Inside View: Flash and RAM

Published on December 29, 2011

Inside View: Flash and RAM


    Foreword


    New Year is a pleasant, bright holiday, in which we all sum up the year bygone, look with hope in the future and give gifts. In this regard, I would like to thank all the habr-residents for the support, help and interest shown in my articles ( 1 , 2 , 3 , 4 ). If you had not once supported the first, there would have been no subsequent (already 5 articles)! Thank! And, of course, I want to make a gift in the form of a popular science-educational article about how it is possible to use rather harsh at first glance analytical equipment with fun (both personal and public). Today, on New Year's Eve, on the festive operating table are: USB-Flash drive from A-Data and Samsung SO-DIMM SDRAM module.



    Theoretical part


    I’ll try to be extremely brief so that we all have time to prepare an Olivier salad with a margin for the festive table, so some of the material will be in the form of links: if you want, read at your leisure ...

    What kind of memory happens?

    At the moment, there are many options for storing information, some of them require constant recharge of electricity (RAM), some are forever “sewn” into the control circuits of the equipment surrounding us (ROM), and some combine the qualities of those and others (Hybrid). The latter, in particular, belongs to flash. It seems to be non-volatile memory, but the laws of physics are difficult to repeal, and periodically you still have to rewrite information on flash drives.

    Here you can learn more about the diagram below and a comparison of the characteristics of various types of "solid state memory". Or here - it’s a pity that I was still a child in 2003, they didn’t allow me to participate in such a project ...


    Modern types of "solid state memory". A source

    The only thing that, perhaps, can combine all these types of memory is a more or less the same principle of operation. There is some two-dimensional or three-dimensional matrix, which is filled in 0 and 1 approximately in this way and from which we can subsequently either calculate or replace these values, i.e. all this is a direct analogue of the predecessor - memory on ferrite rings .

    What is flash memory and what is it (NOR and NAND)?

    Let's start with flash memory. Once upon a time, on the notorious ixbt, a rather detailed article was published about what Flash is and what 2 main varieties of this type of memory are. In particular, there is a NOR (logical NOR) and of NAND (logical not-and) the Flash-memory ( here , too, everything is very detailed), which differ slightly in their organization (eg, NOR - two-dimensional, of NAND may be a three-dimensional) but they have one common element - a floating gate transistor.


    Schematic representation of a floating gate transistor. Source

    So, how does this engineering miracle work? Together with some physical formulas it is described here.. In short, between the control gate and the channel through which current flows from source to drain, we place the same floating gate surrounded by a thin layer of dielectric. As a result, when a current flows through such a “modified” field effect transistor, some of the high-energy electrons tunnel through the dielectric and end up inside the floating gate. It is clear that while the electrons tunneled, wandered inside this shutter, they lost some of their energy and can’t practically return back.

    NB: “practically” is the key word, because without overwriting, without updating the cells at least once every few years, Flash “resets” the same way as RAM, after turning off the computer.

    There, on ixbt, there is another article, which is devoted to the possibility of recording on one transistor with a floating gate several bits of information, which significantly increases the recording density.

    In the case of the flash drive we are considering, the memory will, of course, be NAND and, most likely, a multi-level cell (MLC).

    If it is interesting to continue to get acquainted with the technologies of Flash-memory, then here is a look from 2004 on this issue. And here ( 1 , 2 , 3 ) some laboratory solutions for the memory of a new generation. I don’t think that these ideas and technologies have been put into practice, but maybe someone knows better than me ?!

    What is DRAM?

    If someone forgot what DRAM is, then you are welcome here .

    Again, we have a two-dimensional array, which needs to be filled with 0 and 1. Since it takes quite a long time to accumulate the charge on the floating gate, in the case of RAM a different solution is used. The memory cell consists of a capacitor and a conventional field effect transistor. In this case, the capacitor itself has, on the one hand, a primitive physical device, but, on the other hand, is nontrivially implemented in hardware:


    RAM cell device. Source

    Again on ixbt there is a good article on DRAM and SDRAM. She, of course, is not so fresh, but the fundamental points are described very well.

    The only question that torments me is: can DRAM have, like flash, a multi-level cell? It seems so , but still ...

    The practical part


    Flash

    Those who use flash drives for a long time, probably already saw a “bare” drive, without a case. But I will nevertheless briefly mention the main parts of a USB flash drive:


    The main elements of a USB flash drive: 1. USB connector, 2. controller, 3. PCB multilayer printed circuit board, 4. NAND memory module, 5. crystal oscillator reference frequency, 6. LED indicator (now, however, it is not on many flash drives), 7. write protection switch (similarly, many flash drives are missing), 8. space for an additional memory chip. Source

    Let's go from simple to complex. Crystal oscillator (more about the principle of operation here ). To my deep regret, during polishing the quartz plate itself disappeared, so we can only admire the case.


    Case of a quartz oscillator

    By chance, in the meanwhile, I found what the reinforcing fiber inside the textolite and balls look like, of which textolite consists for the most part. By the way, and the fibers are nevertheless stacked with a twist, this is clearly visible in the upper image:


    Reinforcing fiber inside the PCB (red arrows indicate the fibers perpendicular to the cut), of which the bulk of the PCB is composed.

    And here is the first important part of the flash drive - controller:


    Controller . The upper image was obtained by combining several SEM micrographs.

    I admit honestly, I did not quite understand the idea of ​​the engineers who placed some additional conductors in the chip fill itself. It may be easier and cheaper to do from the point of view of the technological process.

