FIRST AMENDMENT TO
                          AGREEMENT AND PLAN OF MERGER

     This FIRST AMENDMENT TO AGREEMENT AND PLAN OF MERGER is dated as of June 6,
1997, by and among ADVANCED TECHNOLOGY  MATERIALS,  INC., a Delaware corporation
("Buyer"),   WELK  ACQUISITION   CORPORATION,   a  Delaware  corporation  and  a
wholly-owned subsidiary of Buyer ("Buyer Sub"), ATMI HOLDINGS,  INC., a Delaware
corporation  and  wholly-owned   subsidiary  of  Buyer   ("Holdings")   LAWRENCE
SEMICONDUCTOR  LABORATORIES,  INC., an Arizona corporation ("LSL"), and LAWRENCE
SEMICONDUCTOR  LABORATORIES  MARKETING AND SALES,  INC., an Arizona  corporation
("LSLMS"; LSL and LSLMS are referred to collectively as "Lawrence");  and all of
the  parties are  referred to  collectively  as the  "Companies."  Buyer Sub and
Lawrence are referred to  collectively  as the  "Constituent  Corporations"  and
individually as a "Constituent Corporation."

     The  Companies  are parties to that  certain  Agreement  and Plan of Merger
dated  May 17,  1997 (the  "Merger  Agreement"),  and wish to amend  the  Merger
Agreement in order to permit the following:  (i) the possible  transfer by Buyer
of the issued and outstanding  shares of capital stock of Buyer Sub to Holdings,
(ii) the transfer by Lamonte H. Lawrence of the issued and outstanding shares of
capital  stock of LSLMS to LSL, and (iii) the adoption by Holdings or Buyer,  as
appropriate,  of a stock  option plan for 900,000  shares of common  stock.  The
Companies  also wish to amend the Merger  Agreement  in order to  eliminate  any
offset of the intercompany indebtedness against the purchase price.

     In  consideration  of the  foregoing  and the  respective  representations,
warranties,  covenants and  agreements  set forth  herein,  the parties agree as
follows:

     1. Capitalized  terms not otherwise  defined herein shall have the meanings
provided in the Merger Agreement.

     2. Section 1.1 is hereby  amended to insert the following at the end of the
final  sentence:  "unless Buyer elects in its discretion to contribute  prior to
the Effective Time all of the issued and outstanding  shares of capital stock of
Buyer  Sub to  Holdings,  in which  case the  Surviving  Corporation  shall be a
wholly-owned subsidiary of Holdings."

     3. Section 3.1(b) is hereby amended and restated to read in its entirety as
follows:  "Each  share of common  stock of LSL  ("Lawrence  Share")  held in the
treasury of LSL, if any, or by any subsidiary of Lawrence and each such Lawrence
Share  held by  Buyer  or any  subsidiary  of  Buyer  immediately  prior  to the
Effective  Time  shall be  canceled  and  retired  and  cease to  exist,  and no
consideration shall be given in exchange therefor."

     4.  Section 3.4 is hereby  amended to delete the  following  from the first
paragraph: "minus any amounts owed to LSL by any related parties as set forth in
Schedule 5.26."

     5.  Section  3.6(a)(xiv)  is hereby  amended  and  restated  to read in its
entirety as follows:  "Written  evidence  satisfactory  to Buyer that Lamonte H.
Lawrence,  the sole stockholder of LSLMS, has contributed  immediately  prior to
the Effective Time all of the issued and outstanding  shares of capital stock of
LSLMS to LSL, with LSLMS becoming a wholly-owned  subsidiary of LSL and that all
consents and approvals  necessary or appropriate for such contribution have been
obtained."

     6. Section 3.6(a)(xvii) is hereby amended to delete the final sentence.

     7. Section  4.2(k) is hereby amended to insert the following as a new final
sentence:  "Notwithstanding the foregoing,  Buyer may elect in its discretion to
contribute prior to the Effective Time all of the issued and outstanding  shares
of  capital  stock of  Buyer  Sub to  Holdings,  in  which  case  the  Surviving
Corporation shall become a wholly-owned subsidiary of Holdings."

     8. Section 5.2 is hereby amended to delete the second sentence.

     9. Section  6.1(f) is hereby  amended to insert the following as subsection
(iv):  "(iv) Lamonte H. Lawrence,  the sole stockholder of LSLMS, may contribute
all of the issued and outstanding  shares of capital stock of LSLMS to LSL, with
LSLMS becoming a wholly-owned subsidiary of LSL."





     10.  Section   7.15(f)  is  hereby  amended  to  insert  the  following  as
subsections (iii) and (iv): "(iii) Buyer may in its discretion  contribute prior
to the Effective Time all of the issued and outstanding  shares of capital stock
of Buyer Sub to Holdings,  in which case the  Surviving  Corporation  shall be a
wholly-owned  subsidiary of Holdings; and (iv) (A) Buyer may approve a new stock
option plan for up to 900,000  shares of Buyer Common  Stock,  if the closing of
the  transactions  contemplated  by the ADCS  Merger  Agreement  shall  not have
occurred on or before the  Effective  Time,  or (B)  Holdings  may  increase the
authorized  number of shares of Holdings  Common Stock available in its existing
stock  option plan from 750,000 to 900,000,  if the closing of the  transactions
contemplated  by the ADCS Merger  Agreement shall have occurred on or before the
Effective Time."

     11. The final sentence of Section 10.5(a) is hereby amended and restated to
read in its entirety as follows:  "As used herein, "Pro Rata Portion" shall mean
with  respect  to  each  Shareholder  his or  its  percentage  ownership  of LSL
immediately prior to the Effective Time."

     12. Except as modified  herein,  the  Agreement as originally  executed and
previously  amended is hereby  ratified and affirmed and  acknowledged to be the
legal, valid and binding obligations of each of the parties hereto.

     13. This  Amendment  shall be governed by and construed in accordance  with
the laws of the  State of New  York  without  giving  effect  to the  provisions
thereof relating to conflicts of law.

     14. This  Amendment  may be executed in two or more  counterparts,  each of
which shall be deemed to be an original  but all of which shall  constitute  one
and the same agreement.

     IN WITNESS  WHEREOF,  each of Buyer,  Buyer Sub,  Holdings and Lawrence has
caused this  Amendment  to be executed on its behalf by its  officers  thereunto
duly authorized, all as of the date first above written.

Advanced Technology Materials, Inc.,
a Delaware corporation


By: /s/ Daniel P. Sharkey
- ---------------------------------
Name: Daniel P. Sharkey
Title: Vice President, Chief
Financial Officer

Welk Acquisition Corporation, a
Delaware corporation


By: /s/ Daniel P. Sharkey
- ---------------------------------
Name: Daniel P. Sharkey
Title: President

ATMI Holdings, Inc., a Delaware
corporation


By: /s/ Daniel P. Sharkey
- ---------------------------------
Name: Daniel P. Sharkey
Title: Treasurer

Lawrence Semiconductor Laboratories,
Inc., an Arizona corporation


By: /s/ Lamonte H. Lawrence
- ---------------------------------
Name: Lamonte H. Lawrence
Title: Chief Executive Officer

Lawrence Semiconductor Laboratories
Marketing and Sales, Inc., an
Arizona corporation


By: /s/ Lamonte H. Lawrence
- ---------------------------------
Name: Lamonte H. Lawrence
Title: Chief Executive Officer