DD271777A1 - METHOD FOR PRODUCING VERGRAINED AETZSTOPSCHICHTEN IN SILICIUM - Google Patents

Abstract

Process for producing a buried etch stop layer in minocrystalline silicon. The solution according to the invention can be used in the production of thin silicon membranes and three-dimensional structural elements, e.g. As in the production of transmission masks for X-ray lithography, of sensor elements, especially of primary transducer elements for piezoresistive and piezoelectric transducer elements and of radiation sensors and SOI devices. The aim of the invention is a technically relatively simple process for the production of superabsorbent layers of high homogeneity and selectivity buried in monocrystalline silicon for the production of silicon membranes in a wide range of thicknesses. The object of the invention is to propose a semiconductor process with which a buried layer can be produced in a monocrystalline silicon substrate which has a low etching rate compared to wet-chemical caustics, preferably anisotropic admixtures. According to the invention, the object is achieved by producing SiOx, SixNy or SixNyOz layers buried by ion implantation of oxygen and / or nitrogen.

Description

Field of application of the invention

The solution according to the invention can be used as an automatic etching stop method for the production of three-dimensional micromechanical structure torques by wet-chemical etching, for. In the manufacture of thin silicon membranes as transmission masks for X-ray lithography and as primary transducer elements for semiconductor sensors.

Characteristic of the known technical solutions

In order to achieve a defined etch at the wet chemical etching of three-dimensional micromechanical structures in monocrystalline silicon, preferably at the vertical subsurface in the semiconductor substrate, various technological methods are known (KE Petersen: .Silicon as a mechanical material "Proc. Of the IEEE 70 [1982 In the <echnological methods, the dependence of the etching rate of the etchant on the doping concentration in monocrystalline silicon is preferably exploited.For the most frequently used alkaline anisotropic and selective etching solutions (ethylenediamine-water and potassium hydroxide-water mixtures ), the etching rate decreases, for example, by a factor of 10 with an increase in the boron concentration to N ₈ - 5 × 10 ¹⁹ cm ^-3 . For this reason, to produce thin membranes with defined membrane thicknesses in a silicon epitaxy process, a highly doped p ⁺ layer (boron doped) first forms on a silicon substrate, forming the later buried etch stop layer and then a low-doped monocrystalline silicon nitride of predetermined thickness deposited. Thickness of the buried p ⁺ layer and thickness of the low-doped epitaxial layer give the thickness of the etched-off membrane. Etching stop p ⁺ layers require very high doping with N ₈ > 5 × 10 ¹⁹ cm ^-3 during the epitaxy process and high demands on the process control. The selectivity or etch rate ratio for anisotropic etchers, especially for KOK. is limited to R (<100> Si): R (p ⁺ ) = 10 ... 100, which leads to problems especially with necessary large lateral undercuts for tongue and bridge production.

Practical significance for the defined vertical boundary of etched structures has been achieved as a second technological method of electrochemical etching stop. The etching process for anisotropically acting etching mixtures stops at the pn junction of a Si wafer. This method has the advantage over a p ⁺ topopause that very high dopant concentrations are not necessary. For extremely thin membranes with d <2 pm, the p ⁺ layer is directly applied to the surface of the silicon substrate by boron diffusion or boron implantation.

The electrochemical Ätzstopverfahren in addition to its advantage of good controllability (current measurement) has the disadvantage that the etched structure elements (membranes) can only be prepared as η-conductive monocrystalline Si regions. This strongly limits the choice of doping type (conductivity type) and dopant concentration for the membrane. The commonly used etch masks in the form of SiO ₂ and S ion insulator layers themselves may also serve as etch stop layers in membrane fabrication by etching away the underlying substrate. In the known implementation examples, the insulator layers mentioned serve as support membranes for polycrystalline and amorphous semiconductor layers as well as for a multiplicity of non-semiconductor layers.

In insulator layers deposited superficially with customary methods (Bedainpfung, CVD, oxidation), the membrane thickness is limited to the thickness <? .Μσι. A disadvantage is the lack of perfect monocrystalline Si layers on such membranes (SOI layer structures) noticeable for many applications.

Object of the invention

The aim of the invention is a technically relatively simple process for producing Ätzstopschichten high homogeneity and selectivity for the production of three-dimensional micromechanical structural elements, especially of silicon membranes in a wide range of membrane thickness. In this case, a monocrystalline epitaxial layer of any conductivity type and any desired dopant concentration and thickness should be deposited over the Ätzstopschicht before the pass-chemical etching process as possible.

