Method of forming porous silicon in a silicon substrate, in particular for improving the performance of an inductive circuit

The method is for forming porous silicon in a silicon substrate, in particular for improving the quality factor of an inductive circuit produced on a silicon semiconductor wafer which also incorporates integrated transistors. The rear face of the wafer, already incorporating the transistors and the inductive circuit on its front face, is placed in contact with an acid electrolyte containing hydrofluoric acid and at least one other acid. An anodic oxidation of the silicon of the wafer at the rear face is carried out so as to convert this silicon into porous silicon over a predetermined height from the rear face which is in contact with the electrolyte.

FIELD OF THE INVENTION
 The invention relates to semiconductor processing, and, more particularly,
 to the formation of porous silicon in a silicon substrate, and which may
 be applied advantageously, but without implying any limitation, to the
 production of inductive circuits produced in integrated form on a silicon
 substrate.
 BACKGROUND OF THE INVENTION
 Inductive circuits are an essential component of radio frequency circuits
 which are used particularly in the field of mobile telephones. An
 inductive circuit is an element of an inductive/capacitive tuned resonant
 circuit. Inductive/capacitive tuned circuits are, in particular, used in
 tuned radio frequency amplifiers (generally bandpass radio frequency
 amplifiers).
 One particularly sensitive feature of these amplifiers resides in the
 selectivity of the inductive/capacitive resonant circuit. This is so since
 outside the working frequency band of the amplifier, all the other
 spectral components are considered as noise. Conventionally, the frequency
 response of an inductive/capacitive tuned amplifier contains a peak which
 is centered on the resonant frequency FO and has a width at the
 half-height commonly denoted by .DELTA.f. The resonant frequency FO is
 equal to the inverse of the square root of the product of the inductance
 times the capacitance.
 The ratio FO/.DELTA.f is referred to as the "quality factor" of the
 inductive/capacitive resonant circuit. In the rest of the text, and by a
 convenient oversimplification, the term "quality factor" will be
 associated with the inductive circuit on its own. This quality factor
 should be as high as possible. However, the width .DELTA.f of the
 resonance peak is directly proportional to the energy losses of the
 resonant circuit. Consequently, the higher the losses are, the more the
 quality factor is reduced.
 On a silicon substrate, the inductive circuits are produced by forming a
 metal spiral which rests on the silicon on an insulating layer, typically
 silicon dioxide having, for example, a thickness of 1 micron. However,
 unlike gallium arsenide (GaAs) semiconductor substrates, silicon
 substrates have low resistivity. The result of this is consequently that
 the magnetic field generated by the flow of current in the metal turns
 induces very high eddy currents in the underlying silicon substrate. Some
 of the energy of this magnetic field will therefore be dissipated in the
 form of heat, consequently reducing the value of the quality factor.
 To address this problem, it has in particular been proposed to use
 inductive circuits with a high quality factor which are external to the
 integrated circuit containing the other elements of the tuned amplifier.
 Notwithstanding, such an approach requires extra components and support
 which are incompatible with low production costs. Furthermore, stray
 interference can impair the operation of the amplifier, in particular
 because of the interconnections between the integrated circuit and the
 external inductive circuit.
 It has therefore been found particularly advantageous to arrange all the
 components of the radio frequency amplifier, and, in particular, all the
 passive components, such as inductors and capacitors, in the same
 integrated circuit. In this regard, it has been proposed to make selective
 localized substrate recesses under the inductor zones. This is done by
 localized chemical attack or etching, particularly using potassium
 hydroxide (KOH).
 Unfortunately, such an approach requires specific infrared masks arranged
 on the rear face of the substrate whose alignment with the components
 arranged on the front face is particularly difficult. It also entails
 problems in coating the chip with resin, because of the presence of these
 relatively large cavities in the substrate.
 Another advocated approach includes fully removing the silicon substrate
 and replacing it with a glass substrate. This approach also has a large
 number of drawbacks, in particular because of the difference between the
 expansion coefficients of silicon and the glass, the fragility of the
 substrate, and the difficulty of welding and coating the chip with resin.
