U. Streller

Selectivity in dry etching of Si(100) with XeF2 and VUV light

Abstract Replica of a mask were etched in Si(100) wafers with a sub-micrometer lateral resolution by VUV irradiation using XeF2. A high selectivity is achieved if the spontaneous reaction of XeF2 is suppressed by a buffer gas and if the unselective light induced reaction is avoided. The strength of unselective etching is strongly wavelength dependent and follows the XeF2 gas phase absorption coefficient. Fragments from the XeF2 gas phase dissociation reach the Si surface and thus cause unselective etching. Optimal dry etching is achieved around 120 nm because the selectivity is high and also the quantum efficiency is very large. The absolute quantum efficiency at 120 nm corresponds to 10 etched Si atoms per incident photon.

EFFICIENT DRY ETCHING OF SI WITH VACUUM ULTRAVIOLET LIGHT AND XEF2 IN A BUFFER GAS

Replicas of a mask are etched in Si wafers with a micrometer lateral resolution and typical depths of 200 nm by irradiation with filtered synchrotron radiation using cutoff wavelengths of 105, 122, and 150 nm. An excellent selectivity and anisotropy is obtained by suppressing the spontaneous etching of the XeF2 etch gas (typical 10−2 mbar) with O2 or Ar buffer gas (typical 1 mbar). The efficiency of etching increases by more than two orders of magnitude by reducing the wavelength from longer than 150 nm to the spectral range of 105–122 nm. The number of removed Si atoms per incident photon reaches a value above unity for the short wavelengths. This very high quantum efficiency, which exceeds that in the visible spectral range by more than four orders of magnitude, is attributed to selective electronic excitation of a thin fluorosilyl layer on top of the Si wafer. The low probability of absorption in this layer implies a reaction efficiency far above unity.

Reaction products in synchrotron radiation induced dry etching of Ga and Cu

Abstract Light induced dry etching using synchrotron radiation (SR) in the VUV range and chlorine as etching gas has been applied to metals (Cu) and semiconductors (GaAs). High quality replica of a mask are etched by irradiation with filtered radiation and the reaction products are analyzed by depth dependent Auger spectroscopy (ADP). For Cu, etching leads to thick CuClx product layers which originate from light induced reactions at the interface between chemisorbed chlorine and the product layer combined with a diffusion of Cu from the substrate to this interface which is attributed to an electrostatic potential perpendicular to the films (Cabrera–Mott mechanism). The anisotropy of etching is preserved despite a transport of Cu through a product layer of about 16 μm. The reaction products swell in volume by nearly a factor of two due to the Cl intake. For GaAs etched with Cl2 the product layer itself consists exclusively of Ga. The thickness of the product layer is restricted to about 1 μm, different to the Cu/Cl2 system. The reaction proceeds again at the interface between chemisorbed chlorine and product layer and is limited by the transport of As. This transport of As (alone or together with Ga) follows a Cabrera–Mott mechanism. During etching As reacts completely with the Cl and forms AsClx. All AsClx molecules are volatile and desorb while Ga sticks.

High efficiency in dry etching of Si for wavelengths around 120 nm

Microstructuring of Si with XeF2 can be optimized by increasing the contrast in choosing a wavelength with minimal nonselective etching. The efficiency of selective etching with optimal quality can be increased by factors of 100 and 500 by using wavelengths around 120 and 110 nm, respectively, in comparison to longer wavelengths around 200 nm. The high efficiency of typically 10 removed atoms per photon, the availability of optical materials for imaging and the potentially high spatial resolution at 120 nm compared to the conventional excimer laser and I‐line wavelengths present a perspective for generating line densities required in the Gbit range.

