EXHIBIT 10d
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                                   ----------

                         AMENDED KEY EMPLOYEE AGREEMENT

                                   ----------






           To:  Mr. John M. Tarrh                             As of July 1, 1996
                20 Oakland Street
                Lexington, Massachusetts  02173

         The  undersigned,  Applied  Science and  Technology,  Inc.,  a Delaware
corporation,  as well as its  successors  and  assigns(hereinafter  collectively
referred to as the "Company"), hereby agree with you as follows:

         l. Position and Responsibilities.

            1.1 You  shall  serve as  Senior  Vice  President,  Finance,  of the
Company (or in such other executive capacity as shall be designated by the Board
of Directors  and  reasonably  acceptable  to you) and shall  perform the duties
customarily associated with such capacity from time to time and at such place or
places  as  the  Company  shall  designate  are  appropriate  and  necessary  in
connection  with  such  employment;  provided,  however,  that you  shall not be
required  to  relocate  your place of  employment  beyond a 20 mile  radius from
Woburn, Massachusetts without your prior written consent.

            1.2 You will, to the best of your ability, devote your full time and
best efforts to the  performance  of your duties  hereunder and the business and
affairs of the  Company.  You agree to perform such  executive  duties as may be
assigned  to you by or on  authority  of the  Company's  President  and Board of
Directors  from time to time.  After  receipt of notice of  termination  of your
employment  hereunder,  you shall  continue to be  available to the Company on a
part-time basis to assist in any necessary transition.

            1.3 You will duly, punctually and faithfully perform and observe any
and all  rules and  regulations  which the  Company  may now or shall  hereafter
establish governing the conduct of its business.

            1.4 You will report directly to the Company's President.

         2. Term of Employment.

            2.1 The initial  term of this  Agreement  shall be for the period of
years set forth on Exhibit "A" annexed hereto  commencing  with the date hereof.
Thereafter, this Agreement shall be automatically renewed for successive periods
of one year,  unless you or the Company shall give the other party not less than
three (3) months written notice of non-renewal. Your













employment with the Company may be terminated at any time as provided in Section
2.2 . If the  Company  gives you notice of  non-renewal,  the  Company  shall be
obligated to pay to you as Severance  Benefits an amount set forth in Sections 7
and 8 of Exhibit "A" hereto,  plus payment in full of any amounts  otherwise due
you, less applicable  taxes and other required  withholdings and any amounts you
may owe to the Company.

            2.2 The Company shall have the right,  on written  notice to you, to
terminate your employment:

                (a)  immediately  at any time for  "Cause"  (as  defined  herein
          subject  to your right of cure and right to  dispute  as  provided  in
          Section 2.3 herein); or

                (b) at any  time,  upon not less  than  seven  (7) days  written
          notice, without "Cause" provided the Company shall be obligated to pay
          to you as Severance  Benefits an amount equal to the sums set forth in
          Exhibit "A," plus any sums then due to you, less (i) applicable  taxes
          and other required  withholdings,  and (ii) any amounts you may owe to
          the Company.  Payments  under this Section 2.2 (b) shall not be due or
          payable  if you are  terminated  at any  time  for  "Cause"  or if you
          voluntarily resign from your employment.

            2.3 For  purposes of Section 2.2 (except as provided in Section 8(c)
of  Exhibit  "A"),  the term  "Cause"  shall  mean (a) gross  negligence  in the
performance of assigned  duties;  (b) refusal to perform or discharge the duties
or  responsibilities  assigned by the Board of Directors of Applied  Science and
Technology,  Inc.  provided the same are not illegal,  unethical or inconsistent
with the position of Senior Vice  President,  Finance,  of a corporation and the
failure to correct  such  refusal  and perform  such duties or  responsibilities
within two weeks (14 calendar  days) after written  notice of such failure;  (c)
conviction  of a felony  involving  moral  turpitude;  (d) willful or  prolonged
absence from work not excused by  disability;  and (e) falseness of any warranty
or  representation  by you herein or the breach of your  obligations  under this
Agreement or your duties as an employee of the Company to the material detriment
of the Company.

            2.4 In the event of the  Involuntary  Termination of your employment
with the Company at any time, the Company hereby  irrevocably  agrees to provide
you with  Severance  Benefits  as defined in Section 7 of Exhibit  "A" hereto or
payments  in the event of a "Change  in  Control"  as  defined  in  Section 8 of
Exhibit "B". In this regard, the phrase "Involuntary Termination" shall mean any
termination of your employment by the Company other than for "cause," as defined
in Section 2.3, any notice by the Company not to renew this  Agreement  pursuant
to Section 2.1, or any  termination of your  employment by you due to any of the
following  circumstances:  (a) a reduction  in your Base Salary or  Company-paid
benefits,  (b) a reduction in your  eligibility  for any Company  bonus or other
benefit program,  (c) a material or substantial change in your title,  position,
authority or duties,  or (d) a change of your principal place of employment from
Woburn,   Massachusetts   to  another   location  beyond  20  miles  of  Woburn,
Massachusetts.



                                      -2-










         3.  Compensation.  You shall receive the  compensation and benefits set
forth on Exhibit A hereto  ("Compensation")  for all  services to be rendered by
you hereunder and for your transfer of property  rights pursuant to an agreement
relating  to  proprietary  information  and  inventions  of even  date  herewith
attached  hereto as  Exhibit C between  you and the  Company  (the  "Proprietary
Information and Inventions  Agreement").  Such Compensation  shall be subject to
temporary  or  permanent  reduction by the Board of Directors if the Board shall
determine that economic conditions so warrant.

