 lies in the fact that a chemical attack carried out subsequently on the rear face, that is to say an attack of the layer 13 in the direction of the substrate 1 will stop on the layer 12, which will allow this attack on the rear face to be well controlled.

 This thick layer 13 will therefore serve as a mechanical support in order to be able to carry out the second step of the method which firstly consists, as shown in FIG. 5, in thinning the substrate 1 to a given thickness e as small as possible. This thinning of the substrate is carried out by any process known to those skilled in the art, for example chemical-mechanical. Then developed by epitaxy on the considerably thinned substrate a layer 2 called I of high resistivity silicon with a thickness of the order of a few microns depending on the desired breakdown voltage, then a layer 3 of type P + of thickness 1 '' order from 2 to 4 microns (doping 1019 3 atoms / cm).

 Note that the P + zone can also be produced by diffusion or implantation, without departing from the scope of the invention.

 The next step, represented in FIG. 6, consists in carrying out a chemical attack by aqueous route or a plasma attack also called attack by dry route, under mask, progressing from the semiconductor layers until in the substrate on all its thickness, so as to create deep grooves forming identical cradles 5, of outer periphery, for example rectangular, which delimit the future diodes. In addition, in FIG. 6, the cradle 5 of each diode is produced so as to delimit two semiconductor plates, one of which is for example circular in shape and is intended to constitute the diode proper and the other of which is identified at 14 is in the form of a crescent as illustrated in FIG. 1 a. This attack can either stop on the stop layer 12 or penetrate slightly into the polycrystalline silicon constituting the thick layer 13. As an illustration, the depth of each cradle is of the order of 20 microns for a working frequency equal to 94 GHz.

 Then, each cradle 5 is filled with a dielectric material with good mechanical qualities, such as for example molten glass, and has, once filled, a slightly curved upper surface.

 The next step, represented in FIG. 7, consists in carrying out, for each diode, a chemical or plasma attack on the entire thickness of the semiconductive enclosure in the form of a crescent (14, FIG. 6), so as to completely eliminate its layers P + and I as well as its substrate N +, this attack stopping on the stop layer 12.

15 is shown in Figure 7 the crescent-shaped opening obtained after attack in each of the diodes.

The next step, represented in FIG. 8, consists in depositing, for each diode, a metallization either by photogravure or by a lift-off technique to make grow two beams which assure one 6 the anodic contact and the other 8 the cathodic contact, ohmic, for each diode. In FIG. 8, the beam 6 rests flat on the P + layer of the diode and on the surface of the glass cradle 5 while the beam 8 rests flat on the periphery of the crescent-shaped opening obtained in the previous step, by marrying its shape, and on the surface of the glass cradle 5. This metallization consists in first depositing a layer of platinum, then in forming platinum silicide by heat treatment and to be completed by successive deposits of titanium (for example 1000air of platinum (for example 1500A) and gold (for
o example 5000 A). The chain of deposits is represented by the two beams thus formed 6 and 8 for each diode in FIG. 8
SiPt-Ti-Pt-Or.

 The next step, represented in FIG. 9, consists in carrying out a chemical attack on the rear face in the direction of the remaining substrate 1 so as to completely remove the polycrystalline silicon layer 13, this attack stopping on the stop layer 12 This stop layer 12 is then removed by any known method, for example by ion machining.

 The next step, represented in FIG. 10, consists in locally metallizing at the level of each diode the underside of the structure obtained in the previous step. This metallization consists first of all in depositing a layer of platinum, then in annealing preferably with a laser beam located at each diode, thus forming platinum silicide. The metallization is then supplemented by successive deposits, for example of titanium, platinum and gold of the same thickness as those forming the beam-lead beams 6 and 8. The layer 10 in FIG. 10 represents all of the SiPt-Ti-Pt-Or deposits which covers the substrate 1 on the rear face as well as the bottom of the cradle 5 and which comes into direct contact with the cathode beam 8.

 Therefore, having a short circuit of the metallization 10 on the rear face with the cathode beam 8 and a minimum thickness e of substrate between the metallization 10 and the N + I junction makes it possible to achieve a maximum reduction of the series resistance of each future diode.

 The different diodes are then separated by a chemical attack on the rear face of the silicon so as to overhang each of the two beams 6 and 8 of each diode, as shown in FIG. 10.

