Process and apparatus for plasma treatment of semiconductor materials.

0001The present invention relates to a method and a device for treating a semiconductor material by plasma. It applies in particular to the etching of a semiconductor material, for example of semiconductor wafers intended for the manufacture of integrated circuits, the word "etching" being taken in a very broad sense "of material removal" according to a predetermined geometry. or whatever. The invention also applies to the deposition of a material on the material as well as to the growth of layers on a substrate and, more generally, to the treatment of surfaces.

0002Machines for processing semiconductor materials are already known, using the reactive ion etching process. These machines have the major drawback of carrying out etching very slowly (at a speed of the order of 100 nm / min for silicon and silicides and 50 nm / min for si-_li_vs4.

0003The subject of the present invention is a method and a device for treating a semiconductor material which improve the selectivity or which allow significantly higher etching speeds.

0004Specifically, the subject of the present invention is first of all a process for treating a semiconductor material, characterized in that it consists in placing the material in an electrically insulating enclosure and in subjecting it, in the enclosure, to a plasma of a gaseous compound suitable for treatment, this plasma being obtained by the action of a uniform magnetic field and a radio frequency electromagnetic field whose magnetic component is perpendicular to the uniform magnetic field.

0005By "gaseous compound" is meant a pure gas or a gaseous mixture.

0006By "treatment" is meant both the etching of the material and the deposition of a material thereon.

0007By "radio frequency electromagnetic field" is meant an electromagnetic field whose frequency is less than 10 GHz.

0008Of course, the material can be prepared in an appropriate manner, depending on the treatment which it must undergo and prior to this treatment. For example, in the case of the etching of semiconductor wafers intended for the manufacture of integrated circuits, it may be necessary to cover these wafers with a discontinuous layer (mask) of resin, Si₃NOT₄, silica ..., before treating them according to the invention, in order to etch only predetermined areas of the platelets.

0009According to a particular embodiment of the process which is the subject of the invention, the treatment is an etching of the material and the gaseous compound comprises at least one gas capable of giving, by decomposition, an element capable of combining with at least the one of the elements constituting the material to give a volatile compound.

0010In the case where the material to be etched is silicon, the gaseous compound preferably comprises argon and hydrogen, a mixture which is of great interest as will be seen later.

0011According to another particular embodiment of the process which is the subject of the invention, the treatment is a deposition of a material on the material and the gaseous compound comprises at least one gas capable of giving the material by decomposition.

0012The present invention also relates to a device for processing a semiconductor material, characterized in that it comprises:<ul id="ul0001" list-style="none"><li>- an electrically insulating enclosure,</li><li>means for supporting the material in the enclosure,</li><li>means of introduction into the enclosure and evacuation thereof, of a gaseous compound suitable for treatment.,</li><li>- means for producing a uniform magnetic field in the enclosure, and</li><li>means for producing a radio-frequency electromagnetic field in the enclosure, such that its magnetic component is perpendicular to the uniform magnetic field,</li></ul>so as to treat the material by subjecting it to a plasma obtained by ionization of the gaseous compound present in the enclosure and subjected to the uniform and radio-frequency fields.

0013According to a particular embodiment of the device which is the subject of the invention, the enclosure has the shape of a tube and the uniform field is parallel to the axis of the tube.

0014Finally, according to another particular embodiment, the material is in the form of a plurality of plates and the support means are mobile and capable of holding the plates perpendicular to the axis of the tube.

