Real time monitoring of CMP pad conditioning process

CMP pad conditioning processes have been monitored and controlled by detecting the vibrational spectrum of a sensor mounted on the conditioner support arm. An accelerometer is used as the detector so that vibrational velocity (which correlates with pad wear) can be measured, rather than displacement or acceleration. After the vibrational spectrum has been transformed to its frequency domain equivalent, it is monitored for the presence of abnormal peaks in order to control the conditioning process.

FIELD OF THE INVENTION

The invention relates to the general field of chemical-mechanical polishing (CMP) with particular reference to conditioning the polishing pads after extended use.

BACKGROUND OF THE INVENTION

CMP has, for many years, been the preferred method for planarizing integrated circuits. As its name implies, this process involves mechanical polishing assisted by chemical action. In CMP, wafers are mounted on a suitable holder and then pressed against a polishing pad that has been attached to a rotating platen. A problem with CMP processing is that the throughput may drop because waste particles accumulate at the surface of the polishing pad. To restore the effectiveness of the polishing pads, they are typically ‘conditioned’ by using an abrasive disk to remove the waste matter. Such disks are generally embedded with diamond particles and are mounted so as to be independently moveable and rotatable.

FIG. 1shows a typical arrangement for a CMP pad conditioning disk. Seen there are platen11to which is attached polishing pad12. Conditioning disk13is pressed against pad12by means of overhead support arm14to which it is attached. Generally conditioning disks remove a thin layer of the pad material itself in addition to the waste matter, thereby forming a fresh planarizing surface for the polishing pad.

The conditioning disks themselves may eventually lose their effectiveness by being worn down or by getting plugged up with particulate matter. If a change in effectiveness is not detected, preferably during the conditioning process itself, a sub-standard polishing pad may be inadvertently returned to service, with undesirable consequences. Thus, it is important to minimize the time required for the conditioning process, to maximize the useful life of each polishing pad, and to detect inadequate pads before they are restored to service.

Conventionally, monitoring of CMP pad conditioning processes is performed by trial-and-error wherein different control parameters are varied manually by an operator to achieve optimal conditioning. These control parameters include pressure of the conditioner on the polishing pad, translational and rotational speeds of the conditioner, platen rotation speed, and the number of sweeps over the pad surface made by the conditioner.

The manual adjustments referred to above are based solely on human experience, hence additional manpower is required. Furthermore, it is very difficult to achieve good repeatability for the conditioning process if the manual adjustment is performed by different operators. In view of this, there exists a need for an apparatus and method for monitoring the CMP pad conditioning process that help to extend the life of the polishing pads, reduce human intervention and improve the control of the conditioning process.

A routine search of the prior art was performed with the following references of interest being found:

U.S. Pat. No. 6,755,718, EP 1,063,056A2, WO 01/32,360A1, U.S. Pat. No. 5,708,506, U.S. Pat. No. 6,424,137, and U.S. Pat. No. 5,399,234 all aim to improve the CMP process as well as teaching in-situ monitoring of the pad conditioning process. U.S. Pat. No. 6,424,137 and U.S. Pat. No. 5,399,234 relate to the use of acoustic analysis for monitoring of CMP process. U.S. Pat. No. 6,424,137 teaches detection of wafer vibration characteristics to minimize wafer damage during polishing by placing a sensor directly on the wafer, while U.S. Pat. No. 5,399,234 discloses the use of acoustic waves generated in the polishing slurry to determine the end-point of the polishing process by placing a sensor in the slurry. Both of these prior art references seek to improve the CMP wafer polishing process rather than the pad conditioning process.

In U.S. Pat. No. 5,708,506 the roughness of the pad is determined by employing a light source that impinges on the polishing pad and a light detector for detecting the light emanating from the polishing surface. In WO 01/3,2360A1, ultrasonic transducers are employed to measure the thickness of the layers on the polishing pad and the result used to control process variables. EP 1,063,056A2 relates to a method and apparatus whereby a contactless displacement sensor is used to generate the polishing pad profile and to monitor the pad wear uniformity. In U.S. Pat. No. 6,755,718, a force sensor is mounted on the conditioner to detect the frictional force imparted to the conditioning body by the planarizing medium whereby the detected force is fed back to a controller for monitoring of the conditioning process.

