6.5. Approach Mode

6.5. Approach Mode
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6.5.1. Purpose of the Approach Procedure

The approach procedure is used to bring the tip close to sample's surface, for STM within tunneling distance.

The approach maneuver consists of both coarse and fine approach steps. Coarse approach steps are taken when you are convinced that you are not in close proximity of the sample. In this case, the tip closes in on the sample with relatively large steps, allowing a fast approach. Fine approach steps are taken when you are expecting to be near tunneling distance of the sample. After each fine approach step, measurements are made to determine whether the tip is within tunneling distance of the sample.

For the fine approach the Z-piezo is used. In general you will apply a stepwise ramp-like voltage pattern to the Z-piezo in order to approach the sample step-by-step. With each step measurements are made to decide whether approach criteria that describe the sample vicinity are met. In certain situations, a non-ramp voltage curve might be required.

For the coarse approach a device is used that allows greater traveling distances within a certain time period than a piezo. Camera supports slip-stick motors and stepper motors for coarse approach.

Usually the slip-stick motor is placed on top of the Z-piezo tube, but other constructs could be possible and are irrelevant for our implementation.

Note that a single displacement-step of the slip-stick motor should be less then the range of the Z-piezo tube. This allows to safely execute one slip-stick cycle, without the risk of crashing the tip.

Instead of a slip-stick motor, a stepper motor can be used to achieve coarse displacement of the tip.

6.5.3. Approach Sequence

The approach sequence consists of the following steps:

Approach Initialization: The approach mode is initialized with data retrieved from the previous scan mode (Analog / Analog 3D / Digital). Only the software (the Approach GUI ) is initialized with the retrieved data. Any relevant hardware settings will be applied after the configuration step.
Approach Configuration: Here you can modify any approach parameters using the GUI. None of the changes will be applied to the hardware. Only when you explicitly move from the configuration phase to the execution phase (referred to as "Entering Approach Mode") the hardware settings are applied.
Apply Approach Configuration: The entered or previously retrieved parameters will be applied to the hardware. Note that you can freely switch between Approach Configuration mode and Approach Execution mode (i.e. you can freely go in and out of "Approach Mode" ).
Sense Action: After you have entered Approach Mode you can perform a fine approach step, involving a fine step with the Z-piezo, followed by a measurement action.
Sense Evaluation: The measured data is evaluated using user-defined approach stop criteria. These criteria determine whether the tip is in the vicinity (for STM within tunneling distance) of the sample.
Coarse Approach Action: This step allows you to perform a coarse approach action using the slip-stick motor or stepper motor.

6.5.4. Feedback Handling on Entering and Leaving Approach Mode

Since Camera 4.1.3, the Feedback module is left untouched during approach and sense actions. Instead, changes with respect to the Feedback module are applied on entering approach mode and on leaving approach mode. Which actions are performed, depends on the Keep feedback on setting on the Sense Configuration Tab. See the tables below.

Table 6.1. Feedback Handling on Enter Approach Mode

Feedback held at 0V	Feedback kept active
0. Save feedback state	0. Save feedback state
1. Quickly move Z-offset from current voltage to maximally retract probe	1. Make sure feedback is active
2. Center feedback output to 0V	2. Gently move move Z-offset from current voltage to 0V
3. Quickly move Z-offset from maximally retract probe to sense starting position

Table 6.2. Feedback Handling on Leave Approach Mode

Feedback held at 0V	Feedback kept active
1. Enable feedback	1. Force setpoint to zero to move tip far away from surface
2. Force setpoint to zero	2. Quickly move Z-offset to requested position
3. Release forced center voltage	3. Release forced setpoint
4. Quickly move Z-offset to requested position
5. Restore feedback state

6.5.5. Manual versus Automatic Approach

Camera offers both an automatic and a manual approach option. In manual approach, you can perform single coarse approach or retraction steps by hand. You also can perform a sense action manually (i.e. a fine approach action, including ADC measurement and evaluation on every step).

In automatic approach mode, Camera performs the coarse approach/fine approach sequence until user-defined stop conditions are met.

