A proposal: Dynamic RAMs as particle detectors

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INSTRUMENTS

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A PROPOSAL: DYNAMIC RAMs AS PARTICLE DETECTORS G I A N F R A N C O CEROFOLINI, GIUSEPPE FERLA Direzione Tecnica. Divisione MOS, SGS-ATES, 20041 Agrate, Milano, Italy and A R M A N D O FOGLIO PARA lstituto di lngegneria Nucleare - CESNEF, Politecnico di Milano, via Ponzio 34/3, 20133 Milano, Italy Received 11 June 1979 and in revised form 5 October 1979 Dynamic RAMs can be used as s-particle detectors. They have low cost, can be directly interfaced with computers or microprocessors, and allow very high localization in space ( = 1 0 p m ) with a time resolution of 1-5000 ~s, according to the operation mode.

1. Introduction The MOS dynamic RAM (Random Access Memory) is an array of cells (roughly: capacitors) each of which can be in two states: "inversion" and "deep depletion ''1) (see fig. 1). The cell is in inversion when charges of sign opposite to that of the substrate (i.e., electrons if

the substrate is, as customary, p-type) are accumulated in the storage region of the cell. The cell in inversion is in a stable state. The cell is in deep depletion when no charge is accumulated in its storage region. The cell in deep depletion is in a metastable state as it decays onto the inversion state with a characteristic time Z

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Fig. 1. Schematic cross section and equivalent circuit of a typical dynamic RAM cell (M 4116 P, SGS-ATES). Gate and field oxides are obtained by thermal oxidation; the other layers by chemical vapor deposition (CVD). Thicknesses are roughly in scale and in the case considered are the following: field oxide, 0.85 pro; gate oxide, 800-900 A; first poly Si, 0.3 ttm; oxide between poly Si layers, 0.15-0.2 pro; second poly Si, 0.3 pro; dielectric layer, 0.5 ~ m ; passivation layer, 0.5-0.6/~m; junction depth, 1.4-1.5 ttm. The AI/Si metallization (1-1.2 p m obtained by sputtering) through which the transfer gate is controlled by the word line is not contained in this section because it runs mainly on the intercell field oxide. Writing is obtained by trasmitting a charge packet from the n + source (bit line) to the storage region by applying a suitable potential to the transfer gate; sensing is obtained by charge transfer in the reverse direction.

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which, in the absence of extended crystal defects 2) and with recombination generation centres kept low enough3), is of the order of 1-102 s at room temperature, this value being evaluated on the RAM while it is still in the wafer. The state of each cell of the RAM can be tested, in the wanted succession, by an amplifying and decoding apparatus. If a cell, brought in deep depletion, is tested within a time much shorter than r, the state of the cell is seen to be practically unchanged unless a "critical charge" qc is therein generated in the lapse of time considered. The critical charge is largely variable from one dynamic structure to another. For instance 4) 4K RAMs have been found to have qc between 1.5 x 106 and 2.5 x 106 electron charges; 16K RAMs qc around 1 x 10 6 electron charges; 16K CCDs* qc around 3 x l0 s electron charges and 64K CCDs qc around 1.5 x 105 electron charges. A greater sensitivity of structure is expressed by a lower value of qc; qc of CCDs can be diminished by lowering the operating voltage a n d / o r by increasing the oxide thickness ; qc of RAMs can be diminished by a linear shrink in the storage capacitor dimension. For example a linear shrink of 20% for a 4K RAMs involves a decrease of q~ from about 2 x 106 to 8 x l0 s electron charges. According to the q~ reduction that follows after shrinking of 4K RAMs, we assume q~ = 3 x l0 s as a reasonable estimate of the critical charge for suitably shrunk 16K RAMs: this requires very sensitive amplifiers. We note that chip (and cell) areas, and accordingly qc, as well as overlying layers may change from one process lot to another with no external indication thereof. In spite of this the following considerations are largely independent of the qc value. 2. Operation modes Consider now the two following operation modes: single cell scanning mode: only one cell is enabled and brought in deep depletion ( " w r i t e " operation); after writing the cell state is tested (" read" operation) and newly brought in the initial state of deep depletion. The least duration of this process is A t = l /~s, that hence is the minimum time within which a change of cell state can be allocated. R A M scanning C C D is the acronym for charge coupled devices and denotes a dynamic structure where a charge packet is moved along an array of MOS capacitor plates. Mutatis mutandis what is said for R A M s applies to C C D s too.

et al.