Although the final objectives of all the prior art cited above are similar, none of them mount their sensor on the conditioner support arm nor do they use the measurement and application of vibration signals from the pad conditioner for real-time monitoring and control of the conditioning process.

SUMMARY OF THE INVENTION

It has been an object of at least one embodiment of the present invention to a process for monitoring and controlling a CMP pad conditioning process.

Another object of at least one embodiment of the present invention has been to provide an apparatus for implementing said process.

Still another object of at least one embodiment of the present invention has been to make said process and apparatus easy to implement using any available CMP tool set.

A further object of at least one embodiment of the present invention has been to prolong the lives of both polishing pads and conditioner disks.

A still further object of at least one embodiment of the present invention has been to facilitate detection of abnormal conditioner disks thereby averting possible future damage to wafers by the CMP process.

These objects have been achieved by measuring the vibration of the pad conditioner. In the present invention, an accelerometer was mounted on the support arm of the pad conditioner to measure its vibration frequency. The time dependent signal obtained is analyzed by using Fast Fourier Transform (FFT) to convert it to the frequency domain. Abnormal frequency peaks, if detected, serve as guideline for optimization of the pad conditioning process. Real-time monitoring of the conditioning process is achieved by means of a negative feedback loop for controlling the number of sweeps and head pressure of the conditioning body in response to the changes detected through the vibration signature.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The apparatus of the present invention is schematically illustrated inFIG. 2. It includes vibration sensor21, signal conditioner22, dynamic spectrum analyzer23, and visual display unit24. Our preferred device for vibration sensing has been an accelerometer (for reasons that will be explained below) but related devices such as microphones, reflectometers, Linear Variable Displacement Transformer (LVDT), Strain Gauge, Proximeter (magnetic and inductive), and Laser Vibrometer could also have been used without departing from the spirit of the invention.

Sensor21is mounted on support arm14of pad conditioner13. The signal detected by the sensor21corresponds to vibrational velocity as a function of time, being therefore a time domain representation. It is amplified by signal conditioner22which may be either a Constant Current Line Drive (CCLD) or a charge amplifier. If the CCLD version is selected it may be built-in together with the Dynamic Spectrum Analyser (DSA) and a separate, external signal conditioner would not be required.

The time domain signal is then transformed into its frequency domain equivalent by the DSA (using a Fast Fourier Transform Algorithm).FIG. 3ashows an example of a typical time domain plot whileFIG. 3bshows its frequency domain equivalent. Although vibration amplitude can be measured in terms of displacement, velocity, or acceleration, velocity was selected because it is a function of time and wear rate is also a function of time. This led to an accelerometer becoming our sensor of choice because acceleration signal can be easily (electronically or by software) integrated to obtain velocity and displacement, and also because of its small physical size and weight, it can be installed in small space without mass loading effects. Thus, an increase in vibration velocity amplitude correlates with an increase in the wear rate of the pad and/or pad conditioner. As a result, the process parameters can no longer be optimized once the pad and/or pad conditioner have worn out. When this happens (i.e. process parameter optimization cannot be achieved after several attempts), the control software can readily issue a warning by sounding an alarm.

The next step is the creation and display of a waterfall plot by cascading a succession of frequency domain spectra during one full sweep of the pad conditioning procedure. At this point (if the system is only partly automated) an operator will be able to determine if the process parameters used were optimum, by observing one or more frequency bands in the waterfall plot, particularly (in this example) the 200 Hz band. The operator can then adjust the appropriate pad conditioning control variables such as sweep rate, head pressure and head rotation rate until all abnormal peaks disappear from the 200 Hz and any other selected bands.

FIG. 4shows a typical waterfall plot for a new or fully reconditioned polishing pad whileFIG. 5is a similar plot for a used pad in need of conditioning. It is readily seen that, for the new pad, any peaks that are present have very low amplitude and are randomly scattered, the only exception being a set at about 150 Hz (representing the CMP tool structural vibration. On the other hand, the plot for the used pad, in addition to substantial activity at very low frequencies, shows strong activity at, or near, 200 Hz, thereby providing a straightforward means of distinguishing between pads that are suitable for use and pads that are in need of conditioning.