    After processing this picture, I shouted: “Yayayayaz!” And ran around the room. So, to your attention is the 500 nm process technology in all its glory with perfectly drawn drain borders, the source, control shutter and even the contacts are preserved in relative integrity:


    “Ide!” Microelectronics - the 500 nm process technology of the controller with beautifully drawn separate drains, sources (Source) and control shutters (Gate)

    Now we proceed to the dessert - memory chips. Let's start with the contacts that literally nourish this memory. In addition to the main (in the picture of the "thickest" contact), there are also many small ones. By the way, “thick” <2 diameters of a human hair, so everything in the world is relative:


    SEM images of contacts supplying a memory chip

    If we talk about the memory itself, then here we are also waiting for success. It was possible to shoot individual blocks whose borders are marked by arrows. When looking at an image with maximum magnification, try to tighten your eyes; this contrast is really difficult to distinguish, but it is on the image (for clarity, I marked a separate cell with lines):


    Memory cells 1. The boundaries of the blocks are marked with arrows. The lines denote individual cells.

    At first it seemed to me as an artifact of the image, but after processing all the photos at home, I realized that it was either control gates extended along the vertical axis with an SLC cell, or several cells assembled in an MLC. Although I mentioned MLC above, it’s still a question. For reference, the “thickness” of the cell (ie the distance between two bright points in the lower image) is about 60 nm.

    In order not to dissemble - here are similar photos from the other half of the flash drive. A completely similar picture:


    Memory cells 2. The boundaries of the blocks are highlighted by arrows. Lines denote individual cells.

    Of course, the chip itself is not just a collection of such memory cells, there are some other structures inside it that I could not identify:


    Other structures inside NAND memory chips

    DRAM

    Of course, I didn’t saw the entire SO-DIMM board from Samsung, I only “disconnected” one of the memory modules with a building hair dryer. It is worth noting that one of the tips suggested after the first publication came in handy was to cut it at an angle. Therefore, for a detailed immersion in what you saw, it is necessary to take this fact into account, especially since a cut at 45 degrees allowed us to still obtain “tomographic” sections of the capacitor.

    However, according to tradition, we start with contacts. It was nice to see what the “chip” of the BGA looks like and what the soldering itself is: The “chip” of the BGA


    solder

    And here is the second time it’s time to shout: “Ide!”, As it was possible to see individual solid-state capacitors - concentric circles in the image, marked by arrows. They store our data during computer operation in the form of a charge on their covers. Judging by the photographs, the dimensions of such a capacitor are about 300 nm in width and about 100 nm in thickness.

    Due to the fact that the chip is cut at an angle, some capacitors are cut neatly in the middle, in others cut only "side»:


    the DRAM memory in all its glory

    If anyone doubts that these structures and have the capacitors, then there can be see a more “professional” photo (though without a large tag).

    The only moment that confused me was that the capacitors are located in 2 rows (bottom left photo), i.e. it turns out that 1 bit accounts for 2 bits of information. As mentioned above, information on multi-bit recording is available, but how this technology is applicable and used in modern industry remains a question for me.

    Of course, besides the memory cells themselves inside the module, there are also some auxiliary structures, the purpose of which I can only guess:


    Other structures inside the DRAM memory chip

    Afterword


    In addition to the links that are scattered throughout the text, in my opinion, this review is quite interesting (albeit from 1997), the site itself (and photo gallery, and chip-art, and patents, and many, many things) and this office , which actually engaged in reverse engineering.

    Unfortunately, we could not find a large number of videos on the subject of Flash and RAM production, so you will only have to be content with the assembly of USB-Flash drives:



    PS: Once again, all a Happy New Year to the black water dragon !!!
    It turns out strange: I wanted to write an article about Flash one of the first, but fate decreed otherwise. Crossing our fingers, we hope that the following at least 2 articles (about bio-objects and displays) will be released in early 2012. In the meantime, the seed is carbon tape: The


    carbon tape on which the test samples were mounted. I think that ordinary tape looks similar



    First , a complete list of published articles on Habré:

    Opening the Nvidia 8600M GT chip , a more detailed article is given here: Modern chips - an inside view Inside
    view: CD and HDD
    Inside view: LED bulbs
    Inside view: LED industry in Russia
    Inside view: Flash -memory and RAM
    Inside view: the world around us
    Inside view: LCD and E-Ink displays
    Inside view: digital camera arrays
    Inside view: Plastic Logic
    Inside view: RFID and other marks
    Inside view: graduate school in EPFL. Part 1
    Inside Look: Graduate School at EPFL. Part 2
    Inside view: the world around us - 2
    Inside view: the world around us - 3
    Inside view: the world around us - 4
    Inside view: 13 LED lamps and a bottle of rum. Part 1
    Inside view: 13 LED lamps and a bottle of rum. Part 2
    Inside view: 13 LED lamps and a bottle of rum. Part 3
    Inside Look: IKEA LED Strikes Back
    Inside Look: Are Filament Lamps So Good?

    and 3DNews:
    Microview: comparing displays of modern smartphones

    . Secondly , in addition to the HabraHabr blog , articles and videos can be read and viewed on Nanometer.ru , YouTube , and Dirty .

    Thirdlyif you, dear reader, liked the article or you want to stimulate writing new ones, then proceed according to the following maxim: “pay what you want”

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    Sometimes you can read briefly, and sometimes not so much about the news of science and technology on my Telegram channel - we ask you;)