mode." all RAM cells are sequentially enabled and brought in deep depletion (" write" operation); after writing the state of each cell is sequentially tested (" read" operation) and newly brought in the state of deep depletion. Any change of state of the considered cell can have, taken place between the last write operation and the subsequent read operation; the least duration of this lapse of time is At=5000~s for the 16K RAM. Any other intermediate scanning mode is possible: the greater the number of explored cells, the longer At. In all operation modes the state of each cell tested during the reading step coincides with the state after writing unless the cell is in deep depletion and a sufficiently high number of electron-hole (e-h) pairs are generated in the storage region or in its vicinity. We denote as the depletion layer the storage region in deep depletion. e-h pairs generated in the depletion region are separated by the electric field of the cell: electrons are stored while holes are removed. This process has almost unit efficiency. e-h pairs generated within a diffusion length from the depletion region may contribute to charge it, but with an efficiency that varies with the position where the pair was generated and that can be estimated by numerical solution of the diffusion equation4). Since e-h pairs are the track of the passage of ionizing radiation, the possibility of using dynamic RAMs as particle detectors follows accordingly. This possibility is confirmed by the fact that ~z particles from the package are responsible for a " s o f t " failure mode (i.e., a failure which disappears after writing) of RAMs4'S). It is interesting to observe that the situation, previously described in terms of metastability, amplifying apparatus and so on, runs into the class of situations that allow a quantum measurement to be carried o u t 6 ) . 3. The detection mechanism To elucidate in greater detail the detection mechanism of the RAM detector, we shall limit ourselves in the following to ~z particles. The energy to create an e-h pair in silicon is 3.6 eV so that the particle must lose an energy at least of 1.1 MeV to generate a critical charge of 3 x 105 electrons. If this charge were generated in the depletion region, it would be sufficient to detect the particle. The width of the depletion region is typically 3/~m, value which must be compared with the range R at

DYNAMIC

1.1 MeV. Experimental data on the range of ~z particles in the low energy region are scanty however, so that most of the reported values are based on more or less approximate calculations. See for instance the tables of ref. 7 which give-a range of 4.3 p m for 1 MeV ~z-particles in silicon. This value is remarkably high compared with the range of 3.5 pm which can be obtained from the Bragg-Kleeman rule, starting from a range of 0.5 cm in air. Even adopting the recent extensive calculations of Gibbons et al.8), based on the Lindhard, Scharff and Schiott theory modified by the empirical corrections of Northcliffe and Schilling, we obtain a value higher than 3/,tin. Indeed the previously quoted computations give a projected range Rp = (3.09___ _~_-0.19)pm at 1 MeV and allow an extrapolation Rp= 3.3pm at 1.1 MeV. This confirms that ~ particles cannot generate the whole assumed critical charge in the depletion region, unless they impinge the surface other than perpendicularly. The detection mechanism, for perpendicular incidence, must therefore be attributed to ~z particles with energy greater than 1.1 MeV which create a number of e-h pairs in excess to the assumed critical charge, even if part of these is generated far from the depletion region; the higher number of pairs counterbalances their lower collection efficiency. If the critical charge is higher than the previously considered value, the RAM can again be used but incidence must be not perpendicular: the higher tlhe value of qc, the smaller the incidence angle*. We can also estimate that the detection efficiency of the RAM increases monotonically with ~zenergy, at least for high enough e-h diffusion lengths; the minimum ~z energy for detection is estimated to be around 2 MeV at the entrance of the depletion layer. We must however take into account the unavoidable, functional dead layer of the commercially available RAMs: this is roughly formed by 0.3-0.6 pm of polysilicon and 1-1.5 pm of silicon dioxide (for greater detail see fig. 1). This entrance window augments the ~z particle energy for detec* It is worthwhile noting that m u c h of the ingenuity of the R A M designers is now being directed towards making the dynamic R A M s as poor as possible in their ~z detection: a way to get this is to reduce the diffusion length in the bulk, for instance by epitaxial growth of the active zone with the wanted resistivity ( - 1 0 I 2 c m ) on a substrate with very low resistivity (=0.0001 ,(2 cm)9). All remedies suggested to make up for the soft failure mode of R A M s can be reversed and applied to increase the sensitivity of the R A M detector.

