The field of the invention is chemical-mechanical polishing of material, in particular of integrated circuit wafers.
Since its advent, Chemical-mechanical polishing or planarization (CMP) has become an enabling process technology for IC manufacturing. Implementation of CMP into manufacturing environment calls for the capability to monitor the process and end-point (EP) it, before it erodes into the metal or dielectric underneath the layer that is mean to be removed.
(CMP) processes are used for the manufacturing of high-speed microprocessors, application specific integrated circuits (ASICs), Microelectromechanical Systems (MEMS), and other IC's or MEMS devices. The invention presents a set of simple and precise algorithm to process the raw physical signals emitting from the events of polishing into distinguishable and repeatable high-resolution symbols that provide end-point control for CMP process.
To achieve this, most of the modern CMP tools are equipped with a sensor to detect the thermal (e.g., U.S. Pat. No. 5,196,353), frictional (e.g., U.S. Pat. No. 5,069,002), optical (e.g., U.S. Pat. No. 5,433,651), vibrational (e.g., U.S. Pat. No. 5,222,329), electrochemical (e.g., U.S. Pat. No. 5,637,185), or electrical (e.g., U.S. Pat. No. 6,072,313) signals emanating from the pad or wafer during polishing. Changes in the magnitude of these signals (referred to as discriminants because they are used to discriminate the correct stopping time) suggest the crossover from one material layer to the other, and hence can be interpreted as reaching the interface for the desired end-point. Quite often, however, the signals emitting from the interface is weak, ambiguous, and indistinguishable so that the end-point is overlooked. Detecting such end-point signals is even more challenging when the material layers of interests share similar thermal, optical, and mechanical properties.
In accordance with the invention, a sensor is attached to component(s) of a CMP tool, in order to detect changes in physical or chemical signals during polishing process. Such signals are collected, amplified, and transported to a controller or computer for further processing. Processed signals with a “cut-off” criterion for identifying the end-point are then fed back to the polisher, in order to terminate the polish process. The signals to be detected and collected can be those from the temperature change during polishing, as described in U.S. Pat. No. 5,196,353 entitled Method for Controlling a Semiconductor (CMP) Process by Measuring a Surface Temperature and Developing a Thermal Image of the Wafer of Gurtej. S. Sandhu et al., which is assigned to Micron Technology, Inc; or as in a series of U.S. Pat. Nos. 5,597,442, 5,643,050, and 5,647,952 of Chen et al. on detecting pad temperature change, which is assigned to Industrial Technology Research Institute of Hsinchu, Taiwan. These signals can also be electrical, as described in U.S. Pat. No. 5,337,015 entitled In-situ Endpoint Detection Method and Apparatus for Chemical-mechanical Polishing Using Low Amplitude Input Voltage of Naftali E. Lustig et al., which is assigned to International Business Machines corporation; or optical (e.g., reflectivity from wafer surface layer), as described in U.S. Pat. No. 6,159,073 entitled Method and Apparatus for Measuring Substrate Layer Thickness during Chemical Mechanical Polishing of Andreas N. Wiswesser of Applied Materials, Inc; or platen/carrier torque change (mechanical) as described in U.S. Pat. No. 5,036,015 entitled Method of Endpoint Detection during Chemical/mechanical Planarization of Semiconductor Wafersof Gurtej. S. Sandhu et al. of Micron Technology, Inc; or vibrational/acoustical as described in U.S. Pat. No. 5,222,329 entitled Acoustical Method and System for Detecting and Controlling Chemical-mechanical Polishing (CMP) depths into Layers of Conductors, Semiconductors, and Dielectric Material of Chris C. Yu of Micron Technologies, Inc.
Although mathematicians are aware that the first derivative of a curve identifies changes in slope, all commercially available CMP tools of which the inventors are aware use direct signals, not derivatives.
A feature of the invention is the provision of an algorithm that yields robust, easily distinguishable signals to end-point the CMP process for improved process control and stability.
A feature of the invention is that the signals are collected and transported to a computer or signal processor where their intensity (magnitude) is differentiated with respect to time in-situ, according to the following equation: dl/dt=ε
where l represents the intensity of the detected signals such as temperature, motor current (proportional to torque), acoustics, reflectivity, interference, impedance, electrical current (e.g., eddy current), or capacitance; t is the time in seconds; and ε is the incremental change of the intensity with time.
A feature of the invention is that when a desired end-point signal, εep is reached, the computer sends a command to the polisher to stop the process.
Another feature of the invention is that, when an interface of interest, εin is reached, the computer sends a command to the polisher to continue the polishing process for certain duration of time before stopping it (overpolishing), in order to meet the designed thickness specification.
Yet another feature of the invention is that the collected signals can be further processed to reveal more of the physical events during polishing, in order to assist the detection of desired interface. For example, second derivative of the signals with time, d2l/dt2 can be generated and monitored in conjunction with dl/dt for end-pointing purpose.
Referring to
The invention is directed to the detection of CMP end-point during the polishing of chemically similar material layers comprising, for example, a single damascene or dual damascene thickness of a first dielectric (or multilayer dielectric stack including an embedded etch stop), and multiple spin-on or plasma-enhanced chemically vapor deposited (PECVD) CMP stop layer(s) (or “cap layers”) upon the first dielectric layer. The first dielectric layer may be comprised of SiLK™, GX-3™, porous SiLK™, GX-3p™, Black Diamond™, NCS™, or other non-porous or porous low k dielectric materials. The CMP stop layers may be comprised of one or combination of the following: TEOS-oxide, silane-oxide, SiNx, BLok™, N-BLok™, PECVD-based SiwCxOyHz dielectric materials, AP 6000™, HOSP™, HOSP BESt™, Ensemble™ Etch Stop, Ensemble™ Hard Mask, hydrido silsesquioxanes, hydrido-organo silsesquioxanes copolymers, siloxanes, silsesquioxanes, or other spin-on or CVD material.
The substrate may contain electronic devices such as, for example, transistors and an array of conductor elements. A shallow-trench isolation structure composed of fill oxide, liner oxide, and SiNx CMP stop layer, a tungsten plug structure employing TiN/Ti liner layers, or an interconnect structure composed of low-k dielectrics and Cu wires, in accordance with the invention can be formed on the substrate. Conventional CMP end-point algorithms would require the collection of the signals emitting from the interface as the structure is polished down from layer 3 to layer 2.
The first (or higher) derivative is calculated. A plot of ε vs. t is put forth instantaneously for monitoring purposes. Distinct peaks on this ε vs. t plot flag the times when a change in the intensity of the above signals takes place and hence corresponds to the change in the materials properties across the interface between two layers.
Referring to
In this case, the structure being polished contains only one dielectric layer, Layer 1, which is PECVD fluorine-doped TEOS (F-TEOS) oxide. Layers 3 and 2 are not present in this example. The interface to be detected is thus liner/F-TEOS. The temperature monitored on the pad shows no obvious feature that can be identified as reaching the end of liner polishing for end-pointing purpose.
|dT/dt|≦m and |d2T/dt2|≦n within Δt=10 sec Eq. [1]
where m is a cut-off value (e.g., 0.5 in this case) for |dT/dt|; n is the cut-off value (e.g., 0.05 in this case) for |d2T/dt2|and Δt is the detection time window.
According to the invention, an end-point value is specified based on empirical data and the polishing is stopped when that value is reached.
The dT/dt vs. t traces on
The temperature curve is shown in
It will readily be seen that this situation is not well suited to the use of the temperature as a discriminant, since there is no knee in the curves that is readily visible.
|dT/dt|≦u and |d2T/dt2|≦v within Δt=10 sec Eq. [2]
where u is the cut-off for |dT/dt| (e.g., can be 1.5 in this case); v is the cut-off value for |d2T/dt2| (e.g., can be 0.3 in this case) and Δt is the detection time window.
In another case, the interface between SiNx and SiwCxOyHz stop layer (e.g., layer 3 and layer 2, respectively, based on
While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced in various versions within the spirit and scope of the following claims.