6.5.6. Approach User Interface

6.5.6.1. Approach Control Panel

6.5.6.1.1. Enter Approach Mode Button
6.5.6.1.2. Automatic Approach
6.5.6.1.3. Manual Approach

6.5.6.2. Approach Display Panel

6.5.6.2.1. Approach/Retract Status
6.5.6.2.2. Approach/Retract Steps
6.5.6.2.3. ADC Information Panel
6.5.6.2.4. Primary ADC Selection

6.5.6.3. Tip Control Panel

6.5.6.3.1. Bias Voltage Control
6.5.6.3.2. Z-offset Control

6.5.6.4. Measurement Info Panel

6.5.6.5. Stop Condition Approach Configuration Tab

6.5.6.6. Stop Condition Retract Configuration Tab

6.5.6.7. Coarse Approach Configuration Tab

6.5.6.7.1. Configure Pulse Parameters
6.5.6.7.2. Change Trajectory Curves
6.5.6.7.3. Coarse Approach Curve Section Dialog (Table)
6.5.6.7.4. Coarse Approach File

6.5.6.8. Coarse Retract Configuration Tab

6.5.6.9. Sense Configuration Tab

6.5.6.9.1. Sense Mode Selection
6.5.6.9.2. Sense Parameters
6.5.6.9.3. Sense Approach File

6.5.6.10. Channels Configuration Tab

The Approach mode user interface can be accessed by going into Measurement mode and selecting the Approach tab.

Figure 6.26. Measurement Dialog Approach Mode Tab

The tab is made up of a number of panels and sub-tabs. We distinguish the following panels (the panel names roughly describe the panel contents):

the Approach Control panel at the top-right to set the main approach parameters, and execute approach commands.
the Approach Display panel at the top-middle to display the results of the approach actions.
the Tip Control panel at the bottom-left to control the bias voltage (Z-offset control is disabled).
the Measurement Info panel at the bottom-middle to display parameters for the approach procedure.

The following tabs at the top-left configure specific aspects of the approach procedure:

We will describe each of these in detail below.

6.5.6.1. Approach Control Panel

The Approach Control panel contains the primary controls to execute approach commands, and it contains controls to modify the primary approach parameters.

Figure 6.27. Approach Mode: Control Panel

6.5.6.1.1. Enter Approach Mode Button

Pressing the Enter Approach Mode button starts the approach mode and enables the other buttons for this mode. Pressing the button again stops the approach mode.

6.5.6.1.2. Automatic Approach

On the automatic approach and retract panel you can:

Approach Steps input field: Specify the number of coarse approach steps to be taken before a sense action is done during the automatic approach procedure..
Approach button: Start an automatic approach action. While in progress, the button is colored and disabled.
Retract Steps input field: Specify the number of coarse retract steps to be taken before a sense action is done during the automatic
Retract button: Start the automatic retract procedure. While in progress, the button is colored and disabled. retract procedure.
Stop button: Interrupt an automatic approach or retract procedure. The button is colored while the interrupt is in progress.

6.5.6.1.3. Manual Approach

On the manual approach and retract panel you can:

Single/Multiple Step radio button: Choose to execute a single or multiple manual approach/retract action.
Sense button: Execute one sense action.
Approach button: Execute a coarse approach action.
Retract button: Execute a coarse retract action.
Stop button: Interrupt a manual sense, approach or retract procedure. The button is colored while the interrupt is in progress.

6.5.6.2. Approach Display Panel

On the Approach Display panel you can watch the approach measurement status and approach measurement data.

Figure 6.28. Approach Mode: Display Panel

6.5.6.2.1. Approach/Retract Status

Appr button: When you perform an automatic Approach action, or a manual Sense action, and the user-defined approach condition is reached, the button blinks green. Press the button to acknowledge this condition-reached warning.
Retr button: When you perform an automatic Retract action, and the user-defined Retract condition is reached, the button blinks green. Press the button to acknowledge this condition-reached warning.
Measure button: Press this button to take an instant ADC measurement. The measurement is made as configurated on the Sense Configuration tab (Average).

	Tip
	You can select another Approach action whithout first acknowledging a blinking button.

6.5.6.2.2. Approach/Retract Steps

Approach Steps indicator: The number of coarse approach steps taken since the start of this approach action or reset. Press the Reset button next to this field to reset the counter.
Retract Steps indicator: The number of coarse retract steps taken since the start of this retract action or reset. Press the Reset button next to this field to reset the counter.

6.5.6.2.3. ADC Information Panel

This panel shows the number and the measured ADC value of the user-selected primary ADC. The ADC value is displayed in the physical units configured for that particular ADC.

6.5.6.2.4. Primary ADC Selection

Here you can select the ADC to watch during the sense procedure. This ADC is refered to as the primary ADC. You can choose from the channels that are enabled in the Channels tab.

6.5.6.3. Tip Control Panel

With the tip control panel you can manage tip parameters and tip actions.

