Studies of wafer surface charging using the THOR monitor device

The THOR monitor is a Texas Instruments in-house gate oxide integrity test device used to qualify the charging performance of production implant processes. It has five charge collection anteanna on capacitor structures with antenna to gate ratios ranging from 100 : 1 to 7500 : 1. The gate oxide thickness of 200 A could be considered to be large, but the addition of photoresist makes this device extremely sensitive to charging damage. A sensitive THGR photoresist configuration has been studied in conjunction with advanced beam diagnostics in a situation in which inte:;,;tttent yield was achieved. A P/ 150 keV/ZO mA implant was found to give full yield on the test structure for only approximately half of the implants before enhancements to the flood system overcame this. Yraditional beam diagnostics, such as wheel current, capacitive charge sensor voltage and beam profile width could not predict when yield would be good or bad. However, measurement of slow ion spectra before an implant using a cylindrical mirror analyser ion spectrometer gave good correlation between the slow ion spectrum and THOR yield. The variation in slow ion spectrum could be related to ion source tuning and demonstrates that although present charge neutralization devices are sufficient for todays requirements, fiulu;: charging performance improvements may well come about from understanding the generation of high quality beams 2nd the use of advanced beam diagnostics to aid in automatic implanter tuning.

1. ilntroduction

7500: I. After implantation, device pass or fail is decided either by a gate oxide integrity (GOI) test (the gate must not short zircurt befiow 18 V) or by a leakage current test (the leakage carrem at 12 V must be less than 100 pAI. Thirty six of the dies are usually tested and one failure is allowed before yield loss is reported. It is observed that the smallest antenna ratio is insensitive, aiways having 100% yield. The largest antenna ratio yield is most strongly correlated to that of the production device. The intermediate antenna ratios are more sensitive than the larger site, the opposite of what would be expected. The photoresist pattern has a huge influence on the capacitor ykib. Typically, five of the six test areas in each die are covered by photoresist. In the absence of photoresist, the device can be implanted to full energy (200 keV) and current (- 20 mA) without any yield loss problems. With photoresist coverage. the capacitive structures become vastly more sensitive, and the sensitivity changes with different photoresist patterns. This factor accounts for why B device with what can be considered a thick gate oxide can suffer yield loss. The increased sensitivity of the smaller structures may be explained by their proximity to more photoresist than the larger structures.

The THOR mor$nr I& ~1 ‘i’exas Instruments in-hous*.. charging test device that is used routineiy to qualify implanter charging performance prior to production. Photoresist on the monitor surface makes the device extremely sensitive and it has been necessary to study specific implant conditions that have given intermittent yield loss for this device. Applied Materials has a program investigating the monitoring of beam quality and charging phenomena by ion spectroscopy [l] which has experimentally shown the influence of beam potential on surface potentials [2]. Thor yield studies gave opportunities to investigate these phenomena in relationship to device yield, evaluate new beam quality diagnostics and improve charging solutions.

i. Description of the THOR device THOR test structures consist of 72 repeated dies on 150 mm diameter wafers. Each die consists of six identical test areas. Included in each test area are five capacitor structures with oxide gate thicknesses of 200 i overlain with square charge collection antennae. The antenna to gate area ratios are 100: 1, 1024:1, 2500:1, 5OOO:l and

’ Corresponding

3. THOR yield perfmnance The typical THOR configuration (five out of six test sites photo-resist covered) proved to be difficult to rou-

auhx.

0168-5M3X/95/$09.50 0 SSDf 016H-5X3XI94)O(J453-h

1995 Eisevier

Science-

B.V.

All

rights

reserved

implant cuuld 12% be done without ~ntermitteot yield loss at a beam current of 20 mA. Reducing the current to 13 mA a4WayS R”‘;” ?a vieid ihi dre 4argest r&e, allnwi;~g I/T .Pf: implant to be qudbfitwl ihz, production. Later t4+ that changing the pe- and pl,gt-a~~i~cjetaii~~r ,Ing~: L 4 tiats improved yield, but still did not give f&i yield for the smaller sites. This was slrong evidence suggesting that beam tuning could play a rote in device yield. The complete solution giving full yield for all site sizes was to improve the emission current attainable by the PFS. For the increased emission (enhanced) PPS, the implant conditions were P/50 i- 100 keV/ZO mA. Many months of operation with fuI1 THOR yiefd confirm this has solved the problem.

Fig. 1. A view of a THOR die. The capacitors evaluated during this study

are situated

in the top left hand comer.

tinely implant without damage. With a primary electron flood gun system [3] installed in a PI9500, it was not possible consistently to implant these structures successfuliy at high beam energies and currents. ‘This situation was mainly solved by installing the Plasma Flood System [4], although for a while one P/l50 keV implant remained difficult. The P/150 SeV performance is summariz:rd in Fig. 2. Yields for the laq;est area and the average yield for the intermediate sized capacitor structures are shown over a range of conditions. The groups of implants shown were carried out over a period of several months. In each group of implants, beam tune conditmns were varied (see Section 4.3). The figure shows that, initially, the P/l50 keV

The situ&on of intermittent THOR yield allowed the usefulness of beam quality and charging monitors as yield predictors to be evaluated. Traditional real time charging monitors such as capacitive charge sensor and wheel current readings, and other beam diagnostics such as beam profile width were not found to be useful predictors of yield. ion spectroscopy was a tool being used at Applied Materials to understand charging phenomena [1,2], and so a new beam quality monitor, a cylindrical mirror anafyser (CMA), was introduced into these investigations. 4. I. Tire cylitzdrical

nzii7;3~ cvza/yser

The CMA used in these studies is shown in Fig. 3. Ions enter the front aperture, and are defkc?ed by :I pitive potential applied to the mirror electrode. 0nly ions within

loo 90 80

’ -cf-

I

70

field(7500) -..&.

