Specific Process Knowledge/Lithography/EBeamLithography/Dose Testing

Introduction

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

The MODULAT command can be used to modulate the base dose for a pattern and hence it can be used to generate a dose matrix. In this setup the SDF will contain a base dose defined by the RESIST command, this dose will then be modulated for different pattern instances defined in the referenced JDF. In this setup the SDF will only contain a single sequence as below.

;SDF example
MAGAZIN    'DOSES'         

#1                            
%4B                           
JDF     'dosetest',1        
ACC 100                       
CALPRM '6na_ap5'              
DEFMODE 2                     
RESIST 200                    
SHOT A,20                     
OFFSET(0,0)                   
     
END 1                         

The arrray is setup with the ARRAY command in the JDF and at the end of the ASSIGN command a dose modulation can be applied. In this way each instance assignment can contain a different modulation. In the example below the pattern is instanced in a 10 x 1 matrix, each element has its own modulation table; SHOT1 to SHOT10. The definition of each modulation is stated at the end of the layer definition.

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.

;JDF example
JOB/W    'DOSES',4                          

PATH   DRF5M                                   
  ARRAY       (50,10,50)/(50,1,0)              
    ASSIGN P(1)->((1,1),SHOT1)                 
    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                                      

P(1)  'dtu_logo_um.v30'                     
SPPRM 4.0,,,,1.0,1                            
STDCUR  6.6 ;nA                               

SHOT1: MODULAT (( 0,0))                       
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.