 It has further been proposed, in an article by Y. H. Xie et al., entitled
 "An Approach For Fabricating High Performance Inductors On Low Resistivity
 Substrates", IEEE BCTM 5.3, September 1997, pp. 88-91, to produce an
 inductive circuit on a silicon substrate which is partly porous, so as to
 increase its resistivity. More precisely, a silicon substrate, typically
 having a thickness of 300 microns, is subject to anodic oxidation in an
 aqueous solution of hydrofluoric acid having a concentration of 20% by
 volume with an anodic current density equal to 50 mA/cm.sup.2. This makes
 the silicon porous to a thickness between 50 and 250 microns. An
 insulating layer of silicon dioxide is then deposited on the outer surface
 of the substrate, and covered with a metal spiral so as to form the
 inductive circuit.
 However, the method described in this prior art document is applicable only
 to the production of an inductive circuit. It is unsuitable for the
 simultaneous production, on the same porous silicon substrate, of other
 active components, such as, for example, bipolar transistors and/or
 complementary field-effect transistors with insulated gates (CMOS
 transistors). These are typically needed for producing the other elements
 of an integrated tuned radio frequency amplifier. Indeed, this document
 indicates that the internal surface of the pores of the porous silicon is
 a strong source of contamination. This is so in particular for the gas
 atmospheres of the ovens which would be used to produce the bipolar or
 CMOS transistors on this same porous silicon substrate. What is more,
 further to these problems of contamination, the porous silicon undergoes
 surface deformations when hot, and this is particularly unsuitable for the
 production of bipolar and/or CMOS transistors prior to this phase of
 converting silicon into porous silicon.
 SUMMARY OF THE INVENTION
 The object of the invention is to provide a more satisfactory approach to
 the problems described above.
 One object of the invention is to reduce the eddy-current losses in the
 underlying substrate in an inductive circuit produced in integrated form
 in an integrated circuit which also includes integrated transistors, this
 integrated circuit being produced on a silicon semiconductor wafer. The
 quality factor of the inductive circuit is then increased.
 The invention is directed to a method of reducing the eddy-current losses
 of an inductive circuit produced on a silicon semiconductor wafer which
 also incorporates integrated transistors, in which the rear face of the
 wafer, already incorporating the transistors and the inductive circuit on
 its front face, is placed in contact with an acid electrolyte containing
 hydrofluoric acid and at least one other acid. Anodic oxidation of the
 silicon of the wafer at the rear face is carried out so as to convert this
 silicon into porous silicon over a predetermined height (thickness) from
 the rear face which is in contact with the electrolyte.
 In other words, according to the invention, the conversion of the silicon
 of the substrate into porous silicon is carried out after the substrate
 has undergone all the conventional treatments for producing the
 transistors and the various circuits, such as, for example, by using
 conventional CMOS or biCMOS (bipolar-CMOS) technology. A post-treatment of
 the silicon is therefore carried out, on a wafer already equipped at the
 front face with the various integrated circuits containing the various
 transistors and inductive circuits. This is contrasted to a preconversion
 of silicon into porous silicon carried out on a virgin wafer.
 Notwithstanding, the thickness of the semiconductor wafers customarily used
 is on the order of several hundred microns. However, the electrolytes
 customarily used to carry out anodic oxidations of silicon have
 hydrofluoric acid concentrations generally less than 35% and furthermore
 contain ethanol. Ethanol provides a surfactant allowing the surface
 tension of the electrolyte to be reduced, thus promoting the elimination
 of the hydrogen bubbles resulting from the anodic oxidation with a view to
 obtaining better uniformity of the attack on the silicon. However, with
 such electrolytes, the values of anodic current density which can be
 applied must remain low enough to avoid the phenomenon of electropolishing
 the silicon, which leads to erosion of the substrate.
 This therefore results in rates of conversion of silicon into porous
 silicon which are lower than 10 microns/minute, which leads, for large
 wafer thicknesses, to immersion of its rear face for an extremely long
 time, typically more than an hour. The risk of chemically dissolving the
 silicon increases as the value of the pH of the electrolyte rises.
 Accordingly, another aspect of the invention therefore provides for
 carrying out the anodic oxidation in an electrolyte including hydrofluoric
 acid and at least one other acid so as to reduce the value of the pH of
 the electrolyte. This makes it possible to increase the anodic current
 density, and consequently the rate of conversion of silicon into porous
 silicon, while reducing the risk of chemically dissolving the silicon. In
 this regard, any acid may be employed, in particular hydrochloric acid or
 sulphuric acid.
 In theory, the hydrogen bubbles could be removed by vigorous mechanical
 agitation of the electrolyte. However, such an approach is less effective
 and may lead to attack non-uniformities thereby resulting in less of an
 improvement in the quality factor. This is the reason why, in certain
 applications, it is particularly preferable to add a surfactant to the
 electrolyte.