Photon-induced dry etching of Si(100) in the VUV

Abstract The photon-induced dry etching of Si(100) using synchrotron radiation (SR) in the VUV range and a halogen containing gas (XeF 2 ) has been investigated. Replica of a mask are etched in Si wafers with a sub-micrometer lateral resolution by irradiation with filtered or monochromatized radiation. The total quantum efficiency of etching reaches a value above unity for wavelenghts shorter than 122 nm. This very high efficiency, which exceeds that in the visible spectral range by more than four orders of magnitude, is attributed to a selective electronic excitation of a thin fluorosilyl layer on top of the Si wafer. The topology indicates that single photons initiate chain reactions with an amplification of about 3 · 10 5 for using Ar as buffer gas. The overall quantum efficiency combined with the amplification factor leads to a reaction stimulation probability per incident photon of 6 · 10 −5 . Addition of O 2 increases the amplification factor but reduces the quality, selectivity and excitation probability.

PHOTOCHEMICAL ETCHING WITH TUNABLE VUV RADIATION

Abstract Light induced dry etching of metals (Cu) and semiconductors (GaAs, Si) using synchrotron radiation (SR) in the VUV range and halogen containing gases (Cl 2 , XeF 2 ) has been investigated and the conditions for selective and anisotropic etching are discussed. Replica of a mask are etched by irradiation with filtered or monochromatized radiation with high quality and a sub-micrometer lateral resolution. The absolute quantum efficiency for etching of GaAs and Si reaches values above unity for wavelengths around 120 nm. These very high efficiencies, which exceed those in the visible spectral range by more than four orders of magnitude, are attributed to selective electronic excitations of reactive layers on top of the sample surfaces combined with typical chain reactions.

Desorption in light-induced dry etching of GaAs with Cl2 around 120 nm

Abstract Quantum efficiency for etching corresponding to the removal of more than 50 Ga and As atoms per incident photon is observed in the spectral range from 115 to 130 nm, which exceeds the values at long wavelengths by four orders of magnitude. Replica of good quality and a submicron spatial resolution are produced. Desorption of reaction products (GaClx, AsClx) depends on the wavelength and the thickness of a layer of reaction products increases by one order of magnitude from 123 to 117 nm. XPS studies combined with sputtering indicate that also the composition varies with wavelength. The products can be desorbed by long wavelength irradiation.

Reaction products in light-induced dry etching of GaAs with Cl2 and wavelengths around 120 nm

Abstract An XPS study of GaAs wafers etched by exposure to vacuum ultraviolet light in a Cl 2 atmosphere reveals a strong wavelength dependence of the desorption of reaction products. While the etching depth is not influenced by shifting the cut-off wavelength from 122 nm to 105 nm an increase of the thickness of accumulated reaction products by an order of magnitude is observed. A lack of As at the surface of the reaction products and a smooth increase to the bulk indicates that As (and Ga) diffuse through the layer and near the surface the light induced reaction with Cl 2 occurs. Nearly all AsCl x and most GaCl x compounds formed for a cut-off at 122 nm are volatile and desorb while the shorter wavelength part strongly suppresses desorption of GaCl x and increases partly the sticking of AsCl x . A mask-related product distribution is observed.

SR-induced dry etching of semiconductors for microstructuring

Abstract The selectivity and anisotropy of dry etching using synchrotron radiation (SR) in the vacuum ultraviolet (VUV) range (100 nm≤ λ ≤ 300 nm) has been investigated for semiconductors (GaAs, Si) in different etching gas atmospheres (Cl2, XeF2). For GaAs/Cl2 an excellent quality of the etched replica of a mask was obtained with a lateral resolution in the submicrometer range and the quantum efficiency was extraordinarily high at short wavelength. Similar results were found for Si/XeF2 in suppressing the spontaneous etching of XeF2 by buffer gas (Ar, O2). For Si the efficiency of etching increased by more than two orders of magnitude by reducing the wavelength from longer than 150 nm to the spectral range of 105 to 122 nm. The number of removed Si atoms per incident photon reached a value above unity for the short wavelength range. This very high quantum efficiency which exceeds that in the visible spectral range by more than four orders of magnitude, is attributed to selective electronic excitation of a thin fluorosylil layer on top of the Si wafer.