0015The invention will be better understood on reading the description which follows, of exemplary embodiments given by way of indication and in no way limiting, with reference to the appended drawings in which:<ul id="ul0002" list-style="none"><li>FIG. 1 is a schematic view of a particular embodiment of the device which is the subject of the invention, and</li><li>- Figures 2 and 3 highlight the advantage of the invention as regards the isotropic and anisotropic etchings of silicon respectively.</li></ul>

0016In Figure 1, there is shown schematically a particular embodiment of the device object of the invention. This includes:<ul id="ul0003" list-style="none"><li>- an electrically insulating enclosure 2,</li><li>means 3 for producing a uniform magnetic field B₀ in the enclosure, and</li><li>means 4 for producing a radio-frequency electromagnetic field in the enclosure, such as its magnetic component B_T either perpendicular to the uniform magnetic field B₀.</li></ul>

0017For example, the enclosure 2 consists of a quartz tube of axis Z, the means 3 for producing the uniform field B₀ consist of two magnetic coils 3a and 3b which surround the tube 2 and are suitably supplied with direct electric current by means not shown, and the means 4 for producing the radio frequency field include a quadrifilar antenna 5 and a radio frequency source 6 to feed the antenna. The latter surrounds the tube 2, in the space between it and the two coils 3a and 3b.

0018The antenna 5 can be schematically considered to be composed of two electrically conductive loops forming Helmholtz coils and diametrically opposite on either side of the tube 2, in the field of action of the coils 3a and 3b. The association of tube 2, of uniform field B₀ and the antenna 5, provided to produce an electromagnetic field whose magnetic component is perpendicular to the uniform field B₀, with a view to creating a plasma in the tube, is moreover already known by the article by RW BOSWELL, published in Physics Letters, vol.33A, n ° 7, December 11, 1970, p.457 and 458, but without mention of particular application of plasma.

0019The device shown in FIG. 1 also includes means 7 for supporting the material 8 to be treated in the field of action of the fields B_O and B_T.

0020In FIG. 1, there is also a conduit 13 for introducing into the tube 2 a gaseous compound suitable for treating the material 8 and another conduit 14 for discharging this compound in the direction of pumping means, not shown. Gauges 15 and 16 make it possible to measure the pressure prevailing in the tube. In addition, an airlock, not shown, is provided at least at one end of the tube, to introduce material 8 therein and to evacuate this material once it has been treated.

0021The device described with reference to FIG. 1 belongs to the category of so-called "tubular" machines which work at low pressure (between approximately 0.1 and 10 pascals and in any case less than 10² pascals) and in which the energies involved do not exceed approximately 10 eV and are in any case less than 100 eV. On the other hand, the system described makes it possible to apply a polarization of the substrate holder independently of the creation of the plasma.

0022This device creates a plasma column in the tube, under the action of the radio frequency field BT coupled to the uniform field B₀, the latter having a low intensity, between approximately a few thousandths and a few tenths of tesla. The advantage of the present invention lies in the remarkable efficiency of radio frequency coupling (the industrial frequency of 13.56 MHz being moreover well suited to the invention) of the exciting antenna 5, which leads to the production plasmas of very high densities compared to the electromagnetic power densities used: one can for example obtain densities of the order of a few 10¹² electrons per cm³ with power densities of the order of 0.1 to 1 W per cm³ depending on the type of gaseous compound used. The possibility of generating high electronic densities gives the device of the invention unique characteristics, in particular in the field of etching a semiconductor material such as monocrystalline or polycrystalline silicon, the etching of the silicon preferably being carried out in the invention using a hydrogen-argon gas mixture.

0023With this mixture, the invention allows a very selective etching of the silicon compared to the silica SiO₂ and silicon nitride Si₃NOT₄₁ selectivity (ratio of the etching speed of silicon to the etching speed of SiO₂ or if₃NOT₄) being of the order of 30 to 50, or even greater than these values. The following table I gives experimental conditions leading to such values ​​as well as to speeds V_Yes high etching for polycrystalline silicon.<tables id="tabl0001" num="0001"><img file="EP0146446A2_D0001.tif" /></tables>

0024The hydrogen molecules are dissociated in the plasma and the hydrogen atoms react with the silicon to give the volatile compound SiH, with x less than or equal to four. Argon catalyzes this reaction by promoting the dissociation of hydrogen molecules.