 What is more, it is particularly advantageous in this regard to use acetic
 acid as the other acid. This is because it has been observed that acetic
 acid is also a good surfactant, making it possible to reduce the surface
 tension of the electrolyte, and therefore to promote the elimination of
 the hydrogen bubbles resulting from the anodic oxidation.
 The invention is in this regard noteworthy in that acetic acid makes it
 possible, on the one hand, in combination with hydrofluoric acid, to
 reduce the pH of the electrolyte, consequently allowing the anodic current
 density to be increased to values which, for example, may be as high as
 300 mA/cm.sup.2. At the same time, the acetic acid avoids the phenomenon
 of electropolishing. And on the other hand, the acetic acid avoids the use
 of ethanol which would have the precise result of increasing the pH, (that
 is to say reducing the acidity) which would be contrary to the desired
 effect.
 For example, use may be made of a hydrofluoric acid concentration at least
 on the order of 40% by weight, to which acetic acid may be added in a
 concentration on the order of 5% by weight. It will be noted here that,
 since the surfactant properties of acetic acid are very good, only a small
 percentage by weight may be used.
 In general, the pH of the electrolyte, as well as the value of the anodic
 current density, may advantageously be chosen in such a way as to obtain,
 without electropolishing of the silicon, a rate of formation of the porous
 silicon in excess of 20 microns/minute with a final porosity of less than
 70% void space. By way of explanation, it has been observed that a final
 porosity value in excess of 70% by volume could lead to greater weakening
 of the substrate.
 For example, use may be made of an electrolyte whose pH is less than 1, for
 example, close to zero, or even negative, with an anodic current density
 at least equal to 150 mA/cm.sup.2, for example equal to 300 mA/cm.sup.2.
 Thus, by way of example, an electrolyte formed by an aqueous solution
 containing 40% by weight hydrofluoric acid and 5% by weight acetic acid,
 leads, with an anodic current density of 300 mA/cm.sup.2, to a growth rate
 of the porous silicon of the order of 20 micron/min and to a final
 porosity on the order of 60% void space.
 Such a growth rate can also be obtained with an electrolyte formed by an
 aqueous solution of hydrofluoric acid at a concentration of 45% by weight,
 to which acetic acid has been added at a concentration of less than 5%,
 and to which a small amount of another acid, for example hydrochloric
 acid, has also been added.
 Another characteristic of the invention resides in the fact that the
 conversion of the silicon into porous silicon should be carried out over a
 predetermined height (thickness). This being in particular with a view to
 avoiding attack on the silicon layer which is arranged in the vicinity of
 the front face of the wafer and in which the various active zones of the
 other components, such as the transistors, are produced. It might be
 possible to determine this height of porous silicon by stopping the anodic
 oxidation after a predetermined time, taking into account the growth rate
 of the porous silicon as measured, for example, during a calibration
 phase. However, it is particularly advantageous to measure the potential
 difference (voltage) between the anode and another electrode arranged in
 the electrolyte (the cathode or another reference electrode) and to use
 this measurement of potential difference to determine the time at which to
 terminate the anodic oxidation. More precisely, this anodic oxidation is
 carried out by applying a constant anodic current. The anodic oxidation is
 then advantageously stopped when an increase in the potential difference
 is detected.
 By way of explanation, the transistors which are produced generally include
 buried layers having heavy doping, for example, of the N.sup.+ type, or
 alternatively N type implanted zones. Furthermore, P.sup.+ type insulation
 zones make it possible to insulate two adjacent transistors, and may also
 be found in the substrate. What is more, when the wafer is, for example, P
 type silicon, an increase in the resistance takes place consistent with a
 P-N junction or a P-P.sup.+ junction. Consequently, since the operation
 has been carried out at constant current, an increase in the potential
 difference between the anode and the other electrode indicates that the
 electrolyte has reached the PN junction and that this anodic oxidation
 should therefore be terminated.
 Merely using this porous silicon obtained as such after the anodic
 oxidation gives a significant increase in the quality factor.
