Pulse voltage compensation for plasma processing applications

Abstract

Embodiments provided herein generally include apparatuses (e.g., plasma processing systems) and methods for plasma processing of a substrate in a processing chamber. Some embodiments relate to a method for waveform generation that generally includes delivering a first waveform having an associated setpoint from an energy source; detecting at least one characteristic of the first waveform; estimating voltage attenuation during a portion of the pulse during the first waveform; calculating a compensation factor; and adjusting the at least one characteristic using the compensation factor to adjust the voltage attenuation.

Description

background Technical Field

[0001] Embodiments of the present disclosure generally relate to systems used in the manufacture of semiconductor devices. More specifically, embodiments of the present disclosure relate to plasma processing systems for processing substrates. Background Art

[0002] Reliably forming high aspect ratio features is one of the key technical challenges in manufacturing next generation semiconductor devices. High aspect ratio openings for forming features are typically formed using a plasma assisted etch process such as a reactive ion etch (RIE) process that enables directionally controlled (i.e., anisotropic) material removal to transfer a pattern from a mask layer to an exposed portion of the substrate surface beneath it. As feature sizes continue to shrink and pattern densities continue to increase, the degree of anisotropy and within-substrate processing uniformity of the RIE process are critical factors in forming tightly spaced (fine pitch) high aspect ratio openings.

[0003] For etching processes in which plasma ions play a major role, ion energy control has always been a challenge for the semiconductor equipment industry. In a typical plasma-assisted etching process, a substrate is positioned on an electrostatic chuck (ESC) disposed in a processing chamber, a plasma is formed on the substrate, and ions are accelerated from the plasma across a plasma sheath (i.e., a region depleted of electrons) formed between the plasma and the substrate surface toward the substrate. Traditionally, radio frequency (RF) substrate bias methods (which use a sinusoidal RF waveform to excite the plasma and form a plasma sheath) have been unable to form these smaller device feature sizes as desired. Recently, it has been discovered that delivering high voltage direct current (DC) pulses to one or more electrodes within the processing chamber can be used to desirably control the plasma sheath formed on the surface of the substrate.

[0004] During plasma processing of a substrate, a voltage pulse will typically be configured to generally include a shell collapse phase, an ion current phase, and a shell formation phase, the shell formation phase being disposed between the shell collapse phase and the ion current phase. The shell collapse phase may be achieved by generating a positive voltage (e.g., 100 volts) that is used to collapse a shell formed on a surface of a substrate disposed on a substrate support positioned in a processing chamber. During the ion current phase, ions within the processing chamber flow toward the surface of the substrate due to a generated negative voltage (e.g., -1600 volts) applied to an electrode disposed adjacent to the substrate. The ion current phase may be accompanied by an associated voltage decay (also often referred to as a "droop") in the voltage pulse established on the substrate during this phase, which may result in an ion energy distribution function (IEDF) that is generally undesirable. The amount of voltage decay that occurs during plasma processing may vary due to a number of factors, including chamber pressure variations, process chemistry variations, and the applied voltage and source power.

[0005]

[0006] Therefore, there is a need in the art for apparatus and methods that provide improved control over the characteristics of a plasma sheath formed on a substrate during plasma assisted processing of the substrate. Summary of the invention

[0006] Embodiments herein provide plasma processing chambers and methods configured for fine tuning and controlling a plasma sheath formed during plasma-assisted processing of semiconductor substrates.

[0007] Some embodiments relate to a method for waveform generation. The method generally includes: delivering a first waveform having an associated set point from an energy source; detecting at least one characteristic of the first waveform using at least one sensor; determining a voltage decay value during a portion of a pulse during the first waveform by using the detected at least one characteristic and at least one stored voltage decay value function; determining a shell-coupled voltage value during the portion of the pulse of the first waveform by using the detected at least one characteristic and at least one stored shell-coupled voltage value function; calculating a compensation factor based on the determined voltage decay value and the determined shell-coupled voltage value; and adjusting the at least one characteristic using the compensation factor.

[0008] Some embodiments relate to a method for waveform generation, comprising: delivering a first waveform having an associated set point from an energy source; detecting at least one characteristic of the first waveform using at least one sensor; determining a voltage attenuation value during a portion of a pulse provided within the first waveform by using the at least one detected characteristic and at least one stored voltage attenuation value function; determining a shell coupling voltage value during the portion of the pulse of the first waveform by using the at least one detected characteristic and at least one stored shell coupling voltage value function; calculating a compensation factor based on the determined voltage attenuation value and the determined shell coupling voltage value; and adjusting at least one characteristic of the first waveform by applying the calculated compensation factor to at least one characteristic of the first waveform.

[0009] Some embodiments relate to a waveform generator. The waveform generator generally includes a sensor assembly coupled to an output of a pulser, wherein the sensor assembly includes at least one sensor configured to detect at least one characteristic of a first waveform generated by the waveform generator; and a system controller coupled to the waveform generator. The system controller generally includes a processor and a memory, wherein the memory includes instructions that, when executed by the processor, result in: determining an amount of voltage decay within a portion of a pulse during the first waveform using at least one stored voltage decay value formula; determining an amount of shell-coupled voltage within the portion of the pulse of the first waveform using at least one stored shell-coupled voltage value formula; calculating a compensation factor based on the determined amount of voltage decay and the determined amount of shell-coupled voltage; and adjusting the at least one characteristic using the compensation factor to adjust the voltage decay.

[0010] Some embodiments relate to a waveform generator, comprising: a sensor assembly coupled to an output of a pulser, wherein the sensor assembly comprises at least one sensor configured to detect at least one characteristic of a first waveform generated by the waveform generator; and a system controller coupled to the waveform generator. The system controller comprises: a processor and a memory, wherein the memory comprises instructions that, when executed by the processor, result in: determining an amount of voltage decay within a portion of a pulse during the first waveform using at least one stored voltage decay value function; determining an amount of shell coupled voltage within the portion of the pulse of the first waveform using at least one stored shell coupled voltage value function; calculating a compensation factor based on the determined amount of voltage decay and the determined amount of shell coupled voltage; and adjusting the at least one characteristic of the first waveform based on application of the calculated compensation factor.

