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Offline Charge Detection Principle

Im Dokument Charge sensing (Seite 67-70)

The current measurement using conventional techniques can be replaced by a charge measurement, as is part of section 6.1. The main drawback from the con-ventional current measurement system is the speed of acquisition, yet the speed of acquisition using a charge measurement requires some attention. With a conven-tional setup, the delay time after application of a voltage step is required to charge the capacitive parts of the connections between the measurement equipment and the target device. As a consequence, the behavior of a first order low pass filter is observed, see Figure 3.1 on page 34. This limits the speed for the measurement sig-nificantly. With a charge measurement system, this limitation is lifted. A capacitive connection between the target device and the charge measurement would be part of the parasitic capacitance. Accordingly, care must be taken to avoid excessive error through this parasitic capacitance, but the speed is not degraded. Only when the parasitic capacitance is exhibiting a slow polarization process, a speed limit can occur, see above. The limiting factor for conducting of a current measurement using the charge measurement system stems from the current derivation, see Section 5.3 and Chapter 6.

5.5 Offline Charge Detection Principle

A drawback of the closed loop feedback implementation as laid out in Section 5.1 is the requirement for a power supply. With this power supply, the feedback implemen-tation compensates introduced charge by a voltage across the feedback capacitance.

By this means, the input terminal is kept at a constant potential. In order to allow for an offline charge detection system, the constant input potential can not be sus-tained. Instead, a capacitive voltage divider, as discussed in Section 4.4, exhibiting a non-linearity, is used. The non-linearity in the capacitive voltage divider is not directly reversible allowing for a hysteresis behavior. Because the power supply is not required for this to work, the charge measurement result is not available in-situ. It must be determined at a later point in time. The following structure does not require a power supply for sensing charge, it is labeled “offline charge detection” circuit or implementation.

The tunneling behavior of dielectrics is a strongly non-linear characteristic. Sec-tion 2.5 summarizes two basic tunneling mechanisms, the Fowler-Nordheim (FN) and the Poole-Frenkel (PF) tunneling. Also, this characteristic is not directly reversible as the physical influence to the dielectric allowing the reversal of the tunneling effect must be large. Approximately the same electrical field strength must be applied for the same time in order to reverse the amount of tunneled charge in a symmetric structure. If only one direction of field is available, reversal of the tunneling effect is not possible. The offline charge detection circuit builds on the tunneling effect in capacitive voltage divider from Section 4.4.

CFB

electrostatic

antenna output

feedback

electrometer MOSFETfem DC floating

Figure 5.4: Circuit of a offline charge detection implementation.

The principle of the capacitive voltage divider paired with a tunneling characteris-tic is used within multiple technologies. For example non-volatile memories (NVMs) work with this principle if the cells are programmed or erased by FN tunneling.

Process monitoring has also been suggested for analysis by this means [49, 75] and multiple patents are found [47, 63]. This work extends the use of this structure to a more general scope of applications. Whenever the charge monitoring has to be conducted without a proper power supply, the offline charge detection can be used.

This section presents the circuit used for the experiments throughout this work. In Section 6.3, the application of the offline charge detector to the surface charging of a FIB is presented showing that the use scenarios of this structure can be extended.

Figure 5.4 shows the principle operation of the offline charge detection circuit.

The circuit is a simplified version of the closed loop feedback implementation from Figure 5.1. The feedback loop has been opened and the feedback terminal is separated from the electrometer output. As input probe, an electrostatic antenna is depicted according to Section 4.3 and discussed below. Application of a voltage to the feedback terminal allows for the determination of the charge amount on the input node and for the purposeful modification of this charge amount, according to Section 4.4.

The principle of measurement for this structure is then conducted in a similar way as for the closed loop feedback implementation described in Section 5.1. Yet, this structure does not allow to derive the charge amount in-situ during the application but only after the charge has been removed again and the power supply has been restored. Accordingly, the principle is:

1. Measure the amount of charge on the inputQinput, possibly alter this charge viaVfeedbackas noted below.

2. Remove the supply voltage of the device, apply the charge or field of interest to the antenna and/or input node and remove the charge or field again.

3. Measure the charge again and determine ∆Qinput from this and the initial measurement.

To initially alter the charge during step 1, the voltage at the feedback inputVfeedback can be used according to Section 4.4. This causes the capacitive voltage divider to alter the charge as desired. In order for this charge alteration (and also the charge alteration

5.5. OFFLINE CHARGE DETECTION PRINCIPLE 69 during the measurement phase, step 2), to stay permanently, the input node must not be connected conductively to any other structure exhibiting leakage behavior. This requires the use of an electrostatic antenna, see Section 4.3. Accordingly, connection of resistive elements between the input and ground is not permissible for this structure.

This is in contrast to the closed loop feedback implementation, which would allow such a construction.

The determination of charge on the input Qinput is also done by the feedback terminal. IfFem(Vinput) is the transfer characteristic of the electrometer and if the electrometer has a constant (or determined) input capacitance, then the transfer function between the feedback terminal and the output terminal can be analyzed through the charge present on the input nodeQinput:

QinputCFB(Vinput−Vfeedback)+CemVinput

WhereCem depicts the electrometer input capacitance andVinputthe electrometer input potential.

Vinput Qinput+VfeedbackCFB CFB+Cem

Depending on the type of electrometer circuit,Voutputmight also be implemented as a current instead. Without limiting the applicability, this work assumes the output to be a voltage and labels itVoutput.

Voutput Fem(Qinput 1

CFB+Cem +Vfeedback CFB

CFB+Cem) (5.11) The implemented design uses a MOSFET as electrometer – again based on the same reasoning as before for the closed loop feedback implementation, makingFemwell known, see Equation 4.1 on page 46. Equation 5.11 allows the operation of this circuit as charge detector. Once sufficiently many pairs(Vfeedback;Voutput)are measured, the point whereQinput1/(CFB+Cem)+VfeedbackCFB/(CFB+Cem) 0 holds can be determined mathematically.Femmust be continuous for this, which is the case for the transfer characteristic of the MOSFET if operated above threshold voltage.

Even ifFemwere unknown, the input toFem can be compensated for, by using the feedback input, to yield a constant value, allowing for direct derivation ofQinput from the setting ofVfeedback.

0Qinput 1

CFB+Cem +Vfeedback CFB CFB+Cem

Qinput −VfeedbackCFB (5.12)

This offline charge detection circuit is limited by the leakage behavior in the first place. Measurement of the input charge through the aforementioned procedure can

for example be conducted through determination of the threshold voltage of the MOS transistor. As this is possible without much error using conventional measurement equipment, it is not discussed here. The leakage current possibly present on the input node is important. If the leakage current is present during all the time between steps 1 and 3, then it will sum up for an error in charge reading:

Qinput,leak

tfinal tinitial

Ileak(t) dt

For a constant leakage current, this can be simplified to Qinput,leak Ileak(tfinal−tinitial)

The amount of charge to be measuredQinput would then have to be significantly larger than the charge from leakage:Qinput ≪Qinput,leak. This requirement is much stronger as the actual measurement time is only part of the time-frame valid for the leakage behavior. Accordingly, for the successful implementation of this principle of operation, care must be taken to keep the leakage low.

Im Dokument Charge sensing (Seite 67-70)