 Notwithstanding, it may in certain cases be desirable to stabilize the
 porous silicon by oxidation and thus avoid possible modifications of the
 material. Stabilization treatments using moderate thermal oxidation are
 already known. However, because of the presence of the circuits
 (transistors and the like) already produced on the wafer, the choice of
 the temperature proves particularly critical. This is so because it must
 be low enough not to damage the circuits and also to prevent any
 contamination of the oxidation ovens. This is the reason why the invention
 proposes to carry out, after the step of forming porous silicon, a
 treatment of stabilizing the porous silicon with chemical or
 electrochemical oxidation.
 Further to the application to inductive circuits produced in integrated
 form on silicon, which has just been discussed, other applications of the
 invention may be envisaged. Such may include, in particular the production
 within a silicon substrate, of particularly thick insulating zones formed
 by oxidized porous silicon which thus make it possible to insulate certain
 regions of the substrate from others.
 The invention therefore also proposes a method of forming porous silicon in
 a silicon substrate by anodic oxidation of the substrate in contact with
 an acid electrolyte containing hydrofluoric acid. According to a general
 characteristic of the invention, the electrolyte furthermore contains at
 least one other acid, and advantageously acetic acid which then provides
 both the other acid and also a surfactant.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 In FIG. 1, the reference PL denotes a circular silicon semiconductor wafer,
 having, for example, a diameter of 200 mm, and typically having a
 thickness on the order of 500 microns. The various integrated circuits
 (chips) CI are produced on predetermined zones of the front face FAV of
 this semiconductor wafer PL. These zones are delimited by cutting tracks
 or streets CD along which the wafer will subsequently be sawed to mutually
 separate the integrated circuits which are produced.
 It is assumed here that each integrated circuit CI includes bipolar and/or
 CMOS transistors as well as inductive circuits. For the sake of
 simplicity, FIG. 2 represents a single bipolar transistor T and a single
 inductive circuit L.
 More precisely, in the non-limiting example which is illustrated, the
 formation of the bipolar transistor T includes epitaxial growth of an N
 type silicon layer CEX on the upper surface of the initial silicon
 substrate SB within which an N.sup.+ type buried layer CE1 has been
 implanted beforehand. Side insulation zones, as well as an offset
 collector well C, were then produced. The base B was then epitaxially
 grown on the intrinsic collector and the emitter E was formed.
 The inductive circuit L results, for example, from the formation on the
 epitaxial layer CEX of a thick insulating layer IS, typically on the order
 of 1000 .ANG.. On top of this insulating layer IS there is a metal spiral
 ML obtained by connecting a metallization level of the integrated circuit.
 Of course, it might have been envisaged to produce the active zones of the
 transistors not by epitaxy but by implantation of the corresponding zones
 in the initial substrate SB. In this case, the insulating layer IS
 supporting the metallization ML of the inductive circuit L would rest
 directly on the P type substrate SB.
 The wafer PL including the integrated circuit which has been produced is
 then placed in an electrochemical cell, for example of the type
 schematically illustrated in FIG. 3. This electrochemical cell has a tank
 1 containing an electrolyte 10 in contact with the rear face FAR of the
 wafer PL. This wafer PL is sandwiched between a metal plate 4, in contact
 with the front face of the wafer, and a peripheral seal 2. The peripheral
 seal 2 is in contact with the rear face of the wafer and rests on a wall
 shoulder of the tank.
 In the case when the intention is to carry out an anodic oxidation only of
 the central zone of the rear face of the wafer PL, for example, because
 the integrated circuits containing the inductive circuits are only located
 in this central zone, a mask 8 could then be interposed between the seal
 and the rear face of the wafer. The metal plate 4 is fixed on the bottom
 of the tank by a threaded plug 11 which has a central orifice allowing a
 pad 40 to be passed through for making electrical contact.
 This metal plate provides the anode. This anode has to be in electrical
 contact with the underlying silicon substrate. In this regard, one way of
 making such a metal contact includes using the substrate contact pads
 which exist in all the integrated circuits produced and which connect the
 underlying silicon substrate to the surface of the integrated circuit. All
 the substrate contacts may then, for example, be short-circuited by using
 a metal layer, for example a silver paste, which can then be easily
 removed by dissolving in an organic solvent once the anodic oxidation is
 completed. Instead of silver paste, use could also be made of the last
 metallization level before etching. Of course, any other conventional
 technique can be used to make this anodic contact. For example, the anodic
 contact can be made by a conventional technique based on the use of an
 anodic electrolytic contact.
