Specific Process Knowledge/Lithography/EBeamLithography/Dose Testing

In E-beam lithography it is often necesarry to do a dose test in order to get the required result. In a dose test one will expose critical parts of a pattern with various doses and determine the best dose by SEM analysis of the final pattern. There are several ways to set up a dose test array on the JEOL system, in this section we will describe four different setups, each with their own benefits and drawbacks.

Simple SDF sequencing

The most straightforward method is to create one SDF sequence per dose and simply vary the dose with the RESIST command in the SDF as illustrated below. Each instance must of course be offset with the OFFSET command such that they are not exposed on top of each other. The pattern information in the referenced JDF can contain a PEC modulation table with this method and hence it is suited for proximity corrected designs.

The main benefit of this method is how easy it is to setup and that it supports PEC modulation.

The drawback is however that the system will perform initial calibration between each sequence, thus for each dose the system will run calibration as defined in the PATH of the JDF file, this can add several minutes of execution time to each sequence and for a large dose matrix it can cost a lot of additional time.

;SDF example
MAGAZIN    'DOSES'       

;--------Dose 1--------
#1                            
%4D                           
JDF     'dosetest',1   
ACC 100                       
CALPRM '6na_ap5'              
DEFMODE 2                     
RESIST 200                    
SHOT A,24                     
OFFSET(-100,0)                   

;--------Dose 2--------
#1                            
%4D                           
JDF     'dosetest',1   
ACC 100                       
CALPRM '6na_ap5'              
DEFMODE 2                     
RESIST 220                    
SHOT A,24                     
OFFSET(0,0)                   

;--------Dose 3--------
#1                            
%4D                           
JDF     'dosetest',1   
ACC 100                       
CALPRM '6na_ap5'              
DEFMODE 2                     
RESIST 240                    
SHOT A,24                     
OFFSET(100,0)                   
     
END   

Dose modulation in JDF

MAGAZIN    'FIRSTEBL'         The magazine name is FIRSTEBL; max. 9 capital letters

#8                            Cassette from auto stocker shelf 8 is used
%4B                           4" wafer in position A is exposed
JDF     'myfirstebl',1        Layer block no. 1 of the jdf-file 'myfirstebl.jdf' is exposed     
ACC 100                       Acceleration voltage of 100keV is used (can not be changed)
CALPRM '6na_ap5'              The condition file 6na_ap5 is used, i.e. exposure at 6 nA 
DEFMODE 2                     Both deflectors are used (default)
RESIST 200                    A base dose of 200 µC/cm2 is used 
SHOT A,16                     Shot pitch is 16 units (of 0.25 nm), i.e. 4 nm
OFFSET(0,0)                   An offset of 0 µm is applied in both X and Y
     
END 8                         After exposure, cassette 8 will be remain on the stage

Pattern placement is controlled with the ARRAY command in the JDF. The goal of this example exposure is to create a dose test and thus we will use the ARRAY command to create an array of the pattern (the DTU logo) and we will use the MODULAT command to modulate the dose for each instance in the array. The ARRAY command takes six parameters as ARRAY(x,nx,dx)/(y,ny,dy), where x and y defines the center of the first element, nx and ny defines element numbers and dx and dy defines the array element pitch. The array defined in the example below will create a 10 x 1 array with an x-axis pitch of 50 µm.

Each array element is assigned a dose modulation using the MODULAT command. The MODULAT command takes two parameters as MODULAT(r,v), where r is the shot rank and v is the shot time modulation in %. The shot rank is defined during export from Beamer. For a simple design as used in this example that is not proximity corrected all elements of the pattern will be in shot rank 0. If a design is proximity corrected pattern elements will be assigned to different shot ranks. The shot time modulation is a simple percentage increase to the base dose defined by the RESIST command in the SDF. The modulation table in this example will thus expose with a base dose of 200 µC/cm² in element (1,1) and a dose of 200 µC/cm² + 45% = 290 µC/cm² in element (10,1). The resulting pattern and modulation is visualised below.

JOB/W    'FIRSTEBL',4                          4inch wafer

 
PATH   DRF5M                                   Cyclic calibration definition
  ARRAY       (50,10,50)/(50,1,0)              10-1 array around (50,50) with 50 µm pitch in x-axis
    ASSIGN P(1)->((1,1),SHOT1)                 Pattern and modulation assignment to each array element
    ASSIGN P(1)->((2,1),SHOT2)
    ASSIGN P(1)->((3,1),SHOT3)
    ASSIGN P(1)->((4,1),SHOT4)
    ASSIGN P(1)->((5,1),SHOT5)
    ASSIGN P(1)->((6,1),SHOT6)
    ASSIGN P(1)->((7,1),SHOT7)
    ASSIGN P(1)->((8,1),SHOT8)
    ASSIGN P(1)->((9,1),SHOT9)
    ASSIGN P(1)->((10,1),SHOT10)
  AEND
PEND

LAYER  1                                      Definition of pseudo layer 1

P(1)  'dtu_logo_um.v30'                      Assignment of P(1) to V30 file
SPPRM 4.0,,,,1.0,1                            Beam parameters
STDCUR  2.2 ;nA                               Beam current + 10% overhead

SHOT1: MODULAT (( 0,0))                       Dose modulation tables
SHOT2: MODULAT (( 0,5))
SHOT3: MODULAT (( 0,10))
SHOT4: MODULAT (( 0,15))
SHOT5: MODULAT (( 0,20))
SHOT6: MODULAT (( 0,25))
SHOT7: MODULAT (( 0,30))
SHOT8: MODULAT (( 0,35))
SHOT9: MODULAT (( 0,40))
SHOT10: MODULAT (( 0,45))

END

Resulting setup from the example job. The pattern (DTU logo) is instanced 10 times with a pitch of 50 µm. The dose is modulated between 200 and 290 µC/cm2. Pattern size is increased for visibility, actual size is 5 x 8 µm2.