.. _DocEtch:

The ``etch()`` Method
=====================

The ``etch()`` method is one of the basic methods for describing a process.
It is called on a mask data object. The basic use case is:

.. code-block:: python

    l1 = layer("1/0")
    substrate = bulk()
    mask(l1).etch(0.3, into='substrate')
    output("1/0", substrate)

This simple case removes material from the substrate and leaves a hole
where the mask is drawn..

The ``etch()`` method has up to two positional arguments and a couple of
options which have to be put after the arguments in the usual Python
keyword notation ``name=value``:

``etch(height, lateral=0, option=value, ...)``

The ``height`` argument is mandatory and specifies the depth of the etch.
The ``lateral`` parameter specifies the lateral extension (underetch).
The lateral extension is optional and defaults to 0. The lateral
extension can be negative. In that case, the profile will be aligned
with the mask at the top. Otherwise it is aligned at the bottom.

There are several options:

.. list-table::
    :widths: 10 60
    :header-rows: 1

    * - Option
      - Value
    * - ``mode``
      - | The profile mode. Can be ``'square'`` (default), ``'round'``,
        | and ``'octagon'``.
    * - ``taper``
      - | The taper angle. This option specifies tapered mode and cannot be
        | combined with ``mode``.
    * - ``bias``
      - | Adjusts the profile by shifting it to the interior of the figure.
        | Positive values will reduce the line width by twice the value.
    * - ``into``
      - | A material or an array of materials into which the etch is
        | performed. This specification is mandatory.
    * - ``through``
      - | A material or an array of materials which form the selective
        | material of the etch. The etch will happen only where this
        | material interfaces with air and pass through this material
        | (hence the name).
    * - ``buried``
      - | Applies the etching at the given depth below the surface. This
        | option allows to create cavities. It specifies the vertical
        | displacement of the etch seed and there may be more applications
        | for this feature.

``mode``, ``taper`` and ``bias``
--------------------------------

The effect of the mode and bias interaction is best illustrated with
some examples.

The initial layout is always this in all following examples:

.. image:: ./img/e1.png

The first example if the effect of the plain etch with a thickness of
0.3 um. It will remove a rectangular part at the mask:

.. code-block:: python

    etch(0.3, into=substrate)

.. image:: ./img/e1_xs.png

The next example illustrates the effect of a lateral extension on a
square profile. The 0.1 um extension will remove material left and right
of the main trench:

.. code-block:: python

    etch(0.3, 0.1, into=substrate)

.. image:: ./img/e2_xs.png

In ``'round'`` mode, the material will be removed with an elliptical
profile. The vertical axis will be 0.3 um, the horizontal 0.1 um
representing the lateral extension. The trench will become bigger
than the mask by the lateral extension at the bottom:

.. code-block:: python

    etch(0.3, 0.1, mode='round', into=substrate)

.. image:: ./img/e3_xs.png

To avoid overetch, a negative lateral extension can be specified,
resulting in a alignment of patch and mask at the top:

.. code-block:: python

    etch(0.3, -0.1, mode='round', into=substrate)

.. image:: ./img/e4_xs.png

Another mode is ``'octagon'`` which is basically a coarse approximation
of the ellipse and computationally less expensive:

.. code-block:: python

    etch(0.3, 0.1, mode='octagon', into=substrate)

.. image:: ./img/e5_xs.png

A bias value can be specified to fine-tune the position of the top
edge of the trench. A *positive* bias value will *shrink* the figure:

.. code-block:: python

    etch(0.3, 0.1, mode='round', bias=0.05, into=substrate)

.. image:: ./img/e6_xs.png

A special profile can be specified with the ``taper`` option. This option
specifies a taper angle and a conical trench will be created. The taper
angle will be the sidewall angle of the trench. This option cannot be
combined with ``mode`` and the lateral extension should be omitted. It can
be combined with ``bias`` however:

.. code-block:: python

    etch(0.3, taper=10, into=substrate)

.. image:: ./img/e7_xs.png

.. code-block:: python

    etch(0.3, taper=10, bias=-0.1, into=substrate)

.. image:: ./img/e8_xs.png

Step etch profile
-----------------

The following image shows the etch profile of a 30° slope and a
vertical step by an etch in round mode with thickness of 0.3 um and
lateral extension of 0.1 um. The sidewall of the step will be removed
with a thickness of 0.1 um corresponding to the lateral extension.

The solid gray line shows the profile before the etch:

.. code-block:: python

    etch(0.3, 0.1, mode='round', into=substrate)

.. image:: ./img/e10_xs.png

``through`` - selective etch
----------------------------

Normally the etch will happen only at the interface between air and
the ``into`` material, as the following example demonstrates:

.. code-block:: python

    # Prepare input layers
    m1 = layer("1/0")
    m2 = layer("2/0")

    substrate = bulk()

    # Grow a stop layer
    stop = mask(m2).grow(0.05, into=substrate)

    # Grow with mask m1, but only where there is a substrate surface
    mask(m1).etch(0.3, 0.1, mode='round', into=substrate)

    # output the material data to the target layout
    output("0/0", substrate)
    output("2/0", stop)

With the following input:

.. image:: ./img/e12.png

This script will produce the following result:

.. image:: ./img/e12_xs.png

The blue material will prevent etching as it blocks the air/substrate
interface. The ``through`` option reverses that scheme: giving this
``stop`` material as an argument to ``through`` will make the etch happen
at places where this material interfaces with air:

.. code-block:: python

    # Prepare input layers
    m1 = layer("1/0")
    m2 = layer("2/0")

    substrate = bulk()

    # Grow a stop layer
    stop = mask(m2).grow(0.05, into=substrate)

    # Grow with mask m1, but only where there is a substrate surface
    mask(m1).etch(0.3, 0.1, mode='round', into=substrate, through=stop)

    # output the material data to the target layout
    output("0/0", substrate)
    output("2/0", stop)

This script will produce the following result:

.. image:: ./img/e13_xs.png

``buried`` - vertically displaced etch
--------------------------------------

This option shifts the seed of the etch operation into the material.
Without this option, the etch will start at the surface. If a positive
value is given, the etch starts below the surface in a depth given by
this value. The etch will proceed upwards and downwards with the given
features. In the extreme case (below the surface by more than the etch
depth), this feature creates cavities:

.. code-block:: python

    # Prepare input layers
    m1 = layer("1/0")
    m2 = layer("2/0")

    substrate = bulk()

    # Grow with mask m1 into the substrate
    mask(m1).etch(0.3, 0.1, mode='round', into=substrate, buried=0.4)

    # output the material data to the target layout
    output("0/0", substrate)

With the following input:

.. image:: ./img/e14.png

This script will produce the following result:

.. image:: ./img/e14_xs.png