0025The hydrogen-argon mixture used with the invention also makes it possible to etch the silicon by predetermining the etching slope, this slope being a function of the relative concentration of argon in the hydrogen. In particular, a perfectly isotropic etching can be obtained for a low relative concentration of argon, a perfectly anisotropic etching for a relative concentration of argon equal to 50%, as well as intermediate configurations, as shown in Table II below:<tables id="tabl0002" num="0002"><img file="EP0146446A2_D0002.tif" /></tables>

0026By way of example, FIG. 2 shows the isotropic etching of a substrate 17 of Si <100> covered with a mask 18 of SiO₂, etching obtained using a mixture of 95% hydrogen and 5% argon, the etching speed of the silicon being of the order of 0.2 to 0.3 μm / min and the selectivity (compared to Si0₂) at least equal to 50, and in FIG. 3, the perfectly anisotropic etching of a layer 20 of polycrystalline silicon covered with a mask 21 of aluminum and deposited on a layer 22 of SiO₂, itself deposited on a substrate 23 of Si <100>, etching obtained using a mixture of 50% hydrogen and 50% argon, the etching speed of the silicon being of the order from 0.15 to 0.2 µm / min and the selectivity (compared to Si0₂) at least equal to 30, with in both cases (Figures 2 and 3) a power density of the radio frequency ch amp (13.56 MHz) of 300 W / cm³, a uniform field B₀ from 5.10^-3 T, a total pressure of 0.79 Pa and a gas flow of the order of 0.3 dm³/ min.

0027The variation of the profile of the etched silicon is obtained without consumption of the mask or addition of an external polarization on the substrate.

0028By comparison, a conventional reactive ion etching of polycrystalline silicon, carried out using SF₆ pure, with a pressure of 0.66 Pa, a flow rate of 15 cm³/ min, a power density of 40 _{W /}dm³, is anisotropic and takes place with a speed of the order of 0.1 µm / min and a selectivity of the order of 2 to 4 with respect to Si0₂.

0029The invention therefore allows etching of the silicon with a hydrogen-argon mixture which is easy to use (compared to the chlorinated gases used in the installations of the prior art, a gas allowing good selectivity but whose use is delicate, poses problems of safety and behavior of the materials and obliges to imperatively avoid the presence of water vapor in the engraving apparatus) and allows a selectivity, compared to Si0₂ or if₃NOT₄, at least equal to those obtained in the prior art. In addition, with this mixture, the invention allows the etching of silicon with a slope which varies continuously with the relative concentrations of argon and hydrogen.

0030Of course, the invention can be used in the field of etching with gas mixtures other than the hydrogen-argon mixture, for example with a mixture comprising a halogen such as chlorine or fluorine in inert gas form, an oxidant such than oxygen, a reducing agent such as hydrogen and a neutral gas such as a rare gas (argon for example). It is also possible to use with the invention fluorinated, carbon-fluorinated or chlorinated gases as well as all the mixtures obtained from these gases and adjuvants such as H₂ or 0₂. The invention allows for example the very high speed etching of polycrystalline silicon using SF₆ pure as shown in Table III below:<tables id="tabl0003" num="0003"><img file="EP0146446A2_D0003.tif" /></tables>

0031The invention also makes it possible to deposit on a substrate, a material such as silicon, using SiH₄ as a gaseous compound: the latter decomposes into hydrogen and silicon which is deposited on the substrate.

0032One can also use the device according to the invention for etching by creating a plasma, from any fluorinated, carbon-fluorinated or chlorinated gas. In all cases, there is an etching speed significantly higher than that obtained in conventional devices.

0033The invention can therefore be applied:<ul id="ul0004" list-style="none"><li>- the construction of trenches for isolation boxes, of the order of 1 µm in width and 5 µm in depth,</li><li>- at the start of cutting paths before cleavage,</li><li>-the production of alignment marks for electronic masks,</li><li>- engraving the rear face of semiconductor wafers to make a contact,</li><li>- thinning and thickening of the wafers (for example before mounting them in housings), and</li><li>- the cleaning of objects soiled by a semiconductor material: for example the cleaning of quartz nacelles after deposition of polycrystalline silicon, by CVD, LPCVD or PECVD.</li></ul>