Reservation  -     

Center of MicroNanoTechnology CMi

Process Flow for Ebeam Lithography

Table of content

  1. Vistec EBPG5000 overview
  2. What is an electron beam (ebeam) lithography system?
  3. Electrons, resists and substrates
  4. Process Flow for Ebeam Lithography
  5. Data conversion
  6. Running the ebeam

Design

In CMI we try to make it as easy for you to do your work as possible. For the ebeam there are 3 possible ways of designing patterns for ebeam exposure. We will call these options simple, wafer alignment and cell alignment. Wafer and cell alignment use standard marker designs available. This marker design will be available either as a standard CMI mask or as a design for you to include in your pattern.

To do your design, you can first download the design librairies (tdb, cif or gdsii file) available on Mask design web page.

Design hints and tips

The resolution of the ebeam is very high. It is possible, with the right process, to regularly obtain < 10nm features. To achieve this extreme resolution your pattern would have to be split into very small "pixels" and we would need to use the smallest beam available on the machine. If you dont need this extreme resolution then your pattern will write faster if it's split into larger pixels and written with a larger beam.

IMPORTANT: if you have a large range of feature sizes you should separate the small features and the large features by putting them on different gds layers in your design. A good rule would be to keep features within a factor of x10 grouped together i.e. if you have a minimum feature of 50nm all features up to 500nm can be placed of the same layer, larger features than 500nm should be placed on a different layer.

Data Conversion

The ebeam machine does not understand gdsii, DWF or any other design or CAD format directly. Your design needs to be translated into the format the machine uses. You will need to have your design in gdsii format, you will then transfer this to the data conversion computer, convert your pattern, and then transfer to the computer that controls the ebeam machine. The data conversion computer is call CAD026 and the software used for the conversion is called CATS. Some simple instructions for using CATS will come soon. For the moment, while we are in service mode, you should put your design in the ebeam drop box here. file://Cmisrv1/@public/ebeam drop-box

Simple Exposure

With simple exposure one pattern, or an array of patterns is written on the wafer without any alignment, and therefore without any possibility to place a subsequent pattern accurately on the wafer with respect to the first. This type of exposure can be used for simple, single level exposures and also for dose testing of new designs.

smile

Alignment

If youre making a complex multi level device its probable that you would want to use ebeam for one or two critical steps that require both high resolution and accurate placement. In order to get your pattern accurately placed on your wafer with respect to previous device layers the ebeam needs alignment marks.

Unlike photolithography, the ebeam finds these marks automatically provided that it knows what size, and what contrast polarity they are; positive (metal on substrate) negative (etched). To do accurate alignment the ebeam will need to find at least 3 marks (usually 4). If your wafer is perfectly flat and undistorted, and the temperature of the substrate is exactly the same as its environment and your alignment marks are lithographically perfect, then it shouldnt matter how far apart you put the alignment marks on the substrate you will get perfect alignment. Unfortunately this never happens. The smaller the distance between marks the potentially the more accurate the alignment is. This is because there is less possibility of wafer distortion and the wafer is flatter over shorter distances. On our machine if the marks are spaced out 10mm apart the alignment accuracy should be less than 30nm mean+3σ (usually less than 20nm mean+3σ). It is important to check to quality of your alignment marks in the SEM before going to the ebeam. In particular you should check that the marks are the correct size and the edges are well defined.

Wafer Alignment

In this scheme your entire wafer design is aligned on the wafer using using 4 ebeam marks. It is possible then to write a subsequent pattern aligned to the first. Using the method alignment accuracy is not as good as the cell alignment method (discussed next), but would still normally be better than 100nm.

smile

Cell Alignment

Here all you will have to do is design your devices to fit in an area 9.5mmx9.5mm. Within this 9.5x9.5mm design square you can create any design you want. You can have lots of different devices of different shapes and sizes as long as it fits in the design area. You can also create more than one version of the design area.

smile

Because we are giving you a standard marker layout and we are using something called a pre-alignment marker array, the procedure for alignment is very easy. Once your wafer is loaded into the machine we only have to edit a file to change the pattern name of your pattern, change the beam we want to use and dose you require. Its very simple.

For 100mm wafers we have standard designs. This designs gives you the flexibility to use two different types of alignment; wafer and cell. Layer 6 contains the pre-alignment array and MUST be included in any design. If you want to use wafer level alignment you should include layer 7. If you want to use cell alignment you should include layer 8 and optionally layer 9 (which draws a dash line border around each cell).