The 200 Hz value is dependent on the mechanical characteristics of the conditioning system. It will vary with the operating parameters of the CMP such as platen speed, polisher speed and number of sweeps i.e. dependent on the CMP operating parameters. It will also vary if different pad materials or different conditioning disk materials are used for conditioning.

FIG. 6is an expanded version of the basic process flow illustrated inFIG. 2. The measurement of a vibration signal starts when the conditioning disk touches the polishing pad, causing trigger signal61to be emitted. The time domain signal that is collected using the sensor mounted on the support arm of the pad conditioner is passed to DSP62which is controlled by computer program63. The FFT is performed at the DSP to convert the time domain signal to a frequency domain signal, the data collected being stored in data memory63. The assembled output of DSP62is sent to visual display24(FIG. 2). Interface RS 232 (shown as65) serves as a serial communication port for interfacing to other personal computers or modems for networking etc. while watchdog66serves to monitor whether the pre-alarm or alarm has exceeded the preset guardband (i.e. it is a comparator circuit).

In addition to the visual display shown inFIG. 2, the apparatus ofFIG. 6also includes control unit67which adjusts the various parameters associated with the pad conditioning process, thereby replacing the human operator who was mentioned earlier. Optimum values for the conditioning parameters are computed by computer63and then passed to unit67for conversion to the specific voltages needed to control operational variables such as pad conditioner pressure, pad conditioner rotation speed, and platen rotation speed, as well as when to terminate the conditioning process (i.e. the sweep profile).

Experiments were conducted to characterize the impact of the above process parameters in terms of vibration signals. Vibration amplitude was observed to correlate with pad conditioner downward force while increasing pad conditioner rotation speed was seen to result in lower vibration amplitude. Different sweep profiles of the pad conditioner were found to have an effect on the vibration signal. Specifically, increasing the number of sweeps resulted in a lower vibration amplitude. The lowest vibration amplitudes were observed at platen rotation speeds of about 65 rpm. This data allowed the characteristics of process parameters to be modeled and programmed into controller67for real-time monitoring and control of the pad conditioning process.

FIG. 7is an example of a waterfall plot generated during a pad conditioning process in which the conditioning parameters were all optimized as discussed above. As can be seen, the frequency peaks at 200 Hz disappear or are reduced after point72is passed, allowing conditioning to be terminated after about 15 seconds of total conditioning time (symbolized by line71), with full confidence that optimum pad renewal has been achieved. In addition to saving time that is more profitably used for performing CMP, this feature also guarantees that no more of the polishing pad's surface is removed than is absolutely necessary.

FIGS. 8aand8btogether constitute a flow chart of the full process of the invention. Beginning at the box labeled “Start” inFIG. 8a, the time domain signal is transformed into its frequency domain equivalent and the amplitude at one (or more) specific frequencies (e.g. 200 Hz) is measured. If this measured amplitude is below the minimum value of a preset guard band, the conditioning process is allowed to continue. If abnormal peaks do not appear during a preset time period the process is allowed to proceed without changing conditioning parameters.

If the measured amplitude was above the minimum value of the guard band, then the pad is in need of conditioning and we transfer toFIG. 8bvia common box A. Now, if the measured amplitude is above the maximum value of the guard band, this indicates a problem such as a non-recoverable pad or an apparatus malfunction. In such a case, the operation is halted and an alarm is sounded to trigger an investigation. If the peaks are below or on the guard band, the process parameters will be adjusted automatically until the peaks disappear (flow chart loop81). Pad conditioning then continues, based on the final set of parameters, until a preset conditioning time is reached (loop82) or until abnormal peaks appear again (loop83). In the latter case, the new peaks are again compared with the preset guard band and the process parameters are again adjusted to effect their removal.

In conclusion, we note the following advantages of the invention:

Easy to implement

A real time monitoring system can be implemented using any available CMP tool set.

Reduced operating costs

Optimization of the pad conditioning process will prolong the lives of both the polishing pad and conditioner disk.

CMP Process enhancement

Real time monitoring allows better control of pad conditioning throughout the pad's life, thereby maintaining good post CMP wafer uniformity. Real time monitoring facilitates detection of abnormal conditioner disks thereby averting any further damage from CMP induced scratches.