Explanation of the essence of the invention

The invention has for its object to provide a semiconductor process, with which in a monocrystalline silicon substrate, a buried layer can be produced, which has a very low Atzrate especially wet chemical etching solutions, preferably anisotropic etchers.

According to the invention, the object is achieved by producing SiO _x * or Si _x N _y or SixOyN.-layers of different layer composition, which have a very low etch rate for the etching solutions, by ion implantation of oxygen and / or nitrogen. The ion energy and the temperature regime during the

Implantation chosen so that above the implanted layer remains a largely interference-free monocrystalline silicon layer on which an epitaxial layer of any conductivity type and virtually arbitrary Dotantenkonzentration can be deposited with the required thickness without any problems, ion beam-induced substrate temperature and temperature regimes after implantation were chosen so that the implanted layers have amorphous character, ion-implanted buried nitrogen and / or oxygen-rich layers can be used as etch stop layers for allo known isotropic and anisotropic etch mixtures, the selectivity for anisotropic etchers being generally higher. In comparison with other layers, ion-implanted layers are characterized by a high degree of homogeneity over the entire substrate surface and very precisely adjustable layer compositions and layer profiles of low-thermal expansion. For implanted SiOr »Si ₃ Nr and SixOyNi layers of stoichiometric composition, a selectivity higher than that of p ⁺ layers by a factor of 10 to 100 or a higher etching ratio of R (<100> Si): R (SiO ₂ , SI] N ₄ , Si _x OyN ₁ )> 10 ³ . In the dose range D = ^{1.times.10.sup.-1.times.10.sup.15 cm.sup.- 2} , the etch rate of the buried layers can still be set in a defined manner and reaches its minimum for stoichiometric layer compositions.The method of manufacturing the vertical SOI structure (SOI-silicon on insulator ) in combination with wet-chemical structuring method can be advantageously used in addition to the simple membrane production for the production of electrically insulated monocrystalline silicon structures on insulator membranes or active components in monocrystalline silicon layers on insulator membranes.

In this case, the monocrystalline membrane structures etched away by the buried and electrically insulating stoichiometric SiO ₂ , Si ₃ N ₄ and Si _x O _y N, layers and passive and active components produced therein are electrically and thermally insulated from the carrier substrate.

exemplary The invention will be explained in more detail using an exemplary embodiment.

In the example, the <100> -n-Si substrate has a specific / resistance of η << · 1 · 10 cm. The substrate was treated with nitrogen ions the energy E = 330keV and a dose D - implanted 4 x 10 ¹⁷ cm ~ ^z. The mean depth of the SixNy layer is 600 nm and the layer thickness is 300 nm. The ion beam-induced target temperature was 5'JO ⁰ C. After implantation, the substrate was annealed at T = 1000 ° C. and t = ² h in a dry nitrogen atmosphere. At these implantation and annealing conditions, a buried, amorphous Si "N _y layer is formed. On the monocrystalline layer above the buried Si _x Ny layer an undoped epitaxial layer of thickness 5μηη was deposited in the usual catfish. The etching mask used was 1 μm thick thermally grown SiO ₂ . On the backside, the etching mask is opened photolithographically according to a predetermined structure and the etching is carried out in the KOH-water mixture. The etching process is stopped laterally by the slow etching <111> planes and vertically through the implanted Si _x N _y layer. At the chosen implantation dose of D = 4 x 10 "cm" ² , which requires only one third of the implantation time to reach the stoichiometric dose, an etch rate reduction by a factor of 100 was observed at the buried Si _x N ₄ layer. Such a lowering of the etching rate is sufficient for many application examples. 5 μm thick membranes were produced up to an area of 10 × 10 mm ² . Defined structures in the membrane can be produced by the abovementioned methods for producing the etching masks and structuring them.

The etch rate of the buried Si _x N ₄ layers in 40% KOH at T = 8O ⁰ C can be selected with appropriate choice of N * dose in the dose range D · »(1 · 10" ... 1.2 * 10 ^l8 ) cm " ² defines more than three orders of magnitude between the etching rate in the <100> direction of the Si substrate (= 68 μm * tr ¹ ) and the etch rate of stoichiometric Si ₃ N ₄ (<10 nm * h"').

Claims (2)

1. A method for producing buried Ätzstopschichten in silicon, characterized in that in an indefinite depth of the silicon target by means of ion implantation, an amorphous layer is prepared with altered chemical composition. 2. The method according to item 1, characterized in that for the production of the Ätzstopschicht preferably nitrogen and / or oxygen in the energy range E = 60keV ... 100MeV and a dose D = 10 ¹⁷ ... 10 ¹⁹ cm ~ ^{2 are} implanted.