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tion. The dead layer can obviously be increased by means of calibrated thicknesses, so that a rough estimate of the cz particle energy can be obtained. Returning to the " s o f t " failure mode of dynamic RAMs induced by the ~z particles, Monte Carlo calculations4) of the number of soft errors for a particularly sensitive 4K RAM give as a by-product an estimate of the RAM intrinsic detection efficiency for naturally occurring ~z particles from the package: e (" counted" ~z/entered ~z)= 1.5 %. The single cell intrinsic efficiency increases with cell sensitivity and is likely to be close to 100% for high energy ~z particles on very sensitive 16K RAMs. 4. Particle detection When used as particle detectors, dynamic RAMs have particular characteristics such as the features: space resolution, parallel to the surface, A x = Ay =(10-20)/~m; space resolution, perpendicular to the surface, A z = 5 /~m ;

time resolution, A t = l ps (single cell scanning mode); At=5000 ps (RAM scanning mode); total tested area: =0.018 cm 2, i.e., about 12% of the total RAM area A. intrinsic efficiency ¢, for ~z particles striking the detector with the required energy: ~=100% if the RAM is used in the single cell operating mode; e = 12% i.e., the ratio of the active area to the total area in the RAM scanning mode. ~x(ym)

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TABLE 1 Useful area A and response to o~ particles of some typical detectors

A (cm 2) 3x~ (/t m) At(/ts) e

Scintillation counter

Position sensitive detector

Wire chamber

Bubble chamber

Nuclear emulsion

RAM {single cell operating mode)

RAM (RAM scanning mode)

1-103 103-104 10 3 - 1 0 2 100%

1-4 102-5 x 102 10 --3 100%

104-105 102 10 2 - 1 0 i 100%

104 102 103 104 100%

1-103 10 l - 1 >106 100%

10 6 5-20 I 100%

1.8xI0 2 5 20 5x103 10-15%

These data are typical for 16K RAMs with silicon resistivity in the range (10-15),0 cm. These features render RAMs of remarkable interest as particle detectors; a comparison with other detectors is given in table 1 and in fig. 2. Proton energy loss in matter is smaller than for ~zparticles ; in particular the projected range in silicon for a proton energy of 360 keV is 3.3 ~m. This energy is sufficient to create in the depletion region or in its close vicinity a charge of about 105 pairs. This suggests that 64K CCDs can already be used for proton detection. 5. Discussion The previous discussion considers only the effects of the passage of the ionizing particle through the storage zone. Actually other possibilities can occur. For instance the particle can hit the RAM in the field zone or in the bit line. In the first case it does not influence the detection, because pairs can recombine in the p+ region, because of its low resistivity, while in the second case the event may be a source of error. Indeed if the particle strikes the bit line while floating, the charge generated in the depletion region of the junction can be detected and erroneously attributed to an event taking place in the storage zone. Let us call q the ratio of the number of false to true event. 17 is roughly given by the ratio of the lapse of time during which the bit line is floating (10--40 ns per cell) to the duration of each cycle (5000 #s) times the ratio of the bit line area to storage capacitor area. In addition the differ-

ent thicknesses of the dead layer, which vary from one region to another (see again fig. 1) can affect r/. For the considered 16K RAM r/ is higher than 1 unless it operates in the "page m o d e " ; in this mode all the bit lines are enabled simultaneously while the sense amplifiers are sequentially tested: ,7 is now estimated to be about 5-10%. This number can be drastically reduced by masking the bit line by a relatively thick ( - 1 0 / x m ) dead layer. This requires a deposition of the wanted material (e.g. SiO2) and a non-critical masking. The situation is better for both existing 4K and announced 64K RAMs. In fact, in these cases the bit line is not formed by a junction but a metal strip (AI/Si, MoSi2 or poly Si) contacting the transfer transistor only close to the storage region. References ]) H. Taub and D. Schilling, Digital hTtegrated electronics McGraw-Hill, New York, 1977). 2) H. Strack, K. R. Mayer and B. O. Kolbesen, Solid St. Electron. 27 (1979) 135. 3) L. Baldi, G. F. Cerofolini and G. Ferla, Surf. Techn. 8 (1979) 161. 4) T. C. May and M. H. Woods, IEEE Trans. Electron. Dev. 26 (1979) 2. 5) D.S. Yaney, J.T. Nelson and L. L Vanskike, IEEE Trans. Electron Dev. 26 (1979) 10. o) A. Daneri, A. Loinger and G. M. Prosperi, Nuovo Cim. 44B (1966) 119. 7) C . F . Williamson, J.P. Boujot and J. Picard, Report CEA - R 3042 (1966). 8) J.F. Gibbons, W . S . J o h n s o n and S . W . Mylroie, Projected range statistics (Dowden, Hutchinson and Ross, Stroudsburg, Pa., 1975). ~) R. P. Capece, Electronics 52, March 15 (1979) 85.