Electrically tunable fabry-perot interferometer, an intermediate product an electrode arrangement and a method for producing an electrically tunable fabry-perot interferometer

Abstract

Electrically tunable Fabry-Perot interferometers which are produced with micromechanical (MEMS) technology. Producing interferometers with prior art processes includes costly and complicated production phases. Therefore, it has not been possible to apply interferometers in consumer mass products. According to the present solution, the Fabry-Perot cavity is made by removing a sacrificial layer (112) which has been polymer material. A mirror layer (113, 117-120) which is produced above the sacrificial layer can be made with atomic layer deposition technology, for example. According to a preferable embodiment, electrodes (106b, 115b) of the mirror structures are formed by using sputtering or evaporation. With the present solution it is possible to avoid the above mentioned problems related with prior art.

Description

TECHNICAL FIELD [0001]The invention relates to a method for producing an electrically tunable Fabry-Perot interferometer, an electrically tunable Fabry-Perot interferometer, an intermediate product, and an electrode arrangement. More specifically, the invention relates to electrically tunable Fabry-Perot interferometers which are produced with micromechanical (MEMS) technology. The technical field of the invention is specified in the preamble of the independent claims.BACKGROUND TECHNOLOGY [0002]Fabry-Perot interferometers are used as optical filters and in spectroscopic sensors, for example. The Fabry-Perot interferometer is based on parallel mirrors, whereby a Fabry-Perot cavity is formed into a gap between the mirrors. The pass band wavelength of a Fabry-Perot interferometer can be controlled by adjusting the distance between the mirrors i.e. the width of the gap. The tuning is usually made electrically. It is common to use micromechanical technology for producing electrically tu...

AI Technical Summary

Benefits of technology

[0031]Significant advantages can be achieved with the invention when compared to the prior known solutions. The sacrificial layer can be removed with dry etching, and this can be performed after cutting the chips and also after encapsulating the chips. This allows simple cutting and packaging procedures because the movable mirror does not need to be released at that phase and is therefore not sensitive to environmental stress, such as physical pressure, changes of temperature or humidity, contamination, etc. Also, it is possible to transport the interferometers in normal transportation manners because the movable mirrors can be released after the transport.

[0032]Further, it is not necessary to use complicated and expensive ion doping equipment in the production. Thus it is possible to use a simpler component production line, whereby the production costs can be made sufficiently low for mass production. As a consequence, the interferometers according to the invention can be applied in new consumer applications where production quantities are large and production costs need to be minimal.

[0033]When the sacrificial layer is removed by dry etching with oxygen plasma, through holes of the movable mirror, it is possible to use a thin sacrificial layer. Thus it is possible to achieve a small gap between the mirrors, and to provide electrically tunable Fabry-Perot interferometers for short wave lengths. For example, it is possible to provide interferometers for ultraviolet range, which has not been possible with prior art technology.

[0035]In one embodiment of the invention the electrodes of the movable mirror structure are covered by electrically isolating or semi-insulating optical layer(s), which protects the electrodes and prevents electrical short circuits of the opposite electrodes. If electrically semi-insulating layer, such as TiO2, is used for covering the electrode surface it will also prevent electrical charging phenomena at the surfaces of the electrodes. Charging of the optical layer may cause inaccuracy of the controlled mirror position.

[0036]In one aspect of the invention one of the electrodes is electrically floating. This allows the use of alternating voltage for the tuning of the interferometer, and it is not necessary to provide control conductors to one of the mirrors for controlling the gap between the mirrors. The production process of the interferometer is less complicated, and the structure is more reliable when it is not necessary to arrange electrical connections to all electrode layers. This floating electrode structure can be advantageous also in electrically tunable Fabry-Perot interferometers where other than polymer material is used as a sacrificial layer. The floating electrode structure can also be used in other components than interferometers, such as adjustable capacitors. The electrically floating electrode can be an electrode of a movable part or an electrode of a fixed part in the structure of a component. When the electrically floating electrode is located in the upper mirror of an interferometer, the number of lithography phases after applying the polymer layer can be minimized.

[0038]According to one embodiment of the invention the interferometer is integrated with a radiation detector, whereby the detector can be formed into a silicon substrate of the interferometer. This allows the production of small-size, reliable and low-cost spectrometer. Low processing temperatures in the production of the interferometer allow the integration of the interferometer with various other structures / functionalities.

Problems solved by technology

However, there are some disadvantages related with the prior art solutions for production of interferometers and the interferometer components.

The doping of the silicon layers requires the use of an ion doping facility, which is expensive equipment and thus increases the production cost.

Another problem relates to removing the sacrificial layer between the mirrors.

Such a separate process increases the complicity of the production process.

Due to the relatively high production cost of interferometers, it has not been possible to use them in mass product applications where the cost requirements are strict.

A further disadvantage of the prior art technology relates to the inability to provide a gap with short distance between the mirrors.

This is due to the wet etching process wherein providing narrow gaps is difficult.

Such membranes easily become damaged during wet etching.

Therefore the prior art technology is not suitable for producing electrically tunable Fabry-Perot interferometers for short wavelengths such as visible and ultraviolet range.

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