Figure 6.29. Approach Mode: Tip Control Panel

6.5.6.3.1. Bias Voltage Control

Pulse button: No operation.
Voltage slider and input field/spin-buttons: The Slider range is from -10V to +10V. Spin box steps are 10 mV. If you enter an out-of-range value, it is limited to the largest (positive or negative) possible value.
Bandwitch input field: The bandwith ranges between 0 and 52875 (1.0 / ( 2 * pi * 7000 * 430E-12 )). If you enter an out-of-range value, it is limited to the largest (smallest) possible value.

6.5.6.3.2. Z-offset Control

You can select the Z-offset condition on leaving approach via three radio buttons:

Set Z-offset to maximally retract probe
Set Z-offset to position where last contact was found
Restore Z-offset to starting position

6.5.6.4. Measurement Info Panel

On the Measurement Info panel you see all parameters that are relevant to the approach procedure.

Figure 6.30. Approach Mode: Measurement Info Panel

The following parameters can be displayed:

Sense Start Voltage: in [V], see Start Voltage on the Sense Configuration tab.
ADC Measurements per Step: Number of ADC measurements to average, see #Measurements for Average on the Sense Configuration tab.
Sense Mode: Single-Step or Trajectory Sense, see Sense Configuration tab.
Single-Point Sense Voltage: in [V], see Sense Voltage on the Sense Configuration tab.
Single-Point Sense Delay: in [μs], see Sense Delay on the Sense Configuration tab.
Cycles per FIFO Point: Computed?
Cycles after FIFO: Computed?
Delay Before Pulse [APPR]: in [μs], see Wait before Pulse on the Coarse Approach/Retract Configuration tabs.
Delay After Pulse [APPR]: in [μs], see Wait after Pulse on the Coarse Approach/Retract Configuration tabs.
Amplitude [APPR]: -
Delay Before Pulse [RETR]: in [μs], see Wait before Pulse on the Coarse Approach/Retract Configuration tabs.
Delay After Pulse [RETR]: in [μs], see Wait after Pulse on the Coarse Approach/Retract Configuration tabs.
Amplitude [RETR]: -

Right-click in the Measurement info panel to get the Measurement Info Panel Configuration panel that lets you select which items to display and in what order. This panel is similar to the configuration panel of the Digital Scan Measurement info panel.

6.5.6.5. Stop Condition Approach Configuration Tab

On the Stop Appr. configuration tab you can select a stop condition for the approach procedure.

Figure 6.31. Approach Mode: Stop Condition Approach Configuration Tabs

The Stop Appr. tab contains the following items:

STOP Condition indicator: This summarizes the selected stop condition.
ADC selector radio button: The ADC to use for the approach measurements.
Operator buttons: conditions ADC <= Value, or Abs(ADC) >= Value, or ADC >= Value.
value [unit] input field: The value to compare against.
Ignore Stop Condition checkbox (Manual mode only): Do not use the selected stop condition.

	Important
	Currently you can select a stop condition involving a single ADC only. In the future this tab should provide more complex stop conditions.

6.5.6.6. Stop Condition Retract Configuration Tab

On the Stop Retr. configuration tab, you can select the stop condition for the retract procedure. This tab is not shown here, as it is similar to the tab for the approach stop condition.

6.5.6.7. Coarse Approach Configuration Tab

With the following dialogs you can assemble a Coarse Approach trajectory (Coarse Appr. tab). Dialogs and procedure for the Coarse Retract trajectory are similar (Coarse Retr. tab).

Figure 6.32. Approach Mode: Coarse Approach/Retract Configuration Tabs

The dialog contains the following panels:

Coarse Approach Parameters: Here you can change the coarse approach pulse parameters directly or via a dialog. Press the Configure button to get the dialog with a graphical presentation of the pulse and to configure the pulse parameters.
Coarse Approach Curve: When the Allow Non-Zero Start Position checkbox is checked, it is possible to configure a curve with a non-zero start voltage. This makes the button Change visible on the Change Curves dialog. Press the Change Curves button to get the a dialog to configure the coarse approach trajectory table sections or curve form.

6.5.6.7.1. Configure Pulse Parameters

Figure 6.33. Approach Mode: Coarse Trajectory Configure Parameters

6.5.6.7.2. Change Trajectory Curves

With this dialog you can construct, load and save an approach curve.

Note: This dialog to change curves is identical for the Coarse Approach, Coarse Retract and Sense tabs. For help on the Sense Configuration tab, look here instead.

Figure 6.34. Approach Mode: Coarse Trajectory Configuration Dialog

Note: The Change button to change the curve start value is not shown here next tho the Curve Start Value. This indicates that the Allow Non-Zero Start Position checkbox on the coarse approach configuration tab was not checked.

