Method for molecular nitrogen implantation dosage monitoring

A method for assessing stability of an ion implanter, comprising:

• implanting molecular nitrogen into a plurality of semiconductor wafers in a predetermined implantation voltage, the molecular nitrogen having a series of dosages;
• subjecting the semiconductor wafers and a non-implanted semiconductor wafer to thermal process to form an oxide layer on each surface of the semiconductor wafers;
• measuring the thickness of the oxide layer on the surface of the semiconductor wafers;
• computing a suppression ratio from the oxide thickness difference between the implanted and non-implanted semiconductor wafers;
• building a relation between the suppression ratio and the dosages of molecular nitrogen;
• implanting a monitor wafer with a predetermined molecular nitrogen dosage;
• subjecting the monitor wafer to thermal process to grow an oxide layer on a surface of the monitor wafer;
• measuring the thickness of the oxide layer and computing a monitored suppression ratio from the oxides thickness difference to the non-implanted wafer;
• computing an estimative molecular nitrogen dosage by inputting the monitored suppression ratio;
• and
• evaluating the deviation between the estimative molecular nitrogen dosage and the predetermined molecular nitrogen dosage of the monitor wafer, wherein the ion implanter is unstable if the deviation is larger than a specific range.