This invention relates generally to chemical mechanical planarization (CMP) processes used in the manufacture of semiconductor devices. More specifically, the present invention pertains to techniques for determining an endpoint of CMP, and for determining the planarity of a semiconductor device.
As the number of interconnect layers in integrated circuit chips increases, the planarization of dielectric and metal layers has become more critical. Chemical mechanical planarization is a procedure used in the fabrication of semiconductor devices to planarize device topography in multilevel semiconductor processing. However, the process must be closely monitored for endpoint detection to avoid the removal of portions of a functional device layer.
Methods of determining an endpoint generally fall in two categories: in-line endpoint detection and in-situ endpoint detection. In-line endpoint detection measures film thickness prior to CMP and again after the device has been polished a predetermined time. However, this method assumes that CMP is a static process with a polish rate that is uniform through the device from die to die, or through a lot from device to device over multiple lots.
In-situ endpoint detection methods monitor film thickness in real time as films are being polished and/or removed. Such endpoint detection methods include front side laser interferometry, spectrometry-based endpoint, optical monitoring and motor-current monitoring, each of which is known in the art. For example, Applied Materials, Inc. has developed an in-situ removal monitor that employs a spectrometer reflective technique, in which a polishing pad is equipped with a laser and detector. Reflections of a laser beam off the device surface are detected and sampled to determine film thickness. Spectrometer-based endpoint detectors determine film thickness based on a change in intensity of reflected light. Motor-current endpoint detectors are responsive to motor loading changes caused by frictional changes at the device surface that may indicate a stop layer on the device.
Chemical mechanical planarization procedures inevitably generate debris consisting in part of a slurry and sacrificial layer particulate removed from the device surface. Endpoint detection systems have not employed a method for determining a CMP endpoint by analyzing the sacrificial effluent removed from the device surface.
Mass spectroscopy has been used for determining an endpoint for etch procedures in semiconductor device manufacturing but has not been contemplated or adapted for use in CMP processes. By way of example, an endpoint detection system for an etch procedure utilizing an ultraviolet mass spectrometer is disclosed in U.S. Pat. No. 5,504,328. The ""328 patent uses mass spectroscopy to detect changes in the composition of reaction product gases during the etch procedure. The endpoint can be determined by noting a change in the gas composition caused by etching into a film of different material on the semiconductor device. The ""328 patent does not disclose the use of the mass spectrometer for anything other than determining endpoint of an etch process.
In addition, mass spectroscopy has not been used to determine the planarity of semiconductor device topography. Planarity of a semiconductor device as used in this disclosure means the magnitude of the deviation from the mean topographic variance (or xe2x80x9croughnessxe2x80x9d) of the device surface. The mean topographic variance is the average measure of xe2x80x9cevennessxe2x80x9d or xe2x80x9croughnessxe2x80x9d of the device surface. The topographic variance of a semiconductor device may be in the range of plus or minus 1500 A.
Planarity of a device surface is typically measured using in-line methods and is independent of end-point detection. For example, a metrology tool known as a ThermaWave Optiprobe, which is an optical measuring tool, is used to measure planarization after fabrication steps such as CMP. This type of instrument is used to measure oxide thickness and uniformity at several points on a semiconductor device to determine planarity.
The present invention provides an in-situ method for monitoring and determining the endpoint of a chemical mechanical planarization process (or CMP, also referred to as polishing) for semiconductor device manufacturing, and provides a method for monitoring and determining planarity of the device topography. Real time detection of the endpoint avoids the multiple steps required for in-line endpoint detection of CMP. In addition, the present invention utilizes spectral analyses to retrieve data from effluents produced in a CMP process.
For purposes of describing the present invention, a semiconductor device includes multiple layers of materials deposited or xe2x80x9cgrownxe2x80x9d on a wafer substrate and may include one or multiple integrated circuits in what is generally referred to as a wafer. The layers are generally categorized as semiconductor, conductor or insulator layers. Alternatively, the layers incorporate the name of the material composing the layer such as dielectric layer, metal layer or oxide layer. The present invention is not limited to a specific type of layer; therefore, the term xe2x80x9cdevice layer,xe2x80x9d as used herein, includes any type of layer on a semiconductor device that may be subject to chemical mechanical planarization.
The present invention imparts a variation in at least one atomic mass of at least one material within a layer formed over a substrate. The variation in the atomic mass is a function of the thickness of the layer. Removal of the layer, and endpoint of its removal, is monitored to detect the variation in atomic mass, and to measure concentration of the material being removed. Once the concentration of the material reaches, or approaches, a predetermined threshold, the removal of the layer is terminated.
In a preferred embodiment, the layer includes at least two isotopic variations of at least one material. The isotopic variations of the material are imparted within the material, as sacrificial layers. The sacrificial layers may comprise a first sacrificial layer and a last sacrificial layer. Wherein the terms xe2x80x9cfirstxe2x80x9d and xe2x80x9clastxe2x80x9d refer to the order in which the sacrificial layers are removed during polishing. Each of the sacrificial layers is comprised of the material (or layer material) with a spiking material distributed within the device layer. The atomic mass of the spiking material in the first sacrificial layer is different from the atomic mass of the spiking material in the last sacrificial layer.
During the CMP polishing process of the device layer using a suitable slurry material, samples of gas effluents of the slurry material are injected into a mass spectrometer. The mass spectrometer generates data indicative of the amount, or concentration of, the spiking material within the gas effluent which concentration is indicative of the degree of abrading of the sacrificial layer. The mass spectrometer thus provides a measurement of the concentration of selected material, such as the spiking material, over a predetermined time range.
A comparison of the concentration of compounds detected by the mass spectrometer, including the material of the device layer and the spiking material, and/or a comparison of the intensity of each spiking material to the range of distribution within which the materials are detected by the mass spectrometer, measures the planarity of the device surface.
The endpoint of the CMP is determined by a detection of a minimum intensity of the spiking material in the last sacrificial layer toward the end of the time range within which the spiking materials are detected.