Electron Beam Writer

The advantage of electron beam lithography is its high resolution. The pattern has to be exposed in a serial manner which can lead to long writ-ing times. In the followwrit-ing the most important parts of the electron beam writer in our laboratory, a Leica Lion LV-1 from Leica Microsystems Jena, shall be explained.

Low voltage optical column The central part of this electron beam wri-ter is the electron-optical column designed by Integrated Circuit Testing GmbH (ICT GmbH) (Fig. 2.1). It allows setting of the electron beam to a desired voltage level anywhere between 1 kV and 20 kV. Usually it is set to a low acceleration voltage of 2.5 kV to avoid the proximity effect (see 2.1.2). A thermal field emission cathode is used as an electron emitter. It is made of tungsten (W) and coated with zirconiumoxid (ZrO) to lower the work-function and it is heated up to 1600^◦C. For operation, the pressure is kept<10⁻⁸mbar. A suppressor close to the emission cathode with a bias

of typically -300 V suppresses unwanted thermal electrons. The emitted electrons are accelerated by an extractor which is biased to +8000 V. This makes the electron beam largely insensitive to magnetic fields from out-side. A beam blanker consisting of 2 plates which deflect the beam away from the sample by applying a voltage V_bb in addition to the 8000 V to one of the plates, essentially switching it off. With gun alignment coils the beam can by directed through holes of 10, 17, 30, 60, and 120µm on an aperture strip. The 30µm hole is in the optical axis of the column. If any other aperture is chosen, the beam can be brought back on the op-tical axis with aperture alignment coils behind the aperture strip. The column provides a spot size as small as 5 nm (at 1 kV) through the use of a compound objective lens. An electrostatic lens produces a diverging field, while the surrounding magnetic lens converges the beam. The com-plementary lenses reduce chromatic aberration, similar to a compound optical lens.

Stage control The stage motion is measured by a two-coordinate dou-ble-beam metering system. The measurement beams are emitted by a fre-quency stabilized HeNe-laser and the position of the stage is measured with interferometers. The mechanical positioning accuracy is 0.1µm.

Discrepancies in the measured and desired stage position are corrected by an electromagnetic deflector system (beam tracking system / BTS). This leads to an accuracy of 2.5 nm over a range of 20µm.

Exposure control Each substrate holder is fitted with a Faraday cup al-lowing measurement of the actual probe current. Prior to exposure, the beam alignment is carried out on a calibration substrate which is virtu-ally identical to the height of the substrate to be exposed. A difference in height will be sensed and recorded by an optical height meter and corrected. The accuracy of the height measurement is 2.2µm. All ma-jor components are supported in a soft spring suspension to ensure that floor and building vibrations do not affect the accuracy. The line width is adjusted by changing the dose and the defocus of the electron beam

[DH99].

Continuous path control For this work quasi-continuous path follow-ing (Continuous Path Control / CPC) was employed. This means that the stage moves under the beam which remains virtually stationary. Devia-tions of the stage’s actual position from its required position are compen-sated with the help of the beam tracking system (BTS). The maximum possible extension of the structures that can be exposed is dependent on the substrate size alone. All structures subject to exposure are by approx-imation represented as Bézier elements.

Data format The files containing the exposure data are called BEZ files.

They are text files which can be produced with every program or pro-gramming language which is able to provide ASCII-text as an output.

The simplest geometrical structure is a straight line. This can be rep-resented by a1^st order Bézier curve which can be described as

−

→P =−→

P₁(1−t) +−→

P₂t t ∈[0,1]. (2.1) Here, the line is created by varying the parameter t in a continuous manner from 0 to 1 and−→

P₁ marks the beginning and−→

P₂ the end to the line (Fig.2.2(a)).

Curved lines are Bézier curves of higher order. Important for this work were rings or segments of rings. The latter is a special case of a 2^nd order Bézier curve which is given by

−

P₃ represents a guiding point for the curve and is located in the intersections of the tangents at the starting point−→

P1 and end point−→

P2. For a ring segment the factor g is given by g =cos(ϕ

2), (2.3)

Figure 2.2: (a) Bézier curve of 1^st order, (b) Ring segment being a special case of a Bézier curve of2^ndorder, (c) Scheme showing the stage movement during writing of a Bézier curve in Continuous Path Control mode.

ϕ being the angle of the ring section at the center (Fig. 2.2(b)). A full ring comprises, for example, four quarter rings or three one third rings because only sections of rings with ϕ < 180^◦ can be processed by the Lion electron beam software.

Each curved line is approximated by a zigzag of straight lines corre-sponding to the address grid of the electron beam writer (Fig. 2.2(c)). The calculation of the zigzag path from the input Bézier data is done in re-altime during the exposure. Because the grid period is very small, the resist line finally obtained after development has the similar shape as the initially given Bézier curve.