[0011] For the foregoing and related purposes, one or more aspects include features that are fully described hereinafter and particularly pointed out in the claims. The following description and the accompanying drawings set forth in detail certain illustrative features of one or more aspects. However, these features are only indicative of several of the various ways in which the principles of the various aspects can be employed. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Therefore, in order to be able to understand in detail the manner in which the above-mentioned features of the present disclosure are achieved, a more specific description of the present disclosure briefly summarized above may be obtained by reference to embodiments, some of which are shown in the accompanying drawings. However, it should be noted that the accompanying drawings illustrate only exemplary embodiments and are therefore not to be considered limiting of their scope and may allow for other equally effective embodiments.

[0013] Figure 1 is a simplified schematic cross-sectional diagram of a processing system and a signal detection module according to certain embodiments of the present disclosure.

[0014] Figure 2 An example of an under-compensated voltage waveform established on a substrate according to certain embodiments of the present disclosure is illustrated.

[0015] Figure 3 is a process flow diagram illustrating a method of waveform generation according to certain embodiments of the present disclosure.

[0016] Figure 4 An example of a compensated voltage waveform established on a substrate according to certain embodiments of the present disclosure is illustrated.

[0017] Figure 5A Illustrated are a graph representing a detected voltage pulse formed within a pulse train of a voltage waveform and a graph representing a portion of a voltage detected during the pulse train, in accordance with certain embodiments of the present disclosure.

[0018] Figure 5B FIG. 1 shows a diagram of some embodiments according to the present disclosure. Figure 5A The curve of the current detected during the transmission of the voltage pulse formed in the pulse train of the voltage waveform, and the curve representing the current detected during the transmission of the voltage pulse formed in the pulse train of the voltage waveform, and Figure 5A A graph of the current detected during the portion of the pulse train is shown.

[0019] Fig. 6A A voltage decay value curve illustrating a waveform according to certain embodiments of the present disclosure.

[0020] Figure 6B Several plasma sheath coupling curves are illustrated according to certain embodiments of the present disclosure.

[0021] Figure 7 Example compensation factors as applied to a voltage waveform are shown.

[0022] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. DETAILED DESCRIPTION

[0023] Certain aspects of the present disclosure generally relate to techniques for generating a compensated voltage waveform for a plasma processing system. During plasma processing of a substrate, a voltage waveform provided to an electrode disposed within a plasma processing chamber will generally be configured to include a sheath collapse phase and an ion current phase. The sheath collapse phase may be achieved by generating a positive voltage (e.g., 100 volts) that is used to collapse a plasma sheath generated on a surface of a substrate disposed on a substrate support positioned in the processing chamber. During the ion current phase, ions within a plasma formed in the processing chamber begin to flow toward the surface of the substrate due to a negative voltage (e.g., >1000 volts) generated by applying a pulsed voltage waveform to an electrode disposed adjacent to the substrate. The ion current phase may be accompanied by an associated voltage decay or "droop," which will generally result in an undesirable energy distribution function (IEDF) at the surface of the substrate. The amount of voltage decay that occurs during plasma processing may vary due to a number of factors, including chamber pressure changes, process chemistry changes, and the applied voltage and source power. Since the voltage decay between the plasma processing chamber and the processing system varies with the plasma processing recipe, it is necessary to account for the voltage decay during plasma processing and compensate for the decay accordingly.

[0024] In aspects of the present disclosure, the waveform generator may use a ramp to implement voltage compensation during the ion current phase of the pulse to narrow the IEDF, as described in more detail herein. A compensated voltage waveform may be generated by applying a calculated compensation factor to a waveform generated by a pulsed voltage source to correct for voltage decay established on a substrate during plasma processing. Plasma Processing System Examples

[0025] Figure 1is a simplified schematic cross-sectional view of a processing system and a signal detection module according to certain embodiments of the present disclosure. In some embodiments, the processing system is configured for a plasma-assisted etching process, such as a reactive ion etching (RIE) plasma process. However, it should be noted that the embodiments described herein may also be used with a processing system configured for other plasma-assisted processes, such as a plasma-enhanced deposition process (e.g., a plasma-enhanced chemical vapor deposition (PECVD) process, a plasma-enhanced physical vapor deposition (PEPVD) process, a plasma-enhanced atomic layer deposition (PEALD) process), a plasma machining process, or a plasma-based ion implantation process (e.g., a plasma doping (PLAD) process).

[0026] As shown, the processing system includes a processing chamber 100 and a signal detection module 388, which can be used to monitor and control the characteristics of a plasma sheath 101a during substrate processing. The processing chamber 100 includes the signal detection module 388, a support assembly 136, and a bias module 198. The bias module 198 can include one or more pulsed voltage (PV) waveform generators 150 and / or one or more RF generator assemblies 118. The support assembly 136 includes a support base 107, and a substrate support 105 disposed on the support base 107 and thermally coupled to the support base 107. As shown, the processing chamber 100 is configured to generate a capacitively coupled plasma by delivering a radio frequency (RF) signal from an RF power supply 118 to the support base 107. However, it is contemplated that the signal detection module 388 can be used with any number of processing chambers, support assemblies, and bias modules to facilitate monitoring and controlling the plasma sheath 101a during substrate processing.

[0027] Here, electrical signals that can be used to determine one or more characteristics of the plasma sheath 101a are received by the signal detection module 388, which then communicates information about the electrical signals to the system controller 126 for use in controlling aspects of the plasma process. The system controller 126 may include a processor 190, a memory 192, and support circuits 194. The processor 190 may be a general-purpose computer processor that is configured for use in an industrial environment for controlling a processing chamber and subprocessors associated therewith. The memory 192 (which is typically a non-volatile memory) may include a random access memory, a read-only memory, a floppy disk or a hard disk drive, or other suitable form of digital storage (local or remote). The support circuits 194 are conventionally coupled to the processor 190 and include caches, clock circuits, input / output subsystems, power supplies, etc., and combinations thereof. Software instructions (programs) and data may be encoded and stored in the memory 192 for instructing the processor 190. A software program (or computer instructions) readable by processor 190 in system controller 126 determines which tasks are performed by components in the processing system.

[0028] Generally, a program readable by the processor 190 in the system controller 126 may be stored in the memory 192 and include code that, when executed by the processor 190, performs tasks related to the plasma processing schemes described herein. The program may include computer-implemented instructions for controlling various hardware and electrical components within the processing system to perform various process tasks and various process sequences for implementing the methods described herein. In one embodiment, the program includes instructions for performing the following with respect to Figure 3 Instructions for one or more of the operations described.

[0029] In general, the system controller 126 determines one or more characteristics of the plasma sheath 101a based on information received from the signal detection module 388 and compares the determined sheath characteristics to desired sheath characteristics. Based on the difference between the determined sheath characteristics and the desired sheath characteristics, the system controller 126 may use the sheath compensation scheme described herein to adjust the one or more sheath characteristics, for example, by changing the configuration of one or more of the pulsed voltage (PV) waveforms established at the bias electrode 104. For example, the system controller 126 may cause the PV waveform generator 150 to change one or more characteristics of the pulsed voltage waveform delivered to the bias electrode 104. In some embodiments, the second PV waveform generator 150 (not shown) is coupled to the edge electrode 115 and is therefore configured to deliver one or more of the PV waveforms to the edge electrode 115. In this configuration, the system controller 126 may also cause the second PV waveform generator 150 to change one or more characteristics of the pulsed voltage waveform delivered to the edge electrode 115 during plasma processing.

[0030] In some embodiments, the RF generator assembly 118 delivers an RF signal to the support pedestal 107 (e.g., a power electrode or cathode), which can be used to generate (maintain and / or ignite) a plasma 101 in a processing region 129 disposed between the substrate support assembly 136 and the chamber lid 123. In some embodiments, the RF generator 118 is configured to deliver an RF signal to the support pedestal 107, the RF signal having a frequency of greater than 1 MHz or greater or about 2 MHz or greater (such as about 13.56 MHz or greater).

[0031] In some embodiments, the RF generator 118 and the RF generator assembly 160 are configured to deliver a desired amount of continuous wave (CW) or pulsed RF power (often referred to herein as "source power") at a desired generally fixed sinusoidal waveform frequency to the support pedestal 107 of the substrate support assembly 136 based on control signals provided from the system controller 126. During processing, the RF generator 118 and the RF generator assembly 160 are configured to deliver RF power (e.g., an RF signal) to the support pedestal 107 disposed proximate to the substrate support 150 and within the substrate support assembly 136. The RF power delivered to the support pedestal 107 is configured to ignite and sustain a plasma 101 containing a process gas disposed within the processing region 129. In some embodiments, the RF generator assembly 118 may alternatively be configured to deliver an RF signal to the chamber lid 123 to ignite and sustain a plasma 101 containing a process gas disposed within the processing region 129.

[0032] Figure 1The electrical connections between the signal detection module 388 and the node N, the connection point, and the bias electrode 104, the first PV module 196, and the electrostatic chuck clamping network 116 are shown. Figure 1 Only one PV waveform generator 150 is illustrated in FIG. 1 , however, it should be noted that in the embodiments herein, the signal detection module 388 is configured to receive electrical signals through electrical connections to the node N, the connection point, and elements within the bias module 198 .

[0033] As shown, the signal detection module 388 is electrically coupled to various electrical components within the processing chamber 100 using a plurality of signal lines 387. The plurality of signal lines 387 include a plurality of signal traces 392 that are coupled to various electrical components within the processing chamber 100 and are configured to deliver electrical signals to signal detection elements within the signal detection module 388. In general, the signal detection module 388 includes one or more input channels 372 and a fast data acquisition module 320. The one or more input channels 372 are each configured to receive an electrical signal from a signal trace 392 and are electrically coupled to the fast data acquisition module 320. The received electrical signal may include one or more characteristics of a waveform established by the PV waveform generator 150 and / or the RF power supply 118.

[0034] The fast data acquisition module 320 includes one or more acquisition channels 322 that receive signal information from various components of the processing chamber 100a via signal lines 387 and one or more input lines 172. The fast data acquisition module 320 processes the received signal information to determine one or more characteristics of the waveform generated by the bias module, and communicates the processed signal information to the system controller 126.

[0035] Typically, the signal detection module 388 includes a plurality of input channels 372, each of which is electrically coupled to a corresponding acquisition channel 322 of the fast data acquisition module 320. Figure 1 , multiple input channels 372 are coupled to connection points located in various portions of the bias module 198 and the clamping network 116 to measure and collect electrical data from these connection points or nodes N during processing. In some embodiments, the multiple input channels 372 may also be coupled to various electrical sensing elements (such as one or more current sensors) that are configured to measure and collect electrical data at various points within the processing chamber 100.

[0036] Here, the fast data acquisition module 320 includes a plurality of acquisition channels 322, a data acquisition controller 323, and a memory 324 (e.g., a non-volatile memory). The data acquisition controller 323 is electrically coupled to the output of each of the acquisition channels 322 and is configured to receive a digitized voltage waveform from each of the acquisition channels 322. In addition, an algorithm stored in the memory 324 of the data acquisition controller 323 is adapted to determine one or more waveform characteristics of each of the waveforms by analyzing each of the digitized voltage waveforms. The analysis may include comparing information received in the digitized voltage waveform with information about one or more stored waveform characteristics stored in the memory 324, as discussed further below.

[0037] The data acquisition controller 323 may include one or more of an analog-to-digital converter (ADC) (not shown), a processor 321, a communication interface (not shown), a clock (not shown), and an optional driver (not shown). The processor may be any general-purpose computing processor. In addition, the processor may be a field programmable gate array (FPGA). The ADC converts the signal within the output waveform from the analog domain to the digital domain, and the output digital signal of the ADC is provided to the processor 321 for processing. The processor 321 determines one or more waveform characteristics of the output waveform by analyzing the output digital signal provided from the ADC.

[0038] The memory 324 may be any non-volatile memory. The data acquisition controller 323 may be electrically coupled to the memory 324 and configured such that the waveform characteristics are stored in the memory 324. In various embodiments, the memory 324 includes instructions that are executable by the data acquisition controller 323 to cause the data acquisition controller 323 to analyze the received output waveform and / or transmit information corresponding to the determined waveform characteristics based on the analysis of the received output waveform. The waveform analyzer stored in the memory 324 includes instructions that are executable by the data acquisition controller 323 and that, when executed, cause the data acquisition controller 323 to analyze the output waveform to determine the waveform characteristics.

[0039] Information regarding the analyzed waveform characteristics may then be transmitted to one or more of the feedback processor 325 and / or the system controller 126. The analysis performed by the data acquisition controller 323 may include a comparison of the waveform characteristics to one or more waveform characteristic thresholds stored in the memory 324. In some embodiments, the analysis is based on one or more electrical characteristics of the processing chamber 100 that are known and stored in the memory.

[0040] In some embodiments of the process chamber 100, the sensor assembly 170 is disposed between the PV waveform generator 150 and one or more of the bias electrodes 104. The sensor assembly 170 includes a current sensor, which may be an in-line current sensor or disposed within the PV waveform generator 150.

[0041] Typically, the PV waveform generated by the PV waveform generator 150 is configured to generate a PV waveform that is substantially constant over a substantial portion of the PV waveform cycle (e.g., Figure 2 A nearly constant sheath voltage is provided during the "ion current phase" in the process (i.e., the "ion current phase"), which, in combination with the sheath thickness, enables the formation of a desired ion energy distribution function (IEDF) at the surface of the substrate 103. As described in the following method, the ability to compensate the PV waveform to account for the voltage decay at the bias electrode 104 enables fine tuning, control, and customization of the processing results on the surface of the substrate 103.

[0042] During processing, multiple PV waveforms are provided to the bias electrode 104 and ultimately to the composite load within the processing chamber 100 by the PV waveform generator 150 of the bias module 198. Overall control of the delivery of the PV waveforms from the PV waveform generator 150 is controlled using signals provided from the signal detection module 388 and / or the system controller 126, as described below. Example Voltage Waveform

[0043] Figure 2 An example of an under-compensated voltage waveform established on a substrate during plasma processing according to certain embodiments of the present disclosure is illustrated. In some embodiments, the waveform may be presented as a train rather than being applied continuously during the execution of a plasma processing process performed on the substrate. Pulse train 200A is an example of a single pulse train established at substrate 103 as a result of delivering the voltage waveform to bias electrode 104 via PV waveform generator 150. In some embodiments, substrate 103 may be a wired version of substrate 103, referred to herein as a "wired chip." The wired chip may be used to detect one or more characteristics of the waveform and plasma characteristics established on the chip during plasma processing. In some embodiments, the wired chip may be coupled to a signal detection module 388 via wiring 392A so that information about one or more plasma processing characteristics (e.g., substrate voltage) may be determined within the plasma processing chamber, such as at Figure 1. Waveform 200B illustrates an example of two pulses generated within a portion of pulse train 200A, and portions of adjacent pulses on either side of the two pulses. In some embodiments, waveform 200B is a PV waveform that includes a first portion (e.g., portion 204 between T1 and T2, and portion 206 between T3 and T4) within each pulse that includes a positive slope (e.g., voltage per unit time), the positive slope being associated with a "drop" in voltage established on the substrate. The positive slope is formed on the substrate during the ion current phase of the PV waveform. Portions 204, 206 of waveform 200B illustrate an example of an under-compensated voltage waveform established on the substrate, the under-compensated voltage waveform being measured using a wired wafer. A voltage decay step count or stair-like ramp includes a plurality of steps (e.g., ≥2 steps) applied to the PV waveform with a fixed step voltage to form a negative voltage ramp that is used to compensate for a positive voltage ramp formed by a voltage decay or drop. The voltage decay step count may not have been applied to the waveform 200B, and the waveform 200B may not be compensated.

[0044] In some embodiments, Figure 2 Portions 204, 206 of waveform 200B in FIG. 3 represent example sampling windows used by signal detection module 388 to detect and / or determine characteristics of a "dip" in the voltage waveform established on substrate 103 as a result of the voltage waveform generated by PV waveform generator 150. In one example, the sampling window includes between about 50% and about 90% of the 400kHz pulse waveform. Several parameters may affect the 400kHz "dip", including plasma density / species, baseline pulse voltage (PV) set point, temperature, and PV on time. Several parameters may affect string shape / spread, including string on time, source L2L pulsing.

[0045] The pulse in waveform 200B generally includes two main phases: the ion current phase and the shell collapse phase. Figure 2 , the ion current phase and shell collapse phase portions of the waveform established on the substrate 103 are shown. At the beginning of the ion current phase, a voltage drop is formed at the substrate 103 due to the PV waveform generator 150 delivering the negative portion of the PV waveform (e.g., the ion current portion) provided to the bias electrode 104, which forms a high voltage shell above the substrate 103. The high voltage shell allows the positive ions generated by the plasma to be accelerated toward the biased substrate. As more positive ions bombard the substrate surface, a certain amount of positive charge accumulates on the surface of the substrate 103 over time. The increase in positive charge on the substrate surface gradually increases the voltage or "substrate potential" of the substrate, and thus forms a "dip". As shown in FIG. Figure 2As can be seen in FIG. 2 , the voltage of waveform 200B gradually and undesirably increases from a more negative voltage at the beginning of the ion current phase to a less negative voltage during the later portion of the ion current phase. If not controlled, the gradual accumulation of positive charge on the substrate surface results in a gradual discharge of the high voltage sheath and chuck capacitances, slowly reducing the sheath voltage and bringing the substrate potential closer to zero. The voltage difference between the beginning and end of the ion current phase determines the width of the ion energy distribution function (IEDF). The larger the voltage difference, the wider the IEDF width, which is undesirable for a number of reasons, making it more difficult to reliably form high aspect ratio features. Process Monitoring and Control Examples

[0046] Figure 3 1 is a process flow diagram illustrating a method 300 of waveform generation according to certain embodiments of the present disclosure. As described above, the blocks of the method 300 may be performed using the system controller 126. The method 300 may allow the waveform generator 150 to compensate for the voltage decay seen on the substrate 103, such as in Figure 4 The method 300 includes delivering a first waveform having an associated setpoint to an electrode within a process chamber, detecting at least one characteristic of the delivered first waveform, estimating a voltage decay during a portion of a pulse during which the first waveform is delivered to the electrode, calculating a compensation factor, and applying the compensation factor to correct for the voltage decay in a subsequent waveform delivered to the electrode.

[0047] Figure 4 An example of a compensated voltage waveform established on substrate 103 as a result of delivering a voltage waveform to an electrode disposed adjacent to the substrate (e.g., bias electrode 104) according to certain embodiments of the present disclosure is illustrated. Pulse train 400A is an example of a pulse train of a compensated voltage waveform established at a substrate, which can be measured using a wired wafer. Waveform 400B illustrates an example of two pulses formed within pulse train 400A and portions of adjacent pulses on either side of the two pulses. In some embodiments, the compensated voltage waveform illustrated by waveform 400B has region 402 or region 404 within each pulse that has been adjusted to compensate for voltage decay similar to the voltage decay illustrated in waveform 200B.

[0048] To achieve monoenergetic ions and narrower IEDF widths during the ion current phase of the voltage waveform established at the substrate, operations are performed to compensate for the varying substrate potential during the ion current phase and to create a region of substantially flat shape (e.g., close to zero slope) of the established voltage waveform experienced by the substrate during plasma processing. Figure 44B is a waveform 400B in the ion current phase of the waveform 400B in the figure. In order to establish a substantially flat region in the voltage waveform established at the substrate, the PV waveform generator 150 may be used to generate a voltage waveform during the ion current phase (i.e., Figure 4 4. A waveform including a negative slope is delivered to the bias electrode 104 during the ion current portion (occurring in the ion current portion) of the waveform 400B. Driving and / or achieving a negative voltage slope at the bias electrode 104 is also referred to as current compensation, which can be created by using a current source coupled to the bias electrode 104. The negative voltage slope achieved during the ion current portion of the waveform 400B is created by increasing the amount of electrons provided to the bias electrode 104 by the PV waveform generator 150 to offset the increased field that would otherwise be caused by the accumulated positive charge due to incoming ions impacting the substrate during plasma processing. Therefore, by determining the slope (dV / dt) of the voltage waveform established at the substrate 103 using the methods described herein, the system controller 126 can adjust the current provided by the current source and / or change the characteristics of the PV waveform generated by the energy source (e.g., the PV waveform generator 150) to thereby maintain a constant sheath potential throughout the ion current phase of the waveform established at the substrate 103. In some embodiments, a direct current (DC) supply current is used to achieve a ramp having a desired slope during the ion current phase.

[0049] At block 302, a first waveform having associated set points is delivered from an energy source (e.g., PV waveform generator 150) to an electrode. In some embodiments, the first waveform may be a high voltage pulse waveform, and the first waveform may establish a plasma sheath 101a. The associated set points may be a baseline process including a waveform voltage set point, a process pressure set point, an RF power set point, and a substrate temperature set point during one or more phases of delivering a waveform (e.g., waveform 400B) to the electrode, and may include a PV waveform on-time set point (e.g., the length of the ion current phase of the waveform).

[0050] At block 304, a sensor (e.g., a current sensor in the sensor assembly 170) may detect at least one characteristic of the first waveform generated by the PV waveform generator 150. The at least one characteristic may be a magnitude of the current detected by the current sensor (e.g., a current transducer reading obtained from the sensor). In some embodiments, the sensor may be an inline sensor disposed between the energy source (e.g., the PV waveform generator 150) and the bias electrode 104.

[0051] At block 306, a voltage decay during a portion of a pulse formed during delivery of the first waveform may be estimated. Estimating the voltage decay may optionally include determining a voltage decay value during a portion of a pulse formed within the first waveform using the detected at least one characteristic and at least one stored voltage decay value function stored in the memory 192 of the system controller 126, and determining a sheath coupling voltage value during the portion of the pulse of the first waveform using the detected at least one characteristic and at least one stored sheath coupling voltage value function. The sheath coupling voltage value is an empirically determined value that represents the capacitive coupling between the wired wafer and the bias electrode caused by delivery of the voltage waveform from an energy source (e.g., PV waveform generator 150) to the electrode. It is believed that the sheath coupling voltage value will vary depending on one or more plasma processing parameters, which may include, but are not limited to, the magnitude of the applied voltage delivered during the pulse, the process pressure, and characteristics of the hardware in the processing chamber (e.g., the capacitance of a dielectric layer disposed between the substrate and the electrode). For each process chamber within the multi-chamber plasma processing system, a sheath coupling voltage value may be determined by the system controller using an empirically derived function or table of values ​​that are predetermined by taking measurements of previously processed substrates within the process chamber during plasma processing. In some embodiments, the measurements are taken using a wired wafer that is capable of at least detecting a voltage established across a substrate during a plasma processing recipe executed within the process chamber.

[0052] In some embodiments of the method 300, the stored voltage decay value function and the stored shell coupling voltage value function may have been previously determined and / or programmed and stored in the memory 192, and thus may be retrieved from the memory 192 for use in estimating and correcting the voltage decay experienced by the substrate during the execution of the plasma processing recipe executed in the process chamber. In some embodiments of the method 300, as will be discussed below, the stored voltage decay value function and the stored shell coupling voltage value function may be determined using a wired wafer. The stored voltage decay value function and the stored shell coupling voltage value function may each include one value, or may each include more than one value. The stored voltage decay value function and the stored shell coupling voltage value function may each form a library or repository of data associated with a plurality of process conditions and recipes for a given processing chamber.

[0053] Figure 5A and Figure 5BCharacteristic of pulses of a voltage waveform established on substrate 103 by using sensing elements within a wired wafer and sensor assembly 170 according to certain embodiments of the present disclosure are illustrated. Pulse trains 500A and 502A are examples of two different measurements for characterizing the electrical properties of a pulse train established at substrate 103. Pulse train 500A includes a detected varying established voltage formed on the substrate as a result of the voltage waveform within the pulse train being delivered to an electrode (e.g., bias electrode 104) by PV waveform generator 150. Pulse train 502A includes a detected varying current flowing between an electrode and PV waveform generator 150 as a result of the voltage waveform within the voltage pulse train being delivered to the electrode by PV waveform generator 150. Waveform 500B ( Figure 5A ) illustrates a time-varying voltage measured during a portion of a pulse formed within detected pulse train 500A, and waveform 502B ( Figure 5B ) illustrates the time-varying current measured during the portion of the pulse formed within the detected pulse train 502A. In some embodiments, the waveform 500B is measured on the substrate 103 using a wired wafer. In some embodiments, it is desirable to perform similar measurements in each process chamber within the plasma processing system to characterize the actual electrical characteristics of each process chamber. In other embodiments, it is desirable to perform measurements in one process chamber within the plasma processing system and then utilize the measured electrical characteristics of the process chamber as representative electrical characteristics of other process chambers in the plasma processing system.

[0054] The method 300 may optionally include creating a function involving at least one voltage decay value and a function for determining at least one sheath coupling voltage associated with the substrate during the plasma process. In some embodiments, the method 300 may include detecting a pulse train (e.g., Figure 5A The pulse train 500A) of the pulse (for example, Figure 5A The at least one voltage decay value may be determined by using an initial voltage value and a final voltage value established on substrate 103 during a portion of waveform 500B in waveform 500B to determine at least one voltage decay value. The at least one voltage decay value may be detected by using one or more sensing elements coupled to one or more voltage sensing elements coupled to substrate 103. The portion of the pulse of the voltage waveform may be a time interval within the pulse of waveform 500B (e.g., between T1 and T2) and may correspond to a portion or all of the ion current phase of waveform 500B. In some embodiments, the portion of the pulse of the waveform may be selected so as to avoid transition times of the pulse, such as between a shell collapse phase and an ion current phase (e.g., Figure 5A The transition between the portion of the ion current phase just before time T1). Figure 5A, the portion of the pulse being measured begins a short time after the start of the ion current phase, which is generally defined after the shell formation phase of the voltage pulse has occurred (i.e., the vertical drop just before the start of the ion current phase). In some embodiments, the initial voltage value during the ion current phase is a function of the voltage waveform set point set by the PV waveform generator 150. The method 300 may also include determining a voltage delta value during a portion of the pulse by measuring the voltage using the established initial voltage value and a final voltage value on the substrate 103 (e.g., the final voltage minus the initial voltage).

[0055] Method 300 may optionally include detecting a current in waveform 502B ( Figure 5B ), the sensor being configured to detect current flowing between the PV waveform generator 150 and the bias electrode 104. The portion of the pulse of the waveform may be measured at an initial time point (e.g., T1) within the waveform 502B and may correspond to the beginning of the ion current phase of the waveform 502B. In some embodiments, the current sensor may be an inline sensor disposed between the energy source (e.g., the PV waveform generator 150) and the bias electrode 104, or a sensor disposed within a power delivery circuit system within the energy source.

[0056] The method 300 may optionally include storing a plurality of voltage decay values ​​that are a function of the generated measured current value (eg, C1). Fig. 6A As illustrated in , the stored plurality of voltage decay values ​​may be used to form a curve 602 representing a change (i.e., a drop) in voltage across a substrate as a function of a measured current value (e.g., C1) detected during a portion of an ion current phase of a voltage waveform. The voltage decay values ​​may be predetermined by measuring the actual amount of voltage change (i.e., a drop) in the voltage established across the substrate at a plurality of detected measured current values ​​within a processing chamber under different plasma processing conditions (such as different pulsed voltage bias levels applied to the electrodes by a PV waveform generator) using a wired wafer.

[0057] In some embodiments, the method 300 may include storing a plurality of shell coupling values ​​in a memory. The shell coupling values ​​represent a capacitive coupling relationship between the substrate and the bias electrode when a voltage waveform is applied to the electrode (e.g., the bias electrode) by the PV waveform generator 150. The shell coupling value is a value obtained by measuring the current value (e.g., Figure 5BThe shell coupling value is a measure of the difference between the voltage established on the substrate at a certain moment in time (measured at T1 in C1) relative to the voltage applied to the electrode. It has been found that the shell coupling value will vary as a function of the measured current value and the amplitude of the voltage applied to the electrode (e.g., the initial voltage measured at time T1). The generated initial voltage value is a function of the PV set point. For example, Figure 6B Each curve in (e.g., 604, 606, 608, 610, 612, 614) is associated with a different PV set point. The sheath coupling value can be determined by measuring the voltage across the wired wafer, comparing the measured voltage to the voltage delivered to the electrode, and also noting the resulting measured current value (e.g., C1) at the same time. It will be noted that the measured current value C1 is based on or affected by the response of the plasma processing chamber to the applied PV set point and the applied source power used to generate the plasma.

[0058] The voltage attenuation value and the shell coupling value may be stored, for example, in the memory 192 of the system controller 126. The at least one voltage attenuation value and the at least one shell coupling value may be stored, for example, in the form of a table, a graph, an equation, or any other available means for recording a relationship. For example, Fig. 6A and Figure 6B Examples are illustrated that represent voltage decay values ​​and shell coupling values ​​associated with measurements (eg, voltage delta, measured current values) of a waveform as measured on substrate 103 , in accordance with certain embodiments of the present disclosure.

[0059] exist Fig. 6A6, graph 600A illustrates the relationship (e.g., voltage drop) between a voltage decay value (i.e., the slope (dV / dt) of the voltage during the ion current phase) and a measured current value (e.g., C1). In some embodiments, determining a voltage decay value during a portion of a pulse during a first waveform (see block 306) includes finding a matching value detection characteristic of the first waveform (such as the measured current value on graph 600A, or current C1), and then finding a corresponding voltage decay value (e.g., DV1) based on the relationship set by curve 602. As described above, the voltage decay values ​​(which are stored in memory 192 as a function of the measured current values) can be stored in a table, graph, or equation so that when compared to the measured current value C1, the voltage decay value DV1 can be determined. In general, a single voltage decay value is a measurement of the rate at which the projection of the measured current value on the substrate voltage during the ion current phase of the voltage pulse is decayed (i.e., dropped). In some embodiments, the determined voltage decay value DV1 can be used directly during a subsequent compensation factor determination activity (block 308). In some other embodiments, based on the determined voltage decay value DV1, the voltage increment (ΔV) may be determined by multiplying the determined voltage decay value DV1 by the known length of the ion current phase (in which the measured current value C1 is measured). The length of the ion current phase is associated with the PV on-time of the voltage pulse.

[0060] exist Figure 6B , graph 600B illustrates the relationship between the initial voltage and the measured current value (current_i) for various PV set points used to find the shell coupling voltage value. Each of curves 604, 606, 608, 610, 612, and 614 represents a single PV set point, and thus the shell coupling voltage value (i.e., Y axis) can be determined by knowing the measured current value (i.e., X axis) at a known PV set point. Figure 6BIn the example illustrated in FIG. 6 , it will be noted that the magnitude of the PV set points used to generate curves 604, 606, 608, 610, 612, and 614 (e.g., the magnitude of the ion current voltage during the ion current phase) decreases in magnitude from curve 604 to curve 614. In one example, the sheath coupling voltage value during the portion of the pulse of the first waveform can be determined by detecting the measured current value C1 (amperes) and finding the corresponding sheath coupling value DV2 (volts) using the known PV set point illustrated by curve 614 in the sheath coupling graph 600B (see block 306). As described above, the sheath coupling values ​​(which are stored in the memory 192 as a function of the measured current values) can be stored in a table, graph, or equation so that when compared to the measured current value C1, a single sheath coupling value can be determined. In some embodiments, multiple different sets of curves (such as two or more of curves 604, 606, 608, 610, 612, and 614) may be stored in memory and used and / or selected based on adjustments to one or more different plasma processing parameters, such as source power, percent on time of PV pulses, pulse frequency, or other plasma processing parameters that will affect the sheath coupling value.

