Optics: measuring and testing – Dimension – Thickness
Reexamination Certificate
2002-01-28
2004-02-03
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
Dimension
Thickness
C356S503000
Reexamination Certificate
active
06687015
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a method of measuring the thickness of a layer during a layer deposition process, as well as to a device suitable to carry out the method.
The fields of application of the present invention are in all sectors where the continuous or periodic measurement of the thickness of layer during a layer deposition or adsorption process is required or desirable. A preferred field of application is the sector of semiconductor production. Specifically in semiconductor manufacturing processes is precise monitoring and determination of the applied layers is highly important. Such a measurement of the thickness of the layer may then be employed in the automation of processes.
BACKGROUND OF THE INVENTION
In prior art, a great number of measuring methods have become known for measuring layer thicknesses, which serve to determine the thickness of adsorbed, deposited or accumulated layers.
One part of the method is based on a micro weighing operation that is carried out by application of quartz vibrators or SAW (Surface Acoustic Wave) sensors. With both sensor types, the dependence of the resonance frequency on the thickness of the material layer deposited on the sensor is evaluated. Quartz vibrators are preferably used in this process, e.g. in sputtering systems, for determining the deposited mass. SAW sensors are employed, for example, as gas sensors. The sensitive area of the sensor is provided with a gas sorption layer. The variation of the mass of the layer by a sorbed gas mass results in a variation of the resonance frequency that is detected by the method.
Such micro weighing methods involve, however, the disadvantage that the behaviour of the sensors depends also on the modulus of elasticity of the deposited additional mass. Many sensors moreover display a strong non-linear dependence on the temperature.
Apart from this micro weighing method, also acoustic methods and methods of dielectrometric measurement of the thickness of layers are known. In the acoustic methods, a sound or ultrasonic wave, respectively, is applied to the layer to be measured and then its propagation or delay time through the layer is measured. The resolution of measuring instruments presently available in the market on the basis of acoustic measuring methods is within the millimeter to micrometer range.
In the dielectrometric method the fact is utilised that the capacitance of a capacitor depends on the dielectric constant of a material disposed in the electric field of the capacitor. There, the layer to be measured is carried between the electrodes of a capacitor, and then the thickness of the layer is derived from the variation of the capacitance of this capacitor. As a matter of fact, this method is, however, only suitable when the value of the dielectric constant of the layer material is substantially different from 1. When this is not the case, the dielectrometric method cannot be employed for measuring the thickness of the layer.
Another disadvantage of dielectrometric measurement for determining the thickness of a layer consists in the aspect that the dielectric constant of the layer material must be known. Apart therefrom, the dielectrometric method can be applied only with non-metallic layers.
In many fields, optical methods are used as well for measuring the thickness of layers. These methods utilise, for example, parameters of the material of the layer, such as transmissivity, reflection, absorption, refraction or scattering. Examples of methods employed particularly in semiconductor production for determining the thickness of layers are the ellipsometry or spectroscopy.
The further known method of laser interferometry is used especially for monitoring the etching rate and the etching depth in etching processes. In these cases, however, it is necessary that the transparent layer to be etched is transparent to the laser wave length applied.
The methods so far employed specifically in the field of semiconductor production use respective specific materials parameters such as the mass, the sound velocity, etc. and are hence not suitable for measuring any materials whatsoever or for measuring stratified systems consisting of different materials. Other known methods do not achieve the required high precision for the application in semiconductor technology.
OBJECTS AND BRIEF SUMMARY OF THE INVENTION
The problem of the present invention consists in providing a method and a device for measuring the thickness of a layer during a process of depositing a layer, which is suitable for determining the thickness of the layer, particularly in semiconductor manufacturing processes, in a simple manner, independently of the employed materials and with a high precision.
The problem is solved with the method or the device according to the present invention. Expedient embodiments of the method and the device are also disclosed. The inventive device makes it possible, in particular, to open up also further fields of application as hereafter disclosed.
In the inventive method, the known physical principle of diffraction is utilised. To this end, a diffraction object is provided which has at least two opposing or parallel diffracting limits whose mutual spacing is known. This diffraction object is disposed in a process zone of the layer depositing process, where the layer to be measured is deposited or adsorbed, such that a layer is deposited or adsorbed also on the diffracting limits. The thickness of the layer depositing on the diffracting object must present a known relationship with the thickness of the layer deposited on the object to be measured. In a preferred field of application of semiconductor production, the diffraction object is therefore disposed in the process chamber envisaged for depositing the layer, preferably in the fairly close vicinity of the substrate to be coated with the layer. In the depositing process, hence the layer is deposited on both the substrate and on the diffraction object.
In another expedient application of the method, the diffraction object is preferably positioned in the vicinity of the inner wall of the process chamber. In this manner, it is possible to determine automatically the time of cleaning of the chamber, i.e. the time by which the chamber requires cleaning due to a defined thickness of the layer deposited on the chamber wall, by deriving it from the size of the thickness of the layer on the diffraction object.
The diffraction object is illuminated with a coherent light beam, i.e. a laser beam, and projected onto a detection surface. This requires that the diffracting limits of the diffraction object be positioned in the optical path of the laser beam. The distance of the diffracting limits of the diffraction object is therefore so selected that it is smaller than the diameter of the laser beam. To this end, preferably an elongate wire-shaped or rod-shaped structure is used whose diameter is selected as a function of the desired resolution and the measuring scope and is smaller than 500 &mgr;m, preferably ≦300 &mgr;m.
The local intensity distribution of the laser beam, which is present on the detection surface due to diffraction on the diffraction object, is detected by means of an appropriate detector. The respective actual thickness of the diffraction object is determined by deriving it from the intensity distribution caused by diffraction. This thickness varies in the course of the process of depositing the layer as a result of the deposited or absorbed layer. It is thus possible to compute the respective thickness of layer present at any time on the basis of the actually measured respective spacing of the diffracting limits and the known thickness of the diffracting object, i.e. the known distance of the opposing diffracting limits given at the beginning of the process.
With the inventive method, it is possible to detect the thickness of thin layers during the process of layer deposition, independently of the parameters of the material and with a high precision. The method is appropriate for a measuring scope of layer thick
Schnupp Ralf
Waller Reinhold
Breiner & Breiner L.L.C.
Fraunhofer Gesellschaft zur Forderung der angewandten Forschung
Pham Hoa Q.
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