An Approach curve consist of a number of sections. A section can be a line-segment or a table of itself. Table sections are defined via a separate dialog, see Section 6.5.6.7.3, “ Coarse Approach Curve Section Dialog (Table)”.

Each line-segment is defined by:

The starting value, being the end value of the previous section, or 0 in case of the first section
The end value
The number of increment/decrement steps to be taken
The duration of each step [μs]

The Coarse Approach Curve Configuration dialog consists of the following items:

Graphical representation of the active approach curve
Table representing the active approach curve
Controls to manipulate the approach curve parameters
Controls to load/save the active approach curve

Constructing the Approach Curve

The controls next to the approach curve table allow you to manipulate the approach curve.

Insert Before button: Select to insert the new section before the currently selected section.
Insert After button: Select to insert the new section after the currently selected section.
Insert button: Insert a new section. Select Line or Table section from the drop-down list at the right. For Line sections you can enter the relevant section parameters in the entry boxes to the right. For Table sections, a separate dialog appears. Pressing the Apply button confirms the data entered and will add a new section to the table.
Modify button: Modify the parameters for the currently selected section in the table.
Delete button: Delete the currently selected section in the table.
Down Arrow button: Move the currently selected section one postion down.
Up Arrow button: Move the currently selected section one postion down.
Clear All button: Clear all sections from the table.
Reset button: Undo all changes to the active approach curve and reload the previous aproach curve.
Line/Table selection drop-down list: Select the type of section: Line or Table. To specify the table, a separate dialog appears, see below.

Loading/Saving the Approach Curve

Load: By pressing the Load button, you are presented a File-Load dialog, allowing him to select an existing IV curve on disk (extension: .ivc). The selected IV curve will be loaded and presented both in the graph and in the table.
Save: By pressing the Save button, you are presented with a File-Save dialog, allowing him to save the IV curve.

6.5.6.7.3. Coarse Approach Curve Section Dialog (Table)

Figure 6.35. Approach Mode: Coarse Trajectory Curve Section Configuration Dialog

6.5.6.7.4. Coarse Approach File

Camera stores the Coarse Approach/Retract data in a text file that has an extension of .apc, such as Approach.apc. The file is located in the subdirectory apprdir in the project directory.

A Coarse Approach file contains the following parameters:

WFG-clock cycles per FIFO-point
WFG-clock cycles after FIFO-table traverse
Trajectory gain (individual for approach and retract)
User-defined delay before starting the trajectory (individual for approach and retract)
User-defined delay after performing the trajectory (individual for approach and retract)
Non-zero starting value for trajectory (if permitted by the user, and individual for approach and retract)
Coarse Approach trajectory consisting of individual sections (either linear or table sections)
Coarse Retract trajectory consisting of individual sections

The Coarse Approach file has the following layout:

WFG_CLOCK_CYCLES 3
WFG_DELAY_AFTER 0
[COARSE_APPR]
   WFG_AMPLITUDE 1.000000
   WFG_USER_DELAY_BEFORE 30
   WFG_USER_DELAY_AFTER 40
   WFG_NON_ZERO_START 0.120000
   [SECTION_LIN]
      SCALE_TYPE SCALETYPE_MINMAX
      END_VALUE 1.000000
      STEPS 1
      DELAY_PER_STEP 1
   [SECTION_LIN]
      SCALE_TYPE SCALETYPE_MINMAX
      END_VALUE 0.000000
      STEPS 1
      DELAY_PER_STEP 1
[COARSE_RETR]
   WFG_AMPLITUDE 1.000000
   WFG_USER_DELAY_BEFORE 30
   WFG_USER_DELAY_AFTER 40

6.5.6.8. Coarse Retract Configuration Tab

The coarse retract tab is similar to the coarse approach configuration tab.

Figure 6.36. Approach Mode: Coarse Retract Configuration Tabs

Pressing the Configure button opens a dialog to configure several of the coarse retract pulse parameters. It is similar to the one used for the Coarse Approach Configuration Tab in Section 6.5.6.7.1, “ Configure Pulse Parameters ”.

Pressing the Change Curves button opens a dialog to coarse retract trajectory table sections or curve form. The dialog is similar to the one used for the Coarse Approach Configuration Tab in Section 6.5.6.7.2, “ Change Trajectory Curves ”.

6.5.6.9. Sense Configuration Tab

The fine approach and retract procedure or sense action is performed using the Z-piezo. The tip is lowered from its initial position either instantly or stepwise. At each intermediate point, a number of ADCs selected by the user are read by the host computer. The measured values are evaluated within a stop condition.