Yi&d(Gtkj

--

Fig. 2. THOR

yield

for different

capacitor

P/SO+tOOket’ 13mA site sizes compared

to beam potential

Pofential /

10

_,--

/

20mA

Ban

1

P&O+90keV

Enhanced

200lA

PFS

under

different

phosphorus

implant

conditions.

See lext

explanation. I. ADV.?WCED fMP1,ANTIRS/PRQCESS

CONTROL

- Output Aperture

Fig. 4. Typical slow ion spectra observed before or after imolantation. 1 :~rse correspond to the first implants done at P/50 + 100 keV/ZO mA. Spectra with no observable features are not Gown.

Fig. 3. The cylindrical

mirror

analyser.

The dotted

line

plate current using an electrometer. In this CMA, transmitted ions have twice the kinetic energy of the mirror potential. A negatively biased aperture in front of the detector plate minimizes the background current from stray electrons.

indicates

i!:: path of a transmittedion. in small energy range are deflected through the correct angle to exit through a colkntr slit anti ::trike a detection plate. Ail energy jpecirum is collecied by scanning the mirror electrode potential and measuring the detection

-

4.2. St&v ion specmra and yield

The CNA was mounted on the guide tube [4] of a PFS. Other results of this spectrometer relevant to this study are

her Gap

Mis-tuned outer Gap

I 0

10

20

30

40

Ion Energy (ev) Fig. 5. Slow ion spectra

measured

at different

source

gaps.

/8

i

50

60

discussed in an ac&offlpanying paper [2]. When a PFS plasma source was turned an, the beam potentiai was reduced below a level at which it could be measured. Therefore, slow ion energy spectra were measured before each implant ws carried out. The spectra feIl into two main groups, as shown in Fig. 4. Firstly, some spectra showed little or no measured current (broken lines). This can be interpreted as showing a low (near zero) beam potential because ions emitted from the beam are at too low an energy to be efficiently detected by the CMA. Other spectra showed larger ion currents (full lines). The position of the peak in the intense spectra can be interpreted as a measureof the beam potential. Fo!lowing these interpretations, the beam potentials (before the FFS was energised) have been plotted for the implants in Fig. 2. There is a good correlation between high beam potential and lower yield. Further studies to confirm this interpretation of the data are required. Alternative explanations, such as variations in the direction of slow ion emission are also possible. There are no clues from present beam profiles that this is the case, although improved profilers are being developed [5]. Whatever the interpretation, the slow ioil intensity is a predictive diagnostic for THOR yield which could be used to study tuning conditions &z-t produced harmful or good qu&ty beams. 4.3. Slow ion spectva LT. beam tuning

The ion source tuning conditions for the implants in the first two groups shown in Fig. 2 were varied over a wide range of parameters. Oven feed temperatures, arc currents and voltages, support gas conditions and extraction electrode positions were all varied. Once the trend between yield and ion spectra was found, further experiments were carried out to investigate what conditiccs causedthe different ion specira. It was found that ~1given source conditions, the two types of slow ion spectra could be produced by changing the ion source slit to extraction electrode gap, as shown in Fig. 5. This result is not consistent in that, depending on exact source conditions, it is just as likely that the inner gap produces the intense slow ion spectrum. Further studies are required to understand more detail of this phenomenon [2,6,7]. The beam potentials during the Pj60 + 90 keV implants shown in Fig. 2 were controlled by varying the extraction electrode position. The beam tuning conditions for the implants using the enhanced PFS

were also varied in this way, although no s!ow ion specrroscopy was cnrried :.trrt.

5. CQnflusisns The above resultsconfirm some of the theoretical view points expressedover the past few years. In particular, the role of beam qua1ity in device yield has been demonstrated. The study suggests tbat state of the art charging systems are adequate for present needs and that future charging performance improvements may be gained by incorporating advanced diagnostics into implanters that feed back into the beamline tuning automation.

The authors wish to thank Prof. David Armour of Salford University for first suggesting slow ion spectroscopy and the use of the CMA. Some of this work was funded as part of a UK Goverment DTI LINK Scheme.

References

[II J. England, N. Bryan,H. tto, D. Armour,J. Van den Berg,i. Fotheriaghamand P. Kindersley,in: Ion JmplantatiocTechnology - 92, eds, D.F. Downey,M. Farley,KS. Jonrsaad G. Ryding(Ebevier,Amsterdam,1993)p. 613. bl 1. England C. Cook, D. Armourand M. Foad,theseProceedings(IiT”941,Nucl. instr. and Melh. B 96 fi995) 39. 131 M.T. Wauk, N. White, B. Ad&i, M. Currentand J. Strain. Nuci. Instr. and Me&. R 55 (1991) 413. I41

H. Ito, T. Kamata,3. England,1.Fotheringham,F. Plumb and M. Current, these Prrceedings (iIT’94$, Nucl. Instr. and Meth. B 96 (199%

[51

30.

W. Szajnowski, J. England, K. Stephens, I. Fotheringham and D. Scargill, Part II of these Proceedings (IIT’94) iNo&-Holland, Amsterdam, 1995).

b1 M. Foad,D. Armour, 2. Kihnes,B. Hittan, I. Van den Berg, C. Cook, A. Chew, D. Sykes, J. England, A. Devaney, H. Ito, N. Bryan.,F. Plumb,P. I(indersleyand S. Moffatt, Fart fI of theseProceedings (IIT”‘;) (North-Holland,Amsterdam, 1995). I71 H. ito, A. Devaney, N. Bryan, D. Armour and M. Foad, Part 11

of these ProceedingsiIIT’94) (North-Holland,Amsterdam, 1995).

1. ADVANCED IMPLANTERS/PROCESS

CONTRQL