 The cathode includes here a platinum grid 6 which is immersed in the
 electrolyte 10 and emerges out of the tank through an orifice formed in
 the upper closure plug 12 of this tank. A mechanical stirrer 7, and
 another electrode 5 which is immersed in the electrolyte and is used as a
 reference electrode, are also advantageously provided. Although, for the
 sake of simplicity, this is not represented in FIG. 3, this reference
 electrode may thus be arranged very close to the rear face of the wafer so
 as to obtain, as will be seen in more detail below, greater accuracy in
 measuring the potential difference measured between the anode 4 and this
 reference electrode 5.
 According to one embodiment of the method according to the invention, an
 electrolyte 10, formed by an aqueous solution of hydrofluoric acid and
 acetic acid is used. The hydrofluoric acid concentration is 40% by weight,
 while that of the acetic acid is 5% by weight. The pH of such an
 electrolyte is then below 0.1. A voltage is applied between the anode and
 the electrode so as to make a constant anodic current flow, corresponding
 to an anodic current density equal to 300 mA/cm.sup.2. The rate of
 conversion of the silicon into porous silicon is then on the order of 20
 microns/minute and the final porosity obtained is on the order of 60%.
 Although acetic acid serves as a very good surfactant, to facilitate the
 removal of the hydrogen bubbles further, gentle mechanical agitation may
 optionally be carried out using the stirrer 7.
 The anodic oxidation is continued until the height of porous silicon,
 calculated from the rear face FAR, reaches the value H2 (FIG. 2). In fact,
 although a relatively thin N.sup.+ doped layer CE1 has been represented,
 for the sake of simplicity, the person skilled in the art will understand
 that the zone LZD actually doped extends deeper into the substrate with a
 doping profile which decreases when moving into the substrate. The height
 H2 therefore corresponds to the N doping limit, that is to say the
 appearance of the PN junction. At this moment, the resistance increases
 and, since the operation has been carried out at constant current, this
 leads to an increase in the voltage between the anode and the electrode.
 The anodic oxidation is then stopped by cutting off the current.
 A porous silicon zone extending over a height H2 is thus finally obtained.
 Even if the difference between the height H1 (initial height of the wafer
 PL or substrate SB) and the height H2 is on the order of a few tens of
 microns, a porous silicon extending over several tens of microns is
 obtained, and this on its own leads to a substantial improvement of the
 quality factor, typically a 50% increase. Furthermore, destruction of the
 other active components produced on the integrated circuit should be
 avoided.
 Another usable electrolyte, allowing a growth rate which is also on the
 order of 20 microns/minute with a final porosity of the order of 60%, may
 include an aqueous solution of hydrofluoric acid at a concentration of 45%
 by weight, acetic acid at a concentration of less than 5% by weight, and a
 small amount of another acid. The other acid, for example, may be
 hydrochloric acid at a concentration of 1 mole per liter. The pH of such
 an electrolyte is then lower than 0.01.
 Other acid electrolytes may be employed, for example, by using hydrofluoric
 acid and any other acid, such as sulphuric acid, preferably in combination
 with acetic acid. In general, the pH of the electrolyte, as well as the
 value of the anodic current density, can be chosen in such a way as to
 obtain, without electropolishing the silicon, a rate of formation of
 porous silicon in excess of 10 microns/minute. The limit values for anodic
 current density, in particular taking the pH of the electrolyte into
 account, which make it possible to avoid electropolishing the silicon are
 well-known to the person skilled in the art and have formed the
 subject-matter of a number of publications. These publications include,
 for example, the thesis by Claude BERTRAND entitled "Preparation et
 caracterisation du silicium poreux obtenue sur substrata P et N"
 [Preparation and characterization of porous silicon obtained on P and N
 substrates] defended on Apr. 10, 1986 and available from l' Institut
 National Polytechnique de Grenoble (France).
 Once this anodic oxidation has been completed, stabilization of the porous
 silicon obtained is advantageously carried out by chemical or
 electrochemical oxidation. If chemical oxidation is used, the porous
 silicon may be arranged in a 4/4/2 by volume solution of
 SO4H2/H2O2/CH3COOH (sulphuric acid/hydrogen peroxide/acetic acid) for a
 time of the order of 10 minutes.
 For electrochemical oxidation (anodic oxidation), the porous silicon will
 then be arranged in an electrochemical cell of the type illustrated in
 FIG. 3. This cell, as electrolyte, contains a 1 mole/liter strength
 aqueous solution of HCl. In this case, the anodic current density used is
 on the order of 10 mA/cm.sup.2 over a time of 15 minutes.