Deep reactive-ion etching

Reactive Ionentiefenätzen (English deep reactive ion etching, DRIE ), an evolution of the reactive ion etching ( RIE) is a highly anisotropic dry etching for the fabrication of microstructures in silicon with an aspect ratio (ratio of depth to width ) of up to 50:1, wherein structure depths can be achieved by some 100 microns. This is used / required, for example for the production of silicon vias.

History

The reactive Ionentiefenätzen was originally developed in the early 1990s by F. Larmer and A. Schilp in the form of a Trockenätzprozeses for silicon. They were employees of Robert Bosch GmbH, the marketing of the process patent meant that the term Bosch process naturalized as a synonym for the reactive silicon Ionentiefenätzen. In the years following the original procedure with the cooperation partners Surface Technology Systems Plc. (STS ) and Alcatel Vacuum Technology developed further. Furthermore, the complex technology required has been refined, adapted to the process and sold commercially. So marketed STS for some years an improved process along with the system technology under the name of Advanced Silicon Etching (ASE ).

Process Description

The DRIE process is like the original Bosch process a two-stage, alternating dry etching, with alternating etch and passivation steps. The aim is possible anisotropic etching, that is, depending on the direction, perpendicular to the wafer surface. In this way, very narrow trenches can for example be etched.

The processes can be summarized as follows. First, the silicon wafer is masked, eg with photoresist or a hard mask of silicon dioxide, silicon nitride and other materials, which covers those places of the wafer that will not be etched. Then the actual etching process begins. To sulfur hexafluoride (SF6) (usually argon) is introduced into a carrier gas into the reactor with the substrate located therein. By generating a high-energy high-frequency plasma arises from the SF6 reactive gas. Together with the acceleration of the ions in an electric field, a chemical isotropic etching reaction is overshadowed by radicals formed from SF6 and a physical anisotropic removal of material by sputtering using Argon ion.

The etching process is stopped after a short time, and introduced a gas mixture consisting of octafluorocyclobutane ( C4F8 ) and argon as a carrier gas; other gas mixtures are possible, for example CF4/H2. In the plasma of the reactor Octaflourocyclobutan is activated and forms the entire substrate, a polymer passivation layer, that is, both of the mask and on the bottom and vertical sidewalls of the trench / hole. In this way, the side walls are protected in the following procedure further removal of material in order to ensure the anisotropy of the overall process. For the passivation of the horizontal surfaces ( grave ground ) is removed more quickly by the directed physical component (ions) of the etching reaction as the layer on the sidewalls by the subsequent repeated etching with SF6.

Both steps are now repeated until the desired machining depth is reached. It is important to balance between the etching and passivation. For example, the polymer layer is too thick, a large part of the etching gas and the process time for the removal of the polymer is used at the grave ground, which results in higher processing costs. However, the layer is applied or too thin, the electric field is too low for the chosen ion transport to the substrate, the side walls are excessively etched.

The process parameters have a decisive influence on the structure of the side walls; they are not smooth but slightly wavy due to the alternating process flow usually. The strength of the waves can be minimized by suitable choice of the process parameters so that they do not adversely affect the subsequent processes in the manufacturing.

After etching must finally the mask material ( which is also partially etched ) and the passivation layer are removed to the grave walls.

A disadvantage of the method are compared to the wet etching very high investment costs and low production throughput.

In addition to the alternating process, there is also the method of simultaneous etching and passivation, a so-called continuous process.

Applications

Main area of ​​application is the production of microsystems. By using different mask materials several depth levels can be realized, however, their number due to the high cost remains very limited (2 ½ -D structures ).

Other applications are found in semiconductor technology, for example for the production of deep " trenches " for the storage capacitor in some DRAM technologies or for the isolation oxide in the grave insulation ( which will be here does not require high aspect ratios ).

In recent years, advances a further possible application in the center of interest: For a 3D integration of circuits, ie a stack of components ( eg transistors and conductors ), it is necessary conductive channels through the ( thinned ) silicon substrate to produce. These channels must have a ( very ) high aspect ratio greater than 50:1, and are therefore produced with reactive Ionentiefätzen and subsequently filled with copper.