         4. Other Activities During Employment.

            4.1 Except for any outside  employments and directorships  currently
held by you as listed on  Exhibit B hereto,  and except  with the prior  written
consent of the  Company's  Board of  Directors,  you will not during the term of
this  Agreement  undertake  or  engage in any other  employment,  occupation  or
business enterprise other than one in which you are an inactive investor.

            4.2 You hereby agree that,  except as disclosed on Exhibit B hereto,
during your employment hereunder,  you will not, directly or indirectly,  engage
(a) individually,  (b) as an officer, (c) as a director, (d) as an employee, (e)
as a consultant,  (f) as an advisor,  (g) as an agent  (whether a salesperson or
otherwise),  (h) as a broker,  or (i) as a partner,  coventurer,  stockholder or
other  proprietor  owning  directly or  indirectly  more than five  percent (5%)
interest in any firm, corporation,  partnership,  trust,  association,  or other
organization  which  is  engaged  in  the  research,  development,   production,
manufacture  or marketing of equipment or processes in direct  competition  with
the  Company or any other  line of  business  engaged  in or under  demonstrable
development  by  the  Company  (such  firm,  corporation,   partnership,  trust,
association,   or  other  organization  being  hereinafter   referred  to  as  a
"Prohibited Enterprise"). Except as may be shown on Exhibit B hereto, you hereby
represent  that  you are not  engaged  in any of the  foregoing  capacities  (a)
through (i) in any Prohibited Enterprise.

         5. Former Employers.

            5.1 You  represent  and warrant that your  employment by the Company
will not  conflict  with and will not be  constrained  by any  prior or  current
employment,  consulting  agreement or relationship  whether oral or written. You
represent and warrant that you do not possess  confidential  information arising
out of any such employment,  consulting agreement or relationship which, in your
best  judgment,  would be utilized in  connection  with your  employment  by the
Company in the absence of Section 5.2.

            5.2 If, in spite of the second  sentence of Section  5.1, you should
find that confidential information belonging to any other person or entity might
be usable in connection with the Company's business,  you will not intentionally
disclose  to the  Company  or use on  behalf  of the  Company  any  confidential
information  belonging  to  any  of  your  former  employers;  but  during  your
employment  by the  Company you will use in the  performance  of your duties all
information  which is  generally  known and used by persons  with  training  and
experience 



                                      -3-







comparable  to  your  own all  information   which  is  common  knowledge in the
industry or otherwise legally in the public domain.

         6.  Proprietary  Information  and  Inventions.  You  agree to  execute,
deliver  and be bound  by the  provisions  of the  Proprietary  Information  and
Inventions Agreement.

         7. Post-Employment Activities.

            7.1  For a  period  of  two  (2)  years  after  the  termination  or
expiration,  for any reason,  of your  employment  with the  Company  hereunder,
absent the Company's prior written approval, you will not directly or indirectly
engage in activities  similar or reasonably  related to those in which you shall
have engaged hereunder during the two years immediately preceding termination or
expiration for, nor render services similar or reasonably related to those which
you shall have rendered hereunder during such two years to, any person or entity
whether now existing or hereafter  established  which directly competes with (or
proposes or plans to directly compete with) the Company ("Direct Competitor") in
any line of business engaged in or under  development by the Company.  Nor shall
you entice,  induce or encourage any of the Company's  other employees to engage
in any activity  which,  were it done by you, would violate any provision of the
Proprietary  Information and Inventions  Agreement or this Section 7. As used in
this Section 7.1, the term "any line of business engaged in or under development
by the  Company"  shall  be  applied  as at the  date  of  termination  of  your
employment,  or, if later, as at the date of termination of any  post-employment
consultation.

            7.2 For a period of two (2)  years  after  the  termination  of your
employment  with the Company,  the provisions of Section 4.2 shall be applicable
to you  and you  shall  comply  therewith.  As  applied  to  such  two (2)  year
post-employment period, the term "any other line of business engaged in or under
development  by the Company," as used in Section 4.2, shall be applied as at the
date of termination of your  employment with the Company or, if later, as at the
date of termination of any post-employment consultation with the Company.

            7.3 No  provision of this  Agreement  shall be construed to preclude
you from  performing  the same services  which the Company hereby retains you to
perform for any person or entity which is not a Direct Competitor of the Company
upon the expiration or termination  of your  employment (or any  post-employment
consultation)  so long as you do not thereby violate any term of the Proprietary
Information and Inventions Agreement.

         8. Remedies.  Your  obligations  under the Proprietary  Information and
Inventions  Agreement  and  the  provisions  of  Sections  6,7,8  and 9 of  this
Agreement  (as  modified  by  Section  10,  if  applicable)  shall  survive  the
expiration or termination of your employment  (whether  through your resignation
or otherwise)  with the Company.  You  acknowledge  that a remedy at law for any
breach  or  threatened  breach  by  you  of the  provisions  of the  Proprietary
Information  and  Inventions  Agreement or Section 7 would be inadequate and you
therefore agree that the Company shall be entitled to such injunctive  relief in
case of any such breach or threatened

                                      -4-




breach.

         The Company's  obligations  and those of any successors or assignees of
the Company  under this  Agreement,  including  but not limited to the severance
provisions  and other  compensation  and benefits due to you pursuant to Exhibit
"A" hereto,  will be a condition of and are to remain those of any  successor or
assignee.  The  Company  acknowledges  that a remedy  at law for any  breach  or
threatened  breach  by  the  Company,  its  directors  or  agents  of any of the
provisions  of Exhibit  "A"  hereto or of this  Agreement  generally,  or of any
extension  of this  Agreement,  would be  inadequate  and the Company  therefore
agrees  that you  shall be  entitled  to  injunctive  relief in case of any such
breach or  threatened  breach.  In the  event of any  dispute  pursuant  to this
Agreement,  the  prevailing  party in any  litigation  or  arbitration  shall be
entitled to prompt  reimbursement  of reasonable legal fees and related expenses
incurred in connection with such dispute.

