N.D. Cent. Code § 44-09-06

The service must be made upon the accused personally, or if upon diligent inquiry the accused cannot be found within the state, the senate, upon proof of that fact, may order the notice to be served by publication or otherwise in such manner as it may deem proper. The notice as published must require the accused to appear at the specified time and place to answer the articles of impeachment. When the notice to the accused is served by publication or otherwise than personally within the state, the articles of impeachment may be served upon the accused by mail or otherwise as the senate may deem proper.