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                                                                     Exhibit 5.1


                           Kirkpatrick & Lockhart LLP
                            Henry W. Oliver Building
                              535 Smithfield Street
                              Pittsburgh, PA 15222



                                  June 13, 2000


Axcelis Technologies, Inc.
55 Cherry Hill Drive
Beverly, MA  01915

         Re:  Registration Statement on Form S-1 (File No. 333-36330)
              -------------------------------------------------------

Ladies and Gentlemen:

         We are acting as special counsel to Axcelis Technologies, Inc., a
Delaware corporation (the "Company"), in connection with a Registration
Statement on Form S-1 (File No. 333-36330) filed with the Securities and
Exchange Commission by the Company on May 4, 2000 and as subsequently amended
(the "Registration Statement"). The Registration Statement relates to the public
offering (the "Offering") of up to _________ shares (the "Shares") of the
Company's Common Stock, $0.001 par value per share (the "Common Stock"),
including up to __________ Shares that the underwriters will have an option to
purchase from the Company solely for the purpose of covering over-allotments, if
any.

         We are familiar with the Registration Statement. We have examined (i)
the Company's Certificate of Incorporation, as amended to date; (ii) the
Company's By-laws, as amended to date; (iii) the Company's proposed Amended and
Restated Certificate of Incorporation (the "New Certificate") in the form in
which it is to be approved and adopted by the sole stockholder of the Company
and filed with the Secretary of State of the State of Delaware prior to the
consummation of the Offering; (iv) the Company's proposed Amended and Restated
By-laws (the "New By-laws") in the form in which they are to be approved by the
sole stockholder of the Company prior to the consummation of the Offering; (v)
the form of the resolutions (the "Resolutions") to be adopted by the Board of
Directors of the Company with respect to the approval and adoption of the New
Certificate and the approval of the New By-laws; and (vi) the form of the
consent (the "Consent") to be executed by the sole stockholder of the Company
with respect to the approval and adoption of the New Certificate and the
approval of the New By-laws. We have also examined such other documents,
corporate records, certificates of public officials, instruments, statutes and
questions of law as we deemed necessary or appropriate to enable us to express
an informed opinion on the matters hereinafter set forth. In making such
examinations and for purposes of rendering the opinions on the matters set forth
below, we have assumed the genuineness of all signatures, the authenticity of
all documents submitted to us as originals, the conformity to original documents
of all documents submitted to us as certified, conformed, telecopied,
photostatic or other reproduced copies and the authenticity of the originals of
such documents, the due execution and delivery of all such documents, and the
accuracy and completeness of the records of the Company.

         We are opining herein only as to the applicable laws of the State of
Delaware, including the General Corporation Law of the State of Delaware, and as
to the federal laws of the United States of America. With respect to the matters
concerning the laws of the State of Delaware, we call your attention to the fact
that we are not admitted to the Bar of the State of Delaware, and our opinions
are based on our


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Axcelis Technologies, Inc.
June 13, 2000
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reasonable familiarity with Delaware law with respect to the limited matters set
forth herein, which we believe is sufficient to enable us to render an informed
opinion on the matters set forth herein.

         Based upon and subject to the foregoing and subject to the adoption by
the Board of the Directors of the Company of the Resolutions, the execution of
the Consent by the sole stockholder of the Company's capital stock and the
filing of the New Certificate with the Secretary of State of the State of
Delaware, we are of the opinion that the Shares, when issued and sold in
accordance with the plan of distribution set forth in the Registration
Statement, will be validly issued, fully paid and non-assessable.

         We consent to the filing of this opinion as Exhibit 5.1 to the
Registration Statement and to the reference to the undersigned in the prospectus
forming a part thereof under the caption "Validity of Common Stock."

                                       Yours truly,



                                       /s/ Kirkpatrick & Lockhart LLP