         9.  Assignment.  This  Agreement and the rights and  obligations of the
parties  hereto  shall  bind  and  inure  to the  benefit  of any  successor  or
successors of the Company by  reorganization,  merger or  consolidation  and any
assignee of all or substantially all of its business and properties, but, except
as to any such successor or assignee of the Company,  neither this Agreement nor
any rights or  benefits  hereunder  may be  assigned  by the  Company or by you,
except by operation of law.

         10.  Interpretation.  IT IS THE INTENT OF THE PARTIES  THAT in case any
one or more of the provisions contained in this Agreement shall, for any reason,
be held to be invalid, illegal or unenforceable in any respect, such invalidity,
illegality  or  unenforceability  shall not affect the other  provisions of this
Agreement,  and this Agreement shall be construed as if such invalid, illegal or
unenforceable  provision had never been contained  herein.  MOREOVER,  IT IS THE
INTENT OF THE PARTIES THAT in case any one or more of the  provisions  contained
in this  Agreement  shall for any reason be held to be  excessively  broad as to
duration,  geographical  scope,  activity or subject,  such  provision  shall be
construed  by limiting  and  reducing it as  determined  by a court of competent
jurisdiction,  so as to be enforceable to the extent  compatible with applicable
law.

         11. Notices.  Any notice which the Company is required to or may desire
to give you shall be given by personal delivery or registered or certified mail,
return  receipt  requested,  addressed to you at your address of record with the
Company,  or at such  other  place as you may  from  time to time  designate  in
writing.  Any notice which you are required or may desire to give to the Company
hereunder  shall be given by personal  delivery or by  registered  or  certified
mail,  return  receipt  requested,  addressed  to the  Company at its  principal
office,  or at such other office as the Company may from time to time  designate
in  writing.  The date of  personal  delivery  or the date of mailing any notice
under this Section 11 shall be deemed to be the date of delivery thereof.

         12.  Waivers.  If either party should waive any breach of any provision
of this  Agreement,  such party  shall not  thereby be deemed to have waived any
preceding  or  succeeding  


                                      -5-







breach  of the same or any  other  provision  of this Agreement.

         13. Complete Agreement; Amendments. The foregoing including Exhibits A,
B and C hereto,  is the entire  agreement  of the  parties  with  respect to the
subject matter hereof,  superseding any previous oral or written communications,
representations,  understandings,  or agreements with the Company or any officer
or  representative  thereof.  Any  amendment to this  Agreement or waiver by the
Company of any right hereunder shall be effective only if evidenced by a written
instrument  executed by the parties hereto,  upon authorization of the Company's
Board of Directors.

         14.  Headings.  The  headings of the  Sections  hereof are inserted for
convenience  only and shall not be deemed to  constitute  a part  hereof  nor to
affect the meaning of this Agreement.

         15.  Counterparts.  This  Agreement may be signed in two  counterparts,
each of which  shall be  deemed an  original  and both of which  shall  together
constitute one agreement.

         16.  Governing Law. This  Agreement  shall be governed by and construed
under Massachusetts law.

         17.  Arbitration  of Disputes.  Subject to the rights of the parties to
seek injunctive relief as described herein, any controversy or claim arising out
of, or relating to, any provision of this Agreement  shall be settled by binding
arbitration in accordance with the laws of the  Commonwealth of Massachusetts by
three  arbitrators,  one of whom shall be appointed by the Company,  one of whom
shall be appointed by you,  and the third  arbitrator  who shall be appointed by
the first two  arbitrators.  If the first two  arbitrators  cannot  agree on the
appointment of a third arbitrator,  then the third arbitrator shall be appointed
by the American Arbitration  Association in the City of Boston. Such arbitration
shall be  conducted  in the City of Boston in  accordance  with the rules of the
American  Arbitration  Association,  except  with  respect to the  selection  of
arbitrators,  which shall be as provided in this Section.  Judgment on the award
rendered  by the  arbitrators  may be entered in any court  having  jurisdiction
thereof and shall not be appealable.  The prevailing  party in such  arbitration
proceeding  shall  be  entitled  to  reimbursement  by the  other  party  of all
reasonable  legal  fees and other  costs  incurred  by the  prevailing  party in
connection with such proceeding,  including any legal fees and costs incurred in
connection with the enforcement of any award.



                      [THIS SPACE INTENTIONALLY LEFT BLANK]





                                      -6-







         If you are in agreement with the foregoing, please sign your name below
and also at the bottom of the Proprietary  Information and Inventions Agreement,
whereupon  this  Agreement  shall become  binding in accordance  with its terms.
Please then  return  this  Agreement  to the  Company.  (You may retain for your
records the accompanying counterpart of this Agreement enclosed herewith).

                                         Very truly yours,

                                         APPLIED SCIENCE AND
                                         TECHNOLOGY, INC.




                                         By:
                                           ------------------------------------
                                            Richard S. Post, Ph.D., President


Accepted and Agreed:




---------------------------------------
John M. Tarrh


                                      -7-



 








                                                                      EXHIBIT A


                   EMPLOYMENT TERM, COMPENSATION AND BENEFITS
                                OF JOHN M. TARRH

l.       Term.  The term of the Agreement to which this Exhibit A is annexed and
         incorporated  shall  be for a period  from  the date of this  Agreement
         through December 31, 1999.

2.       Compensation.

                  (a) Base  Salary.  Your Base Salary shall be  $103,000.00  per
         annum through June 28, 1997,  payable in accordance  with the Company's
         payroll  policies.  For Fiscal 1998 and 1999,  Base Salary  shall be as
         established  by the Board of Directors but in any event in an amount no
         less than $103,000.00.

                  (b) Bonus.  You shall be entitled to such bonuses as the Board
         of Directors may determine.

                  (c) Stock Option  Grants.  In addition to other stock  options
         now held by you,  you shall be  entitled  to receive  additional  stock
         option grants as determined by the Board of Directors.

3.       Vacation.  You shall be entitled to all legal and  religious  holidays,
         and four weeks paid  vacation  per annum.  Any unused  vacation  may be
         accrued or cashed in based on your then current Base Salary.

4.       Insurance and Benefits.  You shall be eligible for participation in any
         health, dental and other group insurance plans which may be established
         by the Company or which the Company is required to maintain by law.

5.       Retirement  Plan.  You will be eligible to participate in the Company's
         401(k) Plan.

6.       Other Benefits.   You shall be entitled to benefits otherwise available
         to all employees.

7.       Severance Benefits.

         (a) When  provided  for in this  Agreement,  you shall be  entitled  to
"Severance Benefits".  When used in this Agreement,  the term Severance Benefits
shall mean a total  amount  equal to (i) your then  current  annual Base Salary,
plus (ii) your Bonus earned for the  Company's  most recent  fiscal  year.  This
total  amount  shall be paid to you in twelve  (12) equal  monthly  installments
commencing  within  ten (10) days after the date of your  termination  of active
employment with the Company.

                                      A-1






         (b) In  addition,  the term  "Severance  Benefits"  shall  include  the
continuation for you and your family,  during the Severance  Period,  as defined
below,  of all of the other  benefits  which are provided or available to you on
the last day of your actual  service with the Company,  including your continued
accrual  and the  vesting  under the terms of any  pension  or 401(k)  plan then
sponsored by the Company to the maximum extent permitted by law. For purposes of
this  Agreement,  the term  "Severance  Period"  means the period of twelve (12)
months beginning on the last day of your active service with the Company.

         (c) The lump sum payment  referred to above will be in addition to, and
not in  substitution  for,  any accrued and unpaid  salary,  vacation,  pension,
retirement or other benefits,  unreimbursed  expenses or other payments to which
you may be otherwise entitled.

         (d) In the event of your death while you are  employed by the  Company,
your  then  current  Base  Salary  shall  continue  to be  paid  to  your  legal
representative  for a period of 120 days  following the date of your death;  and
for a period of three (3) years following your death, the Company shall continue
to provide to your spouse the health insurance  coverage described above. If you
die during the Severance Period,  all cash amounts which would have been payable
to you under this Exhibit "A", unless  otherwise  provided for herein,  shall be
paid in accordance with the terms of this Agreement to your estate.

         (e) You shall not be required to mitigate the amount of any payment the
Company becomes  obligated to make to you in connection with this Agreement,  by
seeking other employment or otherwise.

         8. Change in Control.

         (a) For purposes of this Agreement, "Change in Control" means and shall
be deemed to occur if any of the following occurs:

             (i) the  acquisition,  after  September 30, 1994, by an individual,
          entity or group [within the meaning of Section 13(d)(3) or 14(d)(2) of
          the Securities  Exchange Act of 1934 as amended (the "Exchange  Act")]
          of beneficial  ownership (within the meaning of Rule 13d-3 promulgated
          under the Exchange  Act) of 20% or more of either (A) the  outstanding
          shares of common stock, par value $ .01 per share, of the Company (the
          "Common  Stock"),  or (B) the  combined  voting  power  of the  voting
          securities of the Company  entitled to vote  generally in the election
          of directors (the "Voting  Securities");  or (ii)  Individuals who, on
          January 1, 1997,  constituted  the Board of  Directors  of the Company
          (the "Incumbent  Board") cease for any reason to constitute at least a
          majority of the Board of Directors of the Company; provided,  however,
          that any individual  becoming a director subsequent to January 1, 1997
          whose   election,   or  nomination   for  election  by  the  Company's
          shareholders,  was  approved  by a vote of at least a majority  of the
          directors  then


                                      A-2








          serving and  comprising  the  Incumbent  Board shall be  considered as
          though  such  individual  were a member of the  Incumbent  Board,  but
          excluding,  for  this  purpose,  any  such  individual  whose  initial
          assumption  of  office  occurs  as a result  of  either  an  actual or
          threatened  election contest (as such terms are used in Rule 14a-11 of
          Regulation 14A promulgated  under the Exchange Act) or other actual or
          threatened  solicitation of proxies or consents;  or (iii) Approval by
          the Board of  Directors  or the  shareholders  of the Company of a (A)
          tender offer to acquire any of the Common Stock or voting  securities,
          (B)  reorganization,  (C)  merger or (D)  consolidation,  other than a
          reorganization,  merger or consolidation  with respect to which all or
          substantially  all  of the  individuals  and  entities  who  were  the
          beneficial owners, immediately prior to such reorganization, merger or
          consolidation,  of the Common Stock and voting securities beneficially
          own, directly or indirectly,  immediately  after such  reorganization,
          merger or consolidation,  more than 80% of the then outstanding common
          stock  and  voting  securities  (entitled  to  vote  generally  in the
          election   of   directors)   of  the  Company   resulting   from  such
          reorganization,  merger or  consolidation  in  substantially  the same
          proportions as their respective  ownership,  immediately prior to such
          reorganization,  merger or consolidation,  of the Common Stock and the
          voting  securities;  or (iv) Approval by the Board of Directors or the
          shareholders   of  the  Company  of  (A)  a  complete  or  substantial
          liquidation or  dissolution  of the Company,  or (B) the sale or other
          disposition of all or substantially  all of the assets of the Company,
          excluding a reorganization of the Corporation under the corporate laws
          of a state or province other than Delaware.

         (b) In the  event  of a  Change  in  Control  during  the  term of this
Agreement  or any  extension  hereof and  provided  you remain  employed  by the
Company for a period of 12 months, you will receive, at the one-year anniversary
of the Change of Control,  a supplemental  amount in a lump sum equal to 150% of
your current Base Salary and Bonuses paid during the preceding  fiscal year, and
the fair  market  value of all other  benefits  then  payable,  irrespective  of
whether you thereafter actually terminate employment with the Company.

         (c)  In  the   event  of  your   actual   termination   of   employment
contemporaneous  with or  following a Change in  Control,  except (x) because of
your death,  (y) by the Company for Cause or Disability  (as each is hereinafter
defined) or (z) by you other than for Good Reason (as hereinafter defined):  (i)
you shall be entitled to receive,  in lieu of the sums described in this Section
7, an amount equal to 299% of Severance  Benefits due  determined  as if payable
under  Section  7  above,  to be paid  in  accordance  with  the  terms  of this
Agreement;  and (ii) the  following  additional  provisions  shall apply  (which
provisions shall supersede any other provisions of the Agreement,  including but
not limited to Section 2 of the  Agreement,  to the extent such  provisions  are
inconsistent with the following provisions):

             (1) Disability.  For purposes of this Section 8(c),  termination by
the Company of your  employment  based on  "Disability"  shall mean  termination
because of your  absence  from 


                                      A-3









your duties with the Company on a full time basis for one hundred  eighty  (180)
consecutive  days as a result  of your  incapacity  due to  physical  or  mental
illness,  unless  within  thirty  (30)  days  after  Notice of  Termination  (as
hereinafter  defined) is given to you  following  such  absence,  you shall have
returned to the full time performance of your duties.

             (2) Cause.  For purposes of this Section 8(c),  termination  by the
Company of your  employment  for "Cause" shall mean  termination or for cause as
defined in Sections 2.2(a), (c) or (d).

             (3) Good Reason.  Termination  by you of your  employment for "Good
Reason" shall mean termination based on:

                 (A) a determination by you, in your reasonable  judgment,  that
there has been a material  adverse  change in your status or  position(s)  as an
executive officer of the Company as in effect immediately prior to the Change in
Control, including, without limitation, a material adverse change in your status
or  position  as a result of a  diminution  in your  duties or  responsibilities
(other than, if applicable,  any such change  directly  attributable to the fact
that the Company is no longer  publicly  owned) or the  assignment to you of any
duties  or   responsibilities   which  are  inconsistent  with  such  status  or
position(s),  or any removal of you from, or any failure to reappoint or reelect
you to, such  position(s)  (except in connection  with the  termination  of your
employment  for Cause or Disability or as a result of your death or by you other
than for Good Reason);

                 (B) a reduction by the Company in your Base Salary as in effect
immediately prior to the Change in Control;

                 (C) the  failure by the  Company to continue in effect any Plan
(as  hereinafter  defined)  in which  you are  participating  at the time of the
Change  in  Control  of the  company  (or  Plans  providing  you  with at  least
substantially  similar benefits) other than as a result of the normal expiration
of any such  Plan in  accordance  with its terms as in effect at the time of the
Change in Control,  or the taking of any  action,  or the failure to act, by the
Company which would adversely affect your continued participation in any of such
Plans on at least as  favorable a basis to you as is the case on the date of the
Change in Control or which would  materially  reduce your benefits in the future
under any of such Plans or deprive you of any material benefit enjoyed by you at
the time of the Change in Control;

                 (D) the  failure by the  Company to provide and credit you with
the number of paid  vacation  days to which you are then  entitled in accordance
with the Company's normal vacation policy as in effect  immediately prior to the
Change in Control;

                 (E) the Company's  requiring you to be based at any office that
is greater than ten miles from where your office is located immediately prior to
the Change in Control except for required travel on the Company's business to an
extent  substantially  consistent


                                      A-4





with the business  travel  obligations  which  you undertook  on  behalf  of the
Company prior to the Change in Control;

                 (F) the failure by the Company to obtain from any Successor (as
hereinafter  defined)  the  assent to this  Agreement  contemplated  by  Section
8(c)(7) hereof;

                 (G) any purported termination by the Company of your employment
which  is not  effected  pursuant  to a Notice  of  Termination  satisfying  the
requirements  of Section  (8)(c)(4)  below (and, if applicable,  Section 8(c)(2)
above); and for purposes of this Agreement,  no such purported termination shall
be effective; or

                 (H) any  refusal  by the  Company to  continue  to allow you to
attend to matters or engage in activities  not directly  related to the business
of the Company which, prior to the Change in Control,  you were permitted by the
Board to attend to or engage in.

         For purposes of this Agreement, "Plan" shall mean any compensation plan
or any employee benefit plan such as a thrift, pension, profit sharing, medical,
disability,  accident, life insurance plan or a relocation plan or policy or any
other plan, program or policy of the Company intended to benefit employees.

             (4) Notice of Termination. Any purported termination by the Company
or by you following a Change in Control shall be  communicated by written notice
to the other party hereto which indicates the specific termination  provision in
this Agreement relied upon (the "Notice of Termination").

             (5) Date of Termination.  "Date of Termination"  following a Change
in Control shall mean (A) if your employment is to be terminated for Disability,
thirty (30) days after Notice of Termination  is given  (provided that you shall
not have returned to the  performance of your duties on a full-time basis during
such thirty (30) day period),  (B) if your employment is to be terminated by the
Company  for any reason  other than death or  Disability  or by you  pursuant to
Sections  8(c)(3)(F)  or 8(c)(7)  hereof or for any other Good Reason,  the date
specified in the Notice of Termination,  or (C) if your employment is terminated
on account of your death,  the day after your death.  In the case of termination
of your  employment  by the Company for Cause  pursuant  to  Subsection  8(c)(2)
hereof,  if  you  have  not  previously  expressly  agreed  in  writing  to  the
termination,  then within thirty (30) days after receipt by you of the Notice of
Termination  with  respect  thereto,  you may notify the Company  that a dispute
exists concerning the Termination,  in which event the Date of Termination shall
be the date set either by mutual  written  agreement  of the  parties or by such
court having the matter before it. During the pendency of any such dispute,  the
Company will continue to pay you your full  compensation in effect just prior to
the time the Notice of  Termination  is given and until the dispute is resolved.
However, if such court issues a final and non-appealable  order finding that the
Company had Cause to terminate you, then you must return all  compensation  paid
to you after the Date of  Termination  specified  in the  Notice of  Termination
previously  received  by  you. 


                                      A-5






             (6)  Compensation  Upon  Termination  or During  Disability;  Other
Agreements.

             (A) During any period  following a Change in Control of the Company
that you fail to perform your duties as a result of  incapacity  due to physical
or mental  illness,  you shall  continue to receive your Base Salary at the rate
then in effect  and any  benefits  or awards  under any Plan shall  continue  to
accrue during such period, to the extent not inconsistent with such Plans, until
and unless your employment is terminated pursuant to and in accordance with this
Section 8(c).  Thereafter,  your benefits shall be determined in accordance with
the Plans then in effect.

             (B) If your  employment is terminated for Cause  following a Change
in Control of the Company, the Company shall pay to you your Base Salary through
the Date of Termination at the rate in effect just prior to the time a Notice of
Termination  is given plus any benefits or awards  (including  both the cash and
stock  components)  which pursuant to the terms of any Plans have been earned or
become payable,  but which have not yet been paid to you.  Thereupon the Company
shall have no further obligations to you under this Agreement.

             (7) Successors, Binding Agreement.

             (A) The  Company  will seek,  by written  request at least five (5)
business  days prior to the time a Person  becomes a Successor  (as  hereinafter
defined),  to have such Person, by agreement in form and substance  satisfactory
to you,  assent to the  fulfillment  of the  Company's  obligations  under  this
Agreement.  Failure of such  Person to furnish  such  assent by the later of (i)
three (3)  business  days prior to the time such Person  becomes a Successor  or
(ii) two (2) business  days after such Person  receives a written  request to so
assent shall constitute Good Reason for termination by you of your employment if
a Change in Control of the Company occurs or has occurred.  For purposes of this
Agreement,  "Successor"  shall  mean any  person  that  succeeds  to, or has the
practical  ability to control (either  immediately or with the passage of time),
the Company's business directly, by merger or consolidation,  or indirectly,  by
purchase  of the  Company's  securities  eligible  to vote for the  election  of
directors, or otherwise.

             (B) This Agreement shall inure to the benefit of and be enforceable
by your personal legal representatives,  executors, administrators,  successors,
heirs,  distributees,  devisees and legatees. If you should die while any amount
would still be payable to you hereunder if you had  continued to live,  all such
amounts,  unless otherwise provided herein, shall be paid in accordance with the
terms of this  Agreement to your  devisee,  legatee or other  designee or, if no
such designee exists, to your estate.

             (C) For purposes of this Section 8, the "Company" shall include any
subsidiaries  of the Company and any  corporation  or other  entity which is the
surviving or continuing  entity in respect of any merger,  consolidation or form
of business combination in which the Company ceases to exist; provided, however,
for purposes of determining whether a Change


                                      A-6





in Control  has  occurred  herein,  the term  "Company"  shall  refer to Applied
Science and Technology, Inc. or its Successor(s).

             (8) Fees and Expenses; Mitigation.

             (A) The Company shall  reimburse you, on a current  basis,  for all
reasonable  legal fees and related  expenses  incurred by you in connection with
the Agreement  following a Change in Control of the Company,  including  without
limitation,  (i) all such fees and expenses,  if any,  incurred in contesting or
disputing any  termination of your  employment or (ii) your seeking to obtain or
enforce  any  right  or  benefit  provided  by this  Agreement,  in  each  case,
regardless  of  whether  or not your  claim is  upheld  by a court of  competent
jurisdiction; provided, however, you shall be required to repay any such amounts
to the  Company to the  extent  that a court  issues a final and  non-appealable
order  setting  forth  the  determination  that  the  position  taken by you was
frivolous or advanced by you in bad faith.

             (B) You shall not be required to mitigate the amount of any payment
the Company becomes  obligated to make to you in connection with this Agreement,
by seeking other employment or otherwise.

             (9) Taxes. All payments to be made to you under this Agreement will
be  subject to  required  withholding  of  federal,  state and local  income and
employment taxes.

             (d) Notwithstanding  any other provision of this Agreement,  in the
event that any payment of benefit  received or to be received by you as a result
of or in connection with a Change in Control,  whether  pursuant to the terms of
this Agreement or any other plan, arrangement or agreement with the Company (all
such payment and benefits being  hereinafter  called the "Total Payments") would
subject you to the excise tax (the "Excise  Tax")  imposed under Section 4999 of
the Internal Revenue Code of 1986, as amended (the "Code"),  then, to the extent
necessary to eliminate any such  imposition of the Excise Tax (after taking into
account any reduction in the Total Payments in accordance with the provisions of
any other plan,  arrangement or agreement,  if any), (a) any non-cash  severance
payments  otherwise  payable to you shall  first be reduced  (if  necessary,  to
zero), and (b) any cash severance payment otherwise payable to you shall next be
reduced. For purposes of the immediately  preceding sentence,  (i) no portion of
the Total Payments the receipt or enjoyment of which you shall have  effectively
waived in  writing  shall be taken  into  account,  (ii) no portion of the Total
Payment    shall   be   taken   into   account   which   in   the   opinion   of
nationally-recognized  tax counsel or certified public accountants (in each case
as selected by you) does not constitute a "parachute payment" within the meaning
of Section 280G of the Code, including, without limitation, by reason of Section
280G(b)(2) or (b)(4)(A) of the Code,  (iii) any payments to you shall be reduced
only to the  extent  necessary  so that the Total  Payments  [other  than  those
referred to in clauses  (i) and (ii)] in their  entirety  constitute  reasonable
compensation  for  services  actually  rendered  within  the  meaning of section
280G(4)(B)  of  the  Code  or are  otherwise  not  subject  to  disallowance  as
deductions,  in the opinion of the tax counsel or the accountants referred to in
clause (ii); and (iv) the value of any 



                                       A-7





non-cash  benefit  or any  deferred  payment or  benefit  included  in the Total
Payments  shall  be  determined  by such  accountants  in  accordance  with  the
requirements of section  280G(d)(3) and (4) of the Code (and such  determination
shall be reviewed by such tax counsel).






                                   A-8






                                                                      EXHIBIT B



                    OUTSIDE EMPLOYMENTS AND DIRECTORSHIPS OF

                                  JOHN M. TARRH



                                      None



                                      B-1






                                                                       EXHIBIT C



--------------------------------------------------------------------------------

                PROPRIETARY INFORMATION AND INVENTIONS AGREEMENT

--------------------------------------------------------------------------------

To:      Applied Science and Technology, Inc.
         35 Cabot Road
         Woburn, Massachusetts  01757

                                                              As of July 1, 1996

         The  undersigned,  in  consideration  of  and  as  a  condition  of  my
employment  or continued  employment  by you and/or by companies  which you own,
control,  or are affiliated with or their successors in business  (collectively,
the "Company"), hereby agrees as follows:

         1. Confidentiality. I agree to keep confidential, except as the Company
may otherwise consent in writing,  and, except for the Company's benefit, not to
disclose  or make any use of at any  time  either  during  or  subsequent  to my
employment, any Inventions (as hereinafter defined), trade secrets, confidential
information,  knowledge,  data or other  information of the Company  relating to
products,  processes,  know-how,  designs,  formulas, test data, customer lists,
business plans,  marketing plans and strategies,  pricing  strategies,  or other
subject  matter  pertaining  to  any  business  of  the  Company  or  any of its
affiliates,  which I may produce, obtain, or otherwise acquire during the course
of my  employment,  except as herein  provided.  I further agree not to deliver,
reproduce or in any way allow any such trade secrets,  confidential information,
knowledge, data or other information,  or any documentation relating thereto, to
be  delivered  to or used by any third  parties  without  specific  direction or
consent of a duly authorized representative of the Company.

          2. Conflicting  Employment;  Return of Confidential  Material. I agree
that  during  my  employment  with the  Company  I will not  engage in any other
employment, occupation, consulting or other activity relating to the business in
which  the  Company  is now or may  hereafter  become  engaged,  or which  would
otherwise  conflict  with  my  obligations  to  the  Company.  In the  event  my
employment  with the Company  terminates for any reason  whatsoever,  I agree to
promptly surrender and deliver to the Company all records, materials, equipment,
drawings,  documents and data of which I may obtain or produce during the course
of my  employment,  and I will not take with me any  description  containing  or
pertaining  to any  confidential  information,  knowledge or data of the Company
which I may produce or obtain during the course of my employment.



                                      C-1













         3.       Assignment of Inventions.

            3.1 I hereby  acknowledge and agree that the Company is the owner of
all Inventions.  In order to protect the Company's rights to such Inventions, by
executing  this  Agreement  I hereby  irrevocably  assign to the  Company all my
right, title and interest in and to all Inventions to the Company.

            3.2 For  purposes  of this  Agreement,  "Inventions"  shall mean all
discoveries,  processes, designs, technologies,  devices, or improvements in any
of the foregoing or other ideas,  whether or not  patentable  and whether or not
reduced to practice,  made or  conceived  by me (whether  solely or jointly with
others) during the period of my employment  with the Company which relate in any
manner to the actual or demonstrably anticipated business, work, or research and
development of the Company, or result from or are suggested by any task assigned
to me or any work performed by me for or on behalf of the Company.

            3.3  Any  discovery,   process,  design,   technology,   device,  or
improvement  in any of the foregoing or other ideas,  whether or not  patentable
and whether or not reduced to practice,  made or conceived by me (whether solely
or jointly with others) which I develop entirely on my own time not using any of
the  Company's  equipment,  supplies,  facilities,  or trade secret  information
("Personal  Invention") is excluded from this  Agreement  provided such Personal
Invention  (a)  does  not  relate  to the  actual  or  demonstrably  anticipated
business,  research and  development  of the  Company,  and (b) does not result,
directly or indirectly, from any work performed by me for the Company.

         4.  Disclosure  of  Inventions.  I agree  that in  connection  with any
Invention,  I will promptly disclose such Invention to my immediate  superior at
the  Company in order to permit the Company to enforce  its  property  rights to
such  Invention  in  accordance  with this  Agreement.  My  disclosure  shall be
received in confidence by the Company.

         5. Patents and Copyrights; Execution of Documents.

            5.1 Upon  request,  I agree to assist the Company or its nominee (at
its  expense)  during  and at any  time  subsequent  to my  employment  in every
reasonable  way to  obtain  for its  own  benefit  patents  and  copyrights  for
Inventions in any and all countries.  Such patents and  copyrights  shall be and
remain the sole and exclusive property of the Company or its nominee. I agree to
perform  such lawful acts as the Company  deems to be  necessary  to allow it to
exercise all right, title and interest in and to such patents and copyrights.

            5.2  In  connection  with  this  Agreement,   I  agree  to  execute,
acknowledge  and deliver to the Company or its nominee  upon  request and at its
expense all  documents,  including  assignments  of title,  patent or  copyright
applications,  assignments  of such  applications,  assignments  of  patents  or
copyrights upon issuance, as the Company may determine necessary or



                                      C-2







desirable to protect the  Company's  or its  nominee's  interest in  Inventions,
and/or to use in obtaining patents or copyrights in any and all countries and to
vest title thereto in the Company or its nominee to any of the foregoing.

         6. Maintenance of Records.  I agree to keep and maintain  adequate and
current  written  records  of all  Inventions  made by me (in the form of notes,
sketches,  drawings and other records as may be specified by the Company), which
records shall be available to and remain the sole property of the Company at all
times.

         7. Prior Inventions.  It is understood that all Personal Inventions, if
any, whether patented or unpatented,  which I made prior to my employment by the
Company, are excluded from this Agreement. To preclude any possible uncertainty,
I have set forth on  Schedule  A attached  hereto a  complete  list of all of my
prior  Personal  Inventions,   including  numbers  of  all  patents  and  patent
applications and a brief description of all unpatented Personal Inventions which
are not the property of a previous  employer.  I represent and covenant that the
list is  complete  and that,  if no items are on the list,  I have no such prior
Personal Inventions.  I agree to notify the Company in writing before I make any
disclosure  or  perform  any work on  behalf of the  Company  which  appears  to
threaten or conflict with proprietary rights I claim in any Personal  Invention.
In the event of my  failure  to give such  notice,  I agree  that I will make no
claim against the Company with respect to any such Personal Invention.

         8. Other Obligations.  I acknowledge that the Company from time to time
may have agreements  with other persons or with the U.S.  Government or agencies
thereof,  which impose  obligations  or  restrictions  on the Company  regarding
Inventions   made  during  the  course  of  work  thereunder  or  regarding  the
confidential  nature of such work.  I agree to be bound by all such  obligations
and  restrictions  and to take all action  necessary to discharge  the Company's
obligations.

         9. Trade Secrets of Others.  I represent that my performance of all the
terms of this  Agreement and as an employee of the Company does not and will not
breach any agreement to keep confidential proprietary information,  knowledge or
data acquired by me in  confidence  or in trust prior to my employment  with the
Company,  and I will not disclose to the Company,  or induce the Company to use,
any  confidential  or  proprietary  information  or  material  belonging  to any
previous  employer  or others.  I agree not to enter into any  agreement  either
written or oral in conflict herewith.

         10.  Modification.  I agree that any subsequent change or changes in my
employment duties,  salary or compensation or, if applicable,  in any Employment
Agreement  between the Company and me, shall not affect the validity or scope of
this Agreement.

         11.  Successors and Assigns.  This  Agreement  shall be binding upon my
heirs,  executors,  administrators or other legal representatives and is for the
benefit of the Company, its successors and assigns.



                                      C-3











         12.  Interpretation.  IT IS THE INTENT OF THE PARTIES  THAT in case any
one or more of the provisions contained in this Agreement shall, for any reason,
be held to be invalid, illegal or unenforceable in any respect, such invalidity,
illegality  or  unenforceability  shall not affect the other  provisions of this
Agreement,  and this Agreement shall be construed as if such invalid, illegal or
unenforceable  provision had never been contained  herein.  MOREOVER,  IT IS THE
INTENT OF THE PARTIES THAT in case any one or more of the  provisions  contained
in this  Agreement  shall for any reason be held to be  excessively  broad as to
duration,  geographical  scope,  activity or subject,  such  provision  shall be
construed by limiting and reducing it in  accordance  with a judgment of a court
of competent jurisdiction, so as to be enforceable to the extent compatible with
applicable law.

         13.  Waivers.  If either party should waive any breach of any provision
of this  Agreement,  he or it shall not  thereby  be deemed to have  waived  any
preceding  or  succeeding  breach  of the same or any  other  provision  of this
Agreement.

         14.  Complete  Agreement,  Amendments.  I  acknowledge  receipt of this
Agreement,  and agree that with respect to the subject  matter  thereof it is my
entire  agreement  with the Company,  superseding  any previous  oral or written
communications, representations,  understandings, or agreements with the Company
or any officer or  representative  thereof.  Any amendment to this  Agreement or
waiver  by  either  party of any  right  hereunder  shall be  effective  only if
evidenced by a written  instrument  executed by the parties hereto,  and, in the
case of the  Company,  upon  written  authorization  of the  Company's  Board of
Directors.

         15.  Headings.  The  headings of the  sections  hereof are inserted for
convenience  only and shall not be deemed to  constitute  a part  hereof  nor to
affect the meaning thereof.

         16.  Counterparts.  This  Agreement may be signed in two  counterparts,
each of which  shall be  deemed an  original  and both of which  shall  together
constitute one agreement.

         17. Governing Law. This Agreement shall be governed and construed under
Massachusetts law.


                      [THIS SPACE INTENTIONALLY LEFT BLANK]


                                      C-4











         18 Employment  Status.  Nothing in this  Agreement  shall affect in any
manner  whatsoever the right or power of the Company to terminate the employment
of the Employee.

                                       EMPLOYEE



                                       --------------------------------------
                                       John M. Tarrh


Accepted and Agreed:

APPLIED SCIENCE AND TECHNOLOGY, INC.


By:
   --------------------------------- 
   Richard S. Post, Ph.D., President


                                      C-5




                                     
                                                                      SCHEDULE A


                            LIST OF PRIOR INVENTIONS


                                         Identifying Number of
Title       Date                          Brief Description
-----       ----                          -----------------

                                      NONE