 It will be noted that the above description has been made with reference to a PIN type diode whose base material was silicon.

Of course, the invention also applies to any other type of diode, Schottky for example for which the layer 3 is made of suitable metal, as well as for a basic material of the mV family,
GaAs for example. In the case of a diode the substrate of which is made of gallium arsenide, the SiO2 stop layer described above during the production process is not obtained by thermal oxidation, but by a deposition of silicon oxide SiO2 optionally mixed with silicon nitride Si3 N4.

 Also falls within the scope of the invention the case where the cantilever part of the beams is eliminated. The beams are then reduced to metal studs used in particular for the reverse welding process of a diode on a base, called "flip chie".

Claims (6)

 1. Microwave diode comprising a semiconductor substrate (1) having two opposite parallel faces respectively called front face (la) and rear face (lob) at least two superimposed layers (2, 3) one of which (2) is made of semiconductor material and partially covers the front face of the substrate and the other (3) of which is made of semiconductor material or metal, these layers forming an island delimited by a cradle (5) extending in the substrate over a given depth and being filled with '' a dielectric material, the cradle being produced so as to delimit in the form of a crescent the part of the front face not covered with the substrate, and two metal beams (6, 8) of external connections, one of which makes a first contact (6 ) is supported flat on the enclosure and on the dielectric material to end in overhang and whose other taking a second ohmic contact (8) is supported flat on the front face of the substrate by marrying its crescent shape and on the dielectric material e to end in overhang, characterized in that the rear face of the substrate (1) is covered with a metallization (10).  2. Microwave diode according to claim 1, characterized in that the substrate (1) remains over a given thickness between its rear face (lb) and the bottom of the part of the cradle (5) located between the island and the metal beam taking ohmic contact.  3. Microwave diode according to claim 1, characterized in that the substrate (1) remains over a given thickness in a first part at the level of the island and in a second crescent-shaped part at the level of the metal beam taking the ohmic contact, the metallization (10) covering the rear face of each of the first and second parts of the substrate as well as the bottom of the part of the cradle (5) located between these parts of substrate.  4. Microwave diode according to claim 1, characterized in that the substrate (1) remains over a given thickness only at the level of the closure of the metallization (10) covering the rear face of the substrate as well as the bottom of the cradle and coming into contact direct with the metal beam taking the ohmic contact (8).  5. Microwave diode according to one of the preceding claims, characterized in that the metallization (10) on the rear face comprises successive layers of titanium, platinum and gold.  6. Method for the collective production of microwave diodes according to claim 4, characterized in that it comprises, from a semiconductor substrate (1), the following steps:  a) depositing a barrier layer (12) on one of the two parallel opposite faces of the substrate, followed by depositing a thick layer of polycrystalline silicon (13) on the barrier layer;  b) thinning of the substrate (1) to a given thickness;  c) a formation of at least two superimposed layers (2, 3) on the other face of the thinned substrate, one of which covering the substrate is made of semiconductor material and the other of which is made of semiconductor material or metal;  d) a first attack of the layers (2, 3) and of the substrate (1) over its entire thickness so as to form cradles (5) delimiting the diodes, the cradle of each diode being produced so as to delimit two semiconductor islands of which one (14) is in the form of a crescent and the other of which is opposite and at the center of the notch of the crescent and is intended to constitute the actual diode;  e) filling each cradle (5) with a dielectric material;  f) a second attack to eliminate the layers and the substrate constituting the crescent-shaped enclosure (14) of each diode, this attack stopping on the stop layer;  g) a deposit of external connections by metallization of two beams (6, 8) for each diode, one of which (6) forming a first contact rests flat on the semiconductor island remaining for each diode once the previous step performed and on the dielectric material and the other (8) forming a second contact rests flat on the crescent-shaped periphery for each diode as obtained in 1 previous step and on the dielectric material;  h) a third attack on the polycrystalline silicon layer (13) so as to remove it completely, this attack stopping on the stop layer, followed by elimination of the stop layer (12); ;  i) metallization of the rear face of each complex structure obtained in the previous step, so that this metallization (10) comes into direct contact with the metal beam forming the second contact (8) for each diode; and  j) separation of the diodes by attack on the rear face so as to overhang each of the two metal beams (6; 8) for each diode.