The subject invention relates to optical filters formed by depositing a number of thin layers on a substrate. More specifically, the invention relates to an in situ monitoring approach for accurately controlling the deposition of the layers.
In the field of optics, it is know to create wavelength selective filters by depositing multiple thin film layers on a substrate. The layers are typically formed from materials which alternate between high and low indices of refraction. The thickness of each layer is selected to be about one-half wavelength of light at wavelengths that are to be transmitted and about one-quarter wavelength of light at wavelengths that are to be reflected.
Such structures are used in optical communication systems to create Dense Wavelength Division Multiplex (DWDM) filters. DWDM filters are of the Fabry-Perot type with up to 200 dielectric layers deposited on a glass substrate or wafer. The DWDM filters operate around the 1550 nm range and have very narrow and sharp transmission notches or bandwidths. Currently, filters are being produced with bandwidths of 100 GHz (xcx9c8 angstroms) and 50 GHz (xcx9c4 angstroms) bandwidth. The goal is to reach 25 GHz (xcx9c2 angstroms) bandwidth in another year or so. This is important since each time the bandwidth is halved, the number of optical channels that can be multiplexed and demultiplexed is doubled.
To achieve such narrow and sharp bandwidths, the thickness of the Fabry-Perot layers must be controlled with extreme precision, typically about 0.1 angstroms. Currently, this is very difficult to achieve with the result that yields of 100 GHz filters is low and production of 50 GHz filters extremely difficult. In most present systems, the thickness of the layers is controlled by setting deposition parameters, such as time and temperature. It is believed that increased accuracy in the deposition of the layers could be achieved if it was possible to actually measure the thin film thickness as it is being deposited. In this manner, the process can be controlled in real time.
In accordance with the subject invention, the control of the deposition process is significantly improved by the use of a narrow band off-axis ellipsometer in an integrated closed-loop control system. One suitable ellipsometer is marketed by Therma-Wave as part of its OPTI-PROBE product and identified as the Absolute Ellipsometer (AE). This ellipsometer is described in more detail in U.S. Pat. No. 5,900,939, incorporated herein by reference) and includes a laser for generating a probe beam of radiation. The probe beam is passed through a polarizer and strikes the sample. Changes in the polarization state of the reflected beam are monitored using an analyzer. In the illustrated embodiment, the reflected beam is passed through a fixed polarizer and a rotating compensator (retarder). Other conventional ellipsometric detection arrangements, such as a rotating polarizer or analyzer, could be used.
It has been demonstrated that this ellipsometric system can operate as an integrated metrology tool in a semiconductor process device and can achieve precisions of better than 0.005 angstroms and repeatabilities of better than 0.05 angstroms for thickness measurements. Thus, the ellipsometer can be used to monitor film growth on the filter in real time. The output signals from the ellipsometer could be used, for example, for end point determinations, i.e. to determine when the layer thickness has reached a predetermined level.
To improve sensitivity and minimize uncertainties in the measurement of films transmitting in the 1550 nm range, it may be desirable to use a probe beam having a wavelength in the 1550 nm region or fractions thereof. Various lasers, such as erbium-glass and certain semi-conductor lasers operate in this wavelength regime and may be suitable to generate the probe beam for the ellipsometer.
It has also been demonstrated that a rotating compensator ellipsometer has the capability for simultaneously measuring the temperatures of the sample during deposition to 1xc2x0 C., thus permitting one to correct for changes in optical parameters with the temperature of deposition and thereby obtain accurate thickness readings. (See, pending application Ser. No. 09/575,295, filed May 19, 2000, also incorporated herein by reference.)
Further objects and advantages will become apparent from the following detailed description, taken in conjunction with the drawings in which: