Patent Application
20240194444
This disclosure relates generally to the field of ion beam current measurement devices, and in particular to ion beam current measurements using faraday cups.
Faraday cups are devices for ion beam current measurement. In ion implanters for wafer processing, Faraday cups are used to calibrate the ion beam dose. Implant dose accuracy and reproducibility are major challenges in semiconductor device fabrication. In particular, high repeatability of day-to-day implant dose and high uniformity of the implant dose across wafers are critical and increasingly stringent requirements imposed by future advanced device technologies.
Faraday cups are subjected to Faraday cup slit degradation resulting in ion beam dose measurement errors. Measurement errors may show up as slowly increasing or decreasing dose shift trends or as sudden changes in the measured dose. It is important to accurately distinguish between corrupted measurement results due to slit degradation of the Faraday cup and true measurement results caused by a real change of the ion beam implant dose.
According to an aspect of the disclosure, an ion beam current measurement device includes a first Faraday cup having a first ion beam entrance slit of a first width W1. The first Faraday cup is configured to generate a first current signal. The ion beam current measurement device further includes a second Faraday cup having a second ion beam entrance slit of a second width W2. The second Faraday cup is configured to generate a second current signal. The slit widths are designed such that W2 is greater than W1.
According to another aspect of the disclosure, an ion beam implantation system includes a process chamber. A substrate holder is disposed in the process chamber and configured to hold a substrate to be subjected to ion implantation. An ion beam generator is configured to generate an ion beam for ion implantation into the substrate. The ion beam implantation system further includes an ion beam current measurement device as recited above, disposed adjacent the substrate holder. A slit width change computation module is configured to calculate a time-dependent slit width change indicator based on the first current signal and the second current signal. An implant dose control system is configured to control the implant dose based on the time-dependent slit width change indicator.
The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts. The features of the various illustrated embodiments can be combined unless they exclude each other and/or can be selectively omitted if not described to be necessarily required. Embodiments are depicted in the drawings and are exemplarily detailed in the description which follows.
According to the disclosure and for reasons which will be described in more detail further below, the second slit width W2 is greater than the first slit width W1.
A distance D between the first entrance slit 112 and the second entrance slit 122 may, e.g., be equal to or less than D=5 cm or 2 cm or 1 cm or 0.5 cm. In practice, the smaller D1 the better the measurement device 100 ensures that the ion implant dose rate and vacuum conditions are quite similar for both entrance slits 112, 122.
For example, the ratio of slit width W2/W1 may, e.g., be equal to or greater than 1.5, 2, 3, 4, or 5. The greater W2/W1 the more sensitive is the measurement device 100 when used to determine slit width degradation effects as considered below.
The measurement device 100 may further include a Faraday cup frame 130. The Faraday cup frame 130 holds the first and second Faraday cups 110, 120 and/or the first and second entrance slits 112, 122 in a fixed positional relationship. For example, the Faraday cup frame 130 may be formed as a common Faraday cup holder. The Faraday cup frame 130 may, e.g., include or be made of an insulating material in order to exclude electrical contact between the first Faraday cup 110 and the second Faraday cup 120. Further, the Faraday cup frame 130 may either be designed as a mounting platform for mounting the first and second entrance slits 112, 122 thereon or the first and second entrance slits 112, 122 may, e.g., be integrally formed in the Faraday cup frame 130.
More specifically, the entrance slits 112, 122 may, e.g., be formed in individual slit plates 132 or in a common slit plate 132, which may, e.g., be held on ground potential. The slit plate (s) 132 may, e.g., be mounted to the Faraday cup frame 130 or may be integral with the Faraday cup frame 130.
The measurement device 100 may be accommodated in a common housing (not shown). The housing may have signal output ports for the first current signal I1 and the second current signal I2 and/or may include circuitry for processing and/or comparing the current signals I1, I2, as will be described in more detail further below.
Each Faraday cup 110, 120 may be individually equipped with electrostatic and/or magnetic suppression means to prevent ions, which have entered the respective cup to escape without being measured and/or to prevent electrons from entering/escaping the Faraday cup. Each Faraday cup 110, 120 is electrically connected separately to enable distinct and/or independent current measurements.
The first and second Faraday cups 110, 120 together with the first and second ion beam entrance slits 112, 122, the (optional) frame 130 and/or slit plates 132 and the (optional) housing as well as appropriate current measurement devices (denoted by A in
In the example shown, the ion beam 150 is swept or scanned in a horizontal direction across the substrate 220 and across the measurement device 100. For example, during operation, the ion beam 150 may be scanned over the substrate 220 in process and over the first and second ion beam entrance slits 112, 122 with a sweeping frequency in, e.g., the kHz range. In other words, the substrate 220 and the measurement device 100 may be exposed to a fanned-out ion beam 150. Sweeping the ion beam 150 in a horizontal direction may, e.g., be performed by magnetic or electrostatic deflection.
In other examples the ion beam 150 may be a stationary stripe-shaped beam instead of an ion beam 150 scanned in horizontal direction. In both cases the measurement device 100 is exposed to the same ion beam 150 (i.e. to the same ion beam implant dose rate) as the substrate 220 (or, more generally, to an ion beam implant dose rate which is a known, predetermined ratio of the ion beam implant dose rate to which the substrate 220 is exposed). Hence, the measurement device 100 is capable of measuring a quantity which is indicative of the implant dose (i.e. the number of ions which hit the implantation target per cm2) applied to the substrate 220. Differently stated, the implant dose is a (known) function of the measured quantity.
The distance between the right edge of the substrate 220 and the left edge of the nearest entrance slit (here, e.g., the first entrance slit 122, see
In some examples, the measurement device 100 may be operated as a closed loop Faraday measurement device. In this case, the implant dose may be measured, e.g., for each ion beam sweep. The substrate 220 is moved in direction of arrow M, e.g., in a direction perpendicular to the sweeping direction below the ion beam 150. For example, the speed of movement may be controlled as a function of the measured quantity (measurement result), e.g. as a function of any of the current signals I1, I2. For example, if I1 decreases during substrate processing, the implant dose rate is considered to be decreasing. In response the speed of movement of the substrate 220 is slowed down to guarantee that the same implant dose is applied across the entire surface of the substrate 220.
The same process of closed loop control to ensure a uniform implant dose exposure across the substrate 220 may be carried out for a stripe-shaped spatially stationary ion beam 150.
In other examples the measurement device 100 may be used as a calibration Faraday measurement device. Calibration Faraday measurement devices 100 are used for the setup of the ion beam implantation system 200, i.e. without having a substrate 220 in the process chamber. During setup the ion beam 150 is passed over the measurement device 100 for setup and calibration purposes. In this case ion current measurements are also repeatedly performed, but the repetition rate may be much smaller, e.g., in a per substrate (wafer) rate or a per day rate.
Accurate control of any dosimetry system is limited by the fact that all dosimetry related parts and, in particular, the ion beam current measurement device 100 are heavily exposed to either the ion beam 150 itself or the processing station environment. This leads to deterioration of these parts and an associated shift in measurement results, affecting the implant dose accuracy performance of the dosimetry system. As a consequence of the degradation of a Faraday cup entrance slit used for ion beam current measurement, the actual ion dose implanted in the substrate 220 deviates from the target implant dose.
Two possible slit degradation mechanisms are illustrated in
Conventionally, a calibration Faraday cup, which is not as heavily in use as the main Faraday cup, has been implemented to calibrate the main Faraday cup used for dosimetry. However, the use of a calibration Faraday cup operated under different conditions and deteriorating over time adds another uncertainty, and the unknown influence of the species affects the accuracy and reliability of the calibration.
The PM intervals are chosen to timely correct or recalibrate the dosimetry system. During PM at least the slit of the Faraday cup is replaced by a new one. The length of a PM interval depends on the operation conditions and the required implant dose accuracy of the ion beam implantation system. Typically, PM has to be carried out on a time scale of weeks or one or several months or (if low accuracy is sufficient) on an annual rate, for example.
The PM procedure shows up in
According to the disclosure, an ion beam current measurement device 100 is provided which avoids the increasing or decreasing measurement data trend between PM intervals by actively controlling and/or recalibrating the implant system for changed conditions of the dosimetry system with respect to Faraday cup slit degradation. The frequency of controlling and/or recalibrating of the system can be chosen in wide range. For instance, controlling and/or recalibrating can be done on a high frequency scale, e.g. for any sweep of the ion beam 150 across the substrate 220 and the ion beam current measurement device 100 (see e.g.
In other examples the control and/or recalibration may be performed on a larger time scale, e.g. per wafer or per day, or week, etc. This time scale may, e.g., also apply to a calibration Faraday which is used during interruptions of the ion implant operation.
Referring to
At S2 a time-dependent slit width change indicator is calculated based on the first current signal I1 and the second current signal I2.
For example, a time-dependent slit width change indicator is calculated based on a ratio of the slit widths W1, W2 and/or a ratio of the first and second current signals I1, I2.
In the following an exemplary way of calculating a time-dependent slit width change indicator y (t) is presented:
The (initial) width W2 of the second slit 122 is defined as W2=x·W1 by the (initial) width W1 of the first slit 112 via a factor x, with x>1 (differently stated, W2 is the width of the wider slit and W1 is the width of the narrower slit—here, only for explanatory reasons, it is assumed that the second slit 122 is the wider slit).
The decrease or increase of the slit width by time is defined by a percentage y(t) of the width of the first slit 121, with −1<y(t)<1.
The widths of both slits 112, 122 increase or decrease exactly the same in absolute number.
Using these conditions, the time dependent ratio between the two currents I1(t), I2 (t) can be expressed by
A rearrangement of the expressions above yields for the time dependent change in slit width (as expressed, e.g., by the time-dependent slit width change indicator y(t)):
Since the change of the slit width is directly proportional to the ratio of the currents I1(t)/I2 (t) and eventually to the implant dose, the implant dose can directly be corrected by y(t).
In other words, the time-dependent slit width change indicator y (t) gives at all times an indicator of the current slit width change as compared to initial conditions (i.e. known slit widths W1, W2). Thus, any error introduced into the dosimetry system by the (unknown) change in slit widths can be corrected at any time.
One possibility of correcting or recalibrating the dosimetry system is to use a corrected current measurement result instead of the actual current measurement result output by the ion beam current measurement device 100 for dosimetry system control. For example, referring, e.g., to I1(t) as the actual current measurement result, the corrected ion beam current measurement result can be expressed by
Any quantity effective for implant dose adjustment may be controlled based on the corrected (i.e. “true”) ion beam measurement result (e.g. I1corr (t)) instead of the actual (i.e. distorted) ion beam current measurement result (e.g. I1(t)). For example, the speed of the motion of the substrate (wafer) 220 may be controlled based on the corrected ion beam current measurement result.
For example, using equation (2) with the measured currents I1, I2 as an input yielding y(t)=−0.01, the implant dose needs to be recalibrated by −1% to remain unaffected by the slit degradation.
Further, means for computing the corrected ion beam current measurement result are disclosed. These means could be implemented in various different entities. In one example, the means may be implemented in the ion beam current measurement device 100 as such. In this case, the ion beam current measurement device 100 may output, e.g., I1corr(t) in addition to or instead of the actual ion beam current measurement result (e.g. I1(t), I2(t)). As will be described in greater detail further below, in other examples, such means could be implemented in external entities such as, e.g., dose adjustment computation module CM2, see
It is to be noted that y (t) as expressed by equation (2) is only one example for a time-dependent slit width change indicator, and other mathematical framework could be used to derive other such indicators. For example, y (t) is related by definition to the width W1 of the first slit 112. The indicator could just as well be related to the second width W2 of the second slit 122, for example.
It can be observed from equation (2) yielding y (t) that I1/I2 decreases if the width W2 of the second slit 122 is increasing and both slits 112, 122 get narrower (as expressed by a negative y(t)). This means that the current I2 measured on the second Faraday cup 120 (i.e. the Faraday cup with the wider second slit 122) is significantly larger than the first current I1 measured on the first Faraday cup 110 with the first slit 112 (the narrower slit). Hence, for the best sensitivity (corresponding to the lowest I1/I2) the width W2 of the second slit 122 should be as large as possible and/or the width W1 of the first slit 112 should be as small as possible (i.e. the ratio W1/W2 should be as small as possible).
In
The slit width change indicator y(t) allows to monitor Faraday cup slit degradation. The slit width change indicator y (t) can be used for many different purposes. For example, it may merely be used to have more information on the status of the Faraday cup hardware FCH degradation for scheduling appropriate maintenance intervals (e.g. PM intervals). However, as already mentioned, the slit width change indicator y (t) my further be used to correct or recalibrate the dosimetry system in accordance with
The ion beam current measurement device 100 may further include a dose adjustment computation module CM2 configured to derive a dose adjustment quantity DAQ based on the time-dependent slit width change indicator y(t). In one example, DAQ may be derived based on equation (3) or given by equation (3).
The ion beam current measurement device 100 may further include an operation fault detection module CM3 configured to derive a fault warning signal FWS based on a comparison of a rate of change dy (t)/dt of the time-dependent slit width change indicator y (t) with a predetermined threshold.
Referring to
Generally, the implant dose of the ion beam 150 may be controlled based on the time-dependent slit width change indicator y(t). The control may be such that dose adjustment computation module CM2 is configured to derive a dose adjustment quantity DAQ based on y (t). A dosimetry system, which is defined as the system used to control the implant dose (and which can be implemented in various different ways as already described) may include or use the dose adjustment computation module CM2 for implant dose control.
The dose adjustment quantity DAQ can be used to control the implant dose by the dosimetry system. In one example, DAQ may be operatively connected to a control input 932 of a drive 930 of a substrate holder 920. In this case, the speed of movement of the substrate holder 920 in the direction of arrow M may be a function (e.g. of a proportional relationship) of DAQ. Other dosimetry systems may, e.g., use other means for controlling the implant dose than the drive 930.
The dose adjustment computation module CM2 may be implemented in an open loop or closed loop dosimetry control system. In both cases, the dosimetry system may be controlled based on the corrected ion beam measurement result (e.g. I1corr(t)) rather than based on the actual ion beam current measurement result (e.g. I1(t)). That is, in both cases the dose adjustment quantity DAQ used to control the dosimetry system reflects the “true” dosimetry measurement result (based, e.g., on I1corr (t)) rather than the “erroneous” dosimetry measurement result (based, e.g., I1(t)) which is distorted by slit degradation. The closed loop control acts to maintain the relative dose constant as shown in the right portion of
Further, the ion beam implantation system 900 may include the operation fault detection module CM3 configured to derive a fault warning signal FWS based on a comparison of a rate of change dy (t)/dt of the time-dependent slit width change indicator y (t) with a predetermined threshold.
Although the increase or narrowing of the first and second ion beam entrance slits 112, 122 are typically identical in absolute values and continuously varying functions in time, there could be a special fault case when material is building up over short time to cause slit narrowing. For example, although at a low probability, there may be the case where a larger “flake” of built-up material can get loose on only one of the two slits 112, 122. This would yield a faulty slit width change indicator y (t) signal.
A method to detect such fault case may comprise deriving a fault warning signal FWS based on a comparison of a rate of change of the time-dependent slit width change indicator y (t) with a predetermined threshold. More specifically, the method may include calculating the change rate of y(t), dy (t)/dt, and defining a threshold for this change rate. Whenever the change rate dy (t)/dt exceeds the set threshold, the operation fault detection module CM3 may output a fault warning signal FWS.
In response to an activation of the fault warning signal FWS, a maintenance procedure may be initiated. The maintenance procedure (e.g. PM) usually includes replacement of the entire Faraday cup hardware FCH or at least the ion beam entrance slits 112, 122.
The following examples pertain to further aspects of the disclosure:
Example 1 is an ion beam current measurement device, comprising: a first Faraday cup having a first ion beam entrance slit of a first width W1, the first Faraday cup is configured to generate a first current signal; and a second Faraday cup having a second ion beam entrance slit of a second width W2, the second Faraday cup is configured to generate a second current signal; wherein W2 is greater than W1.
In Example 2, the subject matter of Example 1 can optionally include wherein a distance between the first ion beam entrance slit and the second ion beam entrance slit is equal to or less than 5 cm, 2 cm, 1 cm, or 0.5 cm.
In Example 3, the subject matter of Example 1 or 2 can optionally include wherein W2/W1 is equal to or greater than 1.5, 2, 3, 4, or 5.
In Example 4, the subject matter of any preceding Example can optionally further include a Faraday cup frame to which the first ion beam entrance slit and the second ion beam entrance slit are mounted or in which they are formed.
In Example 5, the subject matter of any preceding Example can optionally further include a slit width change computation module configured to calculate a time-dependent slit width change indicator based on the first current signal and the second current signal.
In Example 6, the subject matter of Example 5 can optionally include wherein the calculation of the time-dependent slit width change indicator is based on a ratio of the slit widths W1, W2 and a ratio of the first and second current signals.
In Example 7, the subject matter of Example 5 or 6 can optionally further include a dose adjustment computation module configured to derive a dose adjustment quantity based on the time-dependent slit width change indicator.
In Example 8, the subject matter of any of Examples 5 to 7 can optionally further include an operation fault detection module configured to derive a fault warning signal based on a comparison of a rate of change of the time-dependent slit width change indicator with a predetermined threshold.
Example 9 is an ion beam implantation system, including a process chamber; a substrate holder disposed in the process chamber and configured to hold a substrate to be subjected to ion implantation; an ion beam generator configured to generate an ion beam for ion implantation into the substrate; the ion beam current measurement device of any of Examples 1 to 4, disposed adjacent the substrate holder; a slit width change computation module configured to calculate a time-dependent slit width change indicator based on the first current signal and the second current signal; and an implant dose control system configured to control the implant dose based on the time-dependent slit width change indicator.
In Example 10, the subject matter of Example 9 can optionally include wherein the ion beam generator is configured to sweep the ion beam in a first direction across the substrate, wherein the entrance slits of the first and second Faraday cups are oriented in a second direction substantially perpendicular to the first direction.
In Example 11, the subject matter of Example 9 or 10 can optionally include wherein the ion beam current measurement device is disposed adjacent one side of the substrate holder.
In Example 12, the subject matter of any of Examples 9 to 11 can optionally include wherein the implant dose control system comprises a dose adjustment computation module configured to derive a dose adjustment quantity based on the time-dependent slit width change indicator.
In Example 13, the subject matter of any of Examples 9 to 12 can optionally further include an operation fault detection module configured to derive a fault warning signal based on a comparison of a rate of change of the time-dependent slit width change indicator with a predetermined threshold.
In Example 14, the subject matter of any of Examples 9 to 13 can optionally include wherein the slit width change computation module is configured to repeatedly update the time-dependent slit width change indicator during an operating period.
Example 15 is a method of monitoring a process of ion implantation into a substrate, the method comprising: measuring first and second current signals generated by first and second Faraday cups having first and second ion beam entrance slits of first and second widths W1 and W2, respectively, wherein W2 is greater than W1; and calculating a time-dependent slit width change indicator based on the first current signal and the second current signal.
In Example 16, the subject matter of Example 15 can optionally include wherein the calculation of the time-dependent slit width change indicator is based on a ratio of the slit widths W1, W2 and a ratio of the first and second current signals.
In Example 17, the subject matter of Example 15 or 16 can optionally further include computing a dose adjustment quantity to control the dose of an ion beam used in the process of ion implantation, the dose adjustment quantity being based on the time-dependent slit width change indicator.
In Example 18, the subject matter of Example 17 can optionally include wherein the dose adjustment quantity is repeatedly updated during an operating period, in particular a maintenance interval, of an ion beam implantation system used to perform the process of ion implantation.
In Example 19, the subject matter of any of Examples 15 to 18 can optionally further include deriving a fault warning signal based on a comparison of a rate of change of the time-dependent slit width change indicator with a predetermined threshold.
In Example 20, the subject matter of Example 19 can optionally further include initiating a maintenance procedure in response to an activation of the fault warning signal.
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102022133030.3 | Dec 2022 | DE | national |