When the stop condition is met, the tip is assumed to be in close proximity to the sample and the approach procedure ends. To prevent any damage to tip or sample, the tip is retracted by retracting the Z-piezo to its inital position (sense retract action), see Tip Control panel.

Figure 6.37. Approach Mode: Sense Configuration Tab

6.5.6.9.1. Sense Mode Selection

Z Single-Step Sense: The tip is lowered instantly to a user-defined position, ADC measurments are taken and the tip is retracted to its initial position. You can specify the Sense Voltage in V and Sense Delay in µs.
Z Trajectory Sense: The tip is lowered by the Z-piezo following a user-defined trajectory ('sense approach trajectory'). At each intermediate step ADC measurements are taken and the stop condition is evaluated. When the end of the approach trajectory is reached or the stop condition is met, the tip is retracted to its intial position by the Z-piezo following another user-defined trajectory ('sense retract trajectory'). Hereafter the tip will be positioned at a user-defined final position.
Feedback Sense: The feedback module is active during the sense operation and it controls the Z-position. The Z-offset voltage is not changed.

Provided the Trajectory Sense radio button is selected, you can press the Change Curves button to open a dialog to configure the sense ramp form ( Approach or Retract). The dialog is identical to the one used for the Coarse Approach Configuration Tab in Section 6.5.6.7.2, “ Change Trajectory Curves ”.

6.5.6.9.2. Sense Parameters

When the approach mode is entered, and while performing the coarse approach/retract action, the Z-piezo is in a user-defined retract position that is referred to as the Start Voltage, usually +10V. The retraction to this position happens when the approach mode is entered.

Tip repositioning speed: Reposition tip with this speed expressed as Z-offset slewrate in V/s when not following a trajectory. Specify 0 for instantaneous change of Z-offset voltage. This speed is only used when gentle tip movement is required.
Sense repeat count: Number of times to evaluate sense condition to prevent false positives (since version 4.1.8). Was FB to zero delay; in version 4.1.3, the feedback handling changed, making this parameters unnessary.
Keep feedback on: Keep the feedback active during approach and sense actions. Used with Feedback sense. Indicator-only since version 4.1.7.
Start voltage: Usually +10V for fully retracted position.
#Measurements for average: The number of ADC measurements to average over to evaluate the sense condition with.
Sense voltage: The Z-offset voltage to perform the single-point sense evaluation at.
Sense delay: The time in μs to dwell after setting the single-point sense voltage to perform the ADC measurements to evaluate the sense condition with.

6.5.6.9.3. Sense Approach File

Camera stores the Sense Approach/Retract data in a text file that has an extension of .aps. The file is located in the subdirectory apprdir in the project directory.

A Sense Approach File contains the following parameters:

Tip repositioning speed
Sense start voltage
Number of times to evaluate sense condition to prevent false positives
Number of measurements per sense point to average over
Single-sense voltage
Single-sense delay
Sense Approach trajectory consisting of individual sections
Sense Retract trajectory consisting of individual sections

The Sense Approach file has the following layout:

TIP_INIT_REPOSITIONING_SPEED 1.000000
SENSE_START_VOLTAGE 10.000000
SENSE_REPEAT_COUNT 1
NR_ADC_MEAS_AVG 1
SINGLE_SENSE_VOLTAGE -10.000000
SINGLE_SENSE_DELAY 10000
[SENSE_APPR]
   [SECTION_LIN]
      SCALE_TYPE SCALETYPE_MINMAX
      END_VALUE 0.500000
      STEPS 2500
      DELAY_PER_STEP 500
   [SECTION_LIN]
      SCALE_TYPE SCALETYPE_MINMAX
      END_VALUE 0.000000
      STEPS 1250
      DELAY_PER_STEP 500
[SENSE_RETR]
   [SECTION_LIN]
      SCALE_TYPE SCALETYPE_MINMAX
      END_VALUE 0.000000
      STEPS 1250
      DELAY_PER_STEP 500
   [SECTION_LIN]
      SCALE_TYPE SCALETYPE_MINMAX
      END_VALUE 0.000000
      STEPS 2500
      DELAY_PER_STEP 500

6.5.6.10. Channels Configuration Tab

On the Channels configuration tab you can select which ADC must be included in the measurements, as well as their gain setting.

Figure 6.38. Approach Mode: ADC Channels Configuration Tab

ADC Channel Selection checkboxes: Select Adc 2 through 8; Adc1 is always included.
Amplifiers drop-down: Select a gain of 1, 2, 4, 8, 16, 32 or 64.

	Note
	Since version 4.1.1, it is no longer required to select consecutive ADCs that start at channel 1