[0061] In some plasma processing processes performed on a substrate, it is desirable to adjust the PV set point to achieve a desired voltage on the substrate during the ion current phase of an applied PV pulse. In some cases, the actual desired PV set point required to achieve the desired voltage and process results may not be consistent with one of the generated curves 604, 606, 608, 610, 612, and 614 that are stored in memory and used to determine the sheath coupling value DV2 (volts). In some embodiments, as Figure 6B As illustrated in , it is desirable to adjust the PV set point to a value that falls between two known curves, such as curves 612 and 614, to achieve a desired process result on a substrate during a plasma process performed on the substrate. Figure 6B The new PV set point is illustrated as curve 613 in FIG. 6 , which can be determined by interpolating between the two known curves 612 and 614 at the measured current value C1′. Thus, by using the measured current value C1′ ( Figure 6B ) and using the system controller 126 to find the interpolated PV set point value, a new shell coupling value DV2′ ( Figure 6B ). Using this technique, a new shell coupling value DV2' can be determined for any desired PV setpoint.

[0062] At block 308, a compensation factor may be calculated based on the determined voltage increment (ΔV), which is determined from the voltage decay value DV1 and the determined shell coupling value DV2. The compensation factor is calculated based on the voltage increment (ΔV), and the determined shell coupling voltage value DV2 is determined from information received by detecting the measured current value C1 during one or more pulses within the generated waveform. In some embodiments, the compensation factor is used to adjust the amplitude of the voltage applied during the ion current phase as a function of time based on an expected slope determined from the voltage decay value DV1 and an expected voltage offset determined from the shell coupling value DV2. For example, as described above, Figure 4 Examples of optimally compensated voltage waveforms (eg, 402 , 404 ) established on substrate 103 via implementation of voltage decay step counts are illustrated.

[0063] Figure 7 The compensation factor determined in block 308 is shown to be applied to the electrodes (such as Figure 1 An example of the effect of the voltage waveform of the bias electrode 104) shown in FIG. Figure 7 The waveform example shown in FIG. 1 generally includes two major phases (the ion current phase, and the shell collapse phase / ion neutralization phase applied to the electrode). At the beginning of the ion current phase, the drop in applied voltage establishes a high voltage shell above the substrate, which causes positive ions formed in the plasma to flow to the surface of the substrate 103. As previously described, the charging of the substrate surface caused by the flow of positive ions to the substrate surface generates a voltage drop.

[0064] Thus, by applying the compensation factor determined in block 308, the slope of the waveform established on the substrate during the ion current phase may be adjusted so that it is zero or close to zero to improve the ion energy distribution function (IEDF) created during the plasma process. Figure 4 An example of a compensated waveform is shown in B. The compensated portion of the voltage waveform delivered from the power supply (e.g., waveform generator 150) to the bias electrode 104 may include Figure 7 The waveform portion represented by lines 712, 714, and 716.

[0065] In some embodiments, calculating the compensation factor involves using the voltage delta (ΔV) and the determined sheath coupling voltage value DV2, along with the known PV on-time found within the pulses of the voltage waveform. In some embodiments, the PV setpoint on-time of the waveform is stored in the memory 192 and can be used along with the determined voltage decay value DV1 and the sheath coupling voltage value DV2 to calculate the compensation factor. The determined sheath coupling voltage value DV2 sets an offset to the voltage established at the substrate 103 during plasma processing, as shown in FIG. Figure 7 , for example, includes an increased voltage drop (e.g., more negative) by an amount equal to the magnitude of the shell coupling voltage value DV2 from the original voltage drop (V0) formed during the shell formation phase used in the previous pulse. For example, the shell coupling voltage value DV2 can be applied to the initial applied voltage V0 generated by the power supply to shift the voltage to a compensated voltage value (e.g., V0+DV2) to enable a more optimal negative voltage to be applied at the beginning of the ion current phase. The voltage decay value DV1 is also multiplied by the PV set point on time value (seconds) to determine the required voltage increment (ΔV) compensation (e.g., the necessary slope of curves 712, 714, 716) over the length of the PV on time to achieve a more optimal IEDF that compensates for the voltage decay (e.g., drop). Therefore, in some cases, the compensation factor will include a correction amount formed by determining the shell coupling voltage value DV2 and the determined voltage increment (ΔV) derived from the determined voltage decay value DV1.

[0066] Although in Figure 7 , the voltage delta (ΔV) compensation applied during the ion current phase is illustrated as a linear curve (e.g., a first order curve), but this configuration is not intended to limit the scope of the present disclosure provided herein, as the compensation provided during the ion current phase may include a nonlinear shape due to the need to better adjust the plasma process performed during this phase of the PV pulse. In some examples, the voltage delta (ΔV) compensation provided during the ion current phase may include a second order, third order, fourth order, or higher order (e.g., Nth order) correction. In another example, the voltage delta (ΔV) compensation provided during the ion current phase may include decomposing the compensation provided during the ion current phase into a series of interconnected sub-steps that form a staircase, spline, or other desired curve shape. In this example, each of the interconnected sub-steps may include a first order, second order, third order, fourth order, or Nth order correction.

[0067] At box 310, a compensation factor is applied to correct for voltage decay within a subsequent portion of the waveform (such as, during one or more voltage pulses). Correcting the voltage decay within the subsequent waveform may include adjusting at least one characteristic (e.g., bias voltage, current C1) using the compensation factor. In some embodiments, the compensation factor may be applied as a continuous slope. In some embodiments, instead of forming a linear ramp during the ion current phase, the calculated compensation factor may be used to determine a stepped number of voltage decay steps, and the length and / or size of each step to be applied during the ion current phase of each pulse to correct for the voltage decay. In other words, the compensation factor may be applied in steps, as a continuous voltage ramp or a non-linear voltage ramp. In some embodiments, the compensation factor may be applied to the subsequent waveform as a voltage decay step count (e.g., a stepped ramp correction). In Figure 4An example of a compensated waveform comprising three steps established at the substrate is shown in B.

[0068] In some embodiments, voltage decay may be estimated during processing of a wafer (e.g., substrate 103), and a compensation factor may be applied to correct for voltage decay within a subsequent pulse train for the wafer. In other embodiments, voltage decay may be estimated during processing of a wafer, and a compensation factor may be applied to correct for voltage decay within a subsequent train.

[0069] In some embodiments, voltage decay may be estimated during processing of a wafer (e.g., substrate 103), and a compensation factor may be applied to correct for voltage decay in subsequent waveforms on subsequent wafers. However, because of the stored voltage decay values ​​and shell coupling values, subsequent wafers need not be wired wafers and may rely on previously stored voltage decay values ​​and shell coupling values ​​to calculate compensation factors.

[0070] In some examples, such as Figure 7 As shown in the figure, the frequency (1 / T p ) delivers a voltage waveform having a frequency between about 50 kHz and 1000 kHz. The voltage waveform established at the electrode may have an on-time greater than 50% or greater than 70% (such as between 80% and 95%), which is defined as the ratio of the ion current time period (e.g., the length of the ion current phase) to the waveform period T p The ratio of. With a waveform cycle (which has a period T p A voltage waveform of about 2.5 μs (e.g., about 2.5 μs) may be continuously repeated within a waveform train having a train period between about 100 μs and about 10 milliseconds (ms). The PV waveform train may have a train duty cycle between about 5% and 100%, such as between about 50% and about 95%, where the duty cycle is the ratio of the train period divided by the train period plus the non-string period (i.e., no PV waveform is generated) separating the train periods. As shown, the shell collapse phase may have a T SH duration (e.g., off time), which can be approximately 200ns. Additional Considerations

[0071] As used herein, the term "coupled" is used to represent a direct or indirect coupling between two objects. For example, if object A physically contacts object B and object B contacts object C, objects A and C may still be considered coupled to each other - even if objects A and C are not directly physically in contact with each other. For example, a first object may be coupled to a second object even if the first object has never directly physically contacted the second object.

[0072] While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope of the disclosure is determined by the following claims.

Claims

1. A method for waveform generation, comprising: delivering a first waveform having an associated set point from an energy source; detecting at least one characteristic of the first waveform using at least one sensor; determining a voltage decay value during a portion of a pulse provided within the first waveform by using at least one detected characteristic and at least one stored voltage decay value function; determining a sheath coupling voltage value during said portion of said pulse of said first waveform by using said at least one detected characteristic and at least one stored sheath coupling voltage value function; calculating a compensation factor based on the determined voltage attenuation value and the determined shell coupling voltage value; as well as At least one characteristic of the first waveform is adjusted by applying the calculated compensation factor to the at least one characteristic of the first waveform.

2. The method of claim 1, wherein the first waveform is a high voltage pulse waveform and establishes a shell.

3. The method of claim 1, wherein the associated set point is a baseline process, voltage, and temperature (PVT) set point. The method of claim 1 , wherein the at least one characteristic is a current sensor metric.

5. The method of claim 1, wherein the at least one sensor is an inline sensor and is included in a current transformer.

6. The method of claim 1, further comprising: detecting at least one value associated with an initial voltage and at least one value associated with a final voltage during the portion of the pulse of the first waveform by using at least one second sensor coupled to the wired wafer; determining at least one voltage increment value by using the at least one value associated with the initial voltage and the at least one value associated with the final voltage; detecting, by using the at least one sensor, at least one measured current value during the portion of the pulse of the first waveform; as well as The at least one stored voltage decay value representing a relationship between the at least one voltage increment value and the at least one measured current value is stored.

7. The method of claim 6, further comprising: The at least one stored shell coupling voltage value representing a second relationship between the at least one value associated with the initial voltage and the at least one measured current value is stored.

8. The method of claim 1, wherein a process, voltage, and temperature (PVT) turn-on time of the first waveform is known.

9. The method of claim 1, wherein the compensation factor is a voltage decay step count.

10. The method of claim 1, wherein adjusting the at least one characteristic of the first waveform comprises: A continuous voltage ramp is applied to a portion of the pulse.

11. A waveform generator comprising: a sensor assembly coupled to an output of the pulser, wherein the sensor assembly includes at least one sensor configured to detect at least one characteristic of a first waveform generated by the waveform generator; as well as a system controller coupled to the waveform generator, wherein the system controller comprises: A processor and a memory, wherein the memory comprises instructions that, when executed by the processor, result in: determining an amount of voltage decay within a portion of a pulse during said first waveform using at least one stored voltage decay value function; determining an amount of sheath coupling voltage within the portion of the pulse of the first waveform using at least one stored function of sheath coupling voltage values; calculating a compensation factor based on the determined amount of voltage attenuation and the determined amount of shell coupling voltage; and The at least one characteristic of the first waveform is adjusted based on application of the compensation factor.

12. The waveform generator of claim 11, wherein: The first waveform is a high voltage pulse waveform, and The first waveform establishes a shell.

13. The waveform generator of claim 11, wherein the first waveform has an associated set point, and the associated set point is a baseline process, voltage, and temperature (PVT) set point.

14. The waveform generator of claim 11, wherein the at least one characteristic is a current sensor metric.

15. The waveform generator of claim 11, wherein the at least one sensor is an inline sensor.

16. The waveform generator of claim 11, wherein the memory includes further instructions that, when executed by the processor, result in: determining at least one value associated with an initial voltage and at least one value associated with a final voltage during the portion of the first waveform by using at least one second sensor coupled to the wired wafer; determining at least one voltage increment value by using the at least one value associated with the initial voltage and the at least one value associated with the final voltage; determining, using the at least one sensor, at least one measured current value during the portion of the first waveform; as well as The at least one stored voltage decay value function is stored in the memory, the at least one stored voltage decay value function representing a relationship between the at least one voltage increment value and the at least one measured current value.

17. The waveform generator of claim 16, wherein the memory includes further instructions that, when executed by the processor, result in: The at least one stored shell coupling voltage value formula is stored in the memory, the at least one stored shell coupling voltage value formula representing a second relationship between the at least one value associated with the initial voltage and the at least one measured current value.

18. The waveform generator of claim 11, wherein a process, voltage, and temperature (PVT) turn-on time of the first waveform is known.

19. The waveform generator of claim 11, wherein the compensation factor is a voltage decay step count.

20. The waveform generator of claim 11, wherein the compensation factor is applied as a continuous voltage ramp or a non-linear voltage ramp.

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