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Method of setting threshold voltage levels of a multiple-valued mask programmable read only memory

  • US 6,221,723 B1
  • Filed: 09/09/1998
  • Issued: 04/24/2001
  • Est. Priority Date: 09/10/1997
  • Status: Expired due to Fees
First Claim
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1. A method of setting a plurality of different threshold voltage levels to a plurality of cell regions for a mask programmable semiconductor device, said method comprising the steps of:

  • forming gate electrodes surrounded by a first photo-resist mask;

    applying a first ion-implantation dose to simultaneously form drain, source, and channel regions associated with all of the gate electrodes surrounded by the first photo-resist mask;

    while retaining the first photo-resist mask, with a first impurity carrying out a second ion-implantation through all of the gate electrodes to establish a first threshold voltage level associated with each of the gate electrodes of all of the cell regions surrounded by the first photo-resist mask, the first ion-implantation being performed at an energy level lower than the second ion-implantation;

    discarding the first photo-resist mask and providing a second photo-resist mask protecting the drain and source region and plural ones of the gate electrodes, at least one of the gate electrodes being exposed by the second photo-resist mask; and

    with a second impurity applying a first-code selective ion-implantation through at least a first-selected one of the exposed gate electrodes to establish a second threshold voltage level associated with the exposed at least one gate electrode, the second threshold voltage level being different from the first threshold voltage level, wherein said second impurity of said first-code selective ion-implantation is heavier than said first impurity so as to suppress any excess thermal diffusion to avoid variations